GYROSCOPIC SURVEYING.pdf

Baker Hughes INTEQ

Gyroscopic Surveying Level Rotor Gyro Photomechanical Systems

Training Manual Part Number 750-500-071 Rev. A May 1997

Baker Hughes INTEQ Technical Publications Group P.O. Box 670968 Houston, TX 77267-0968 USA 713-625-4415

The information contained herein is believed to be accurate and, where appropriate, based on sound engineering principles. However, Baker Hughes INTEQ makes no warranties or representations to that effect. All such information is furnished “as is”, and use of such information is entirely at the risk of the user. Unauthorized copying and/or use of the information contained herein is prohibited, and subject to penalties under copyright and other laws of the United States and other countries.

1997 Baker Hughes INTEQ All Rights Reserved

Table of Contents

Table of Contents Chapter 1

Gyroscopic Surveys & Equipment Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 The Baker Hughes INTEQ Gyroscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3 Orientation of the Gyro Compass Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 Example of Orientation Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 Gyro Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5 Gyros Used by Baker Hughes INTEQ. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5 Gyro Multishot Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7 Running Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7 Warm-Up Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-9 Control Sub . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10 Monitor Box. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10 Chapter 2

Gyro Multishot Surveys Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 Pre-Job Information Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 Running a Gyro Multishot Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2 Rigsite Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2 GMS Survey Running Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 Chapter 3

Processing and Calculating a GMS Survey Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 Orientation Correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 Orientation Correction Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 Orientation Correction Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3 Reference Manual 750-500-071 Rev. A / April 1997

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Table of Contents

Gyroscopic Surveying

Drift Corrections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3 Example of Drift During a Drift Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 Drawing a Drift Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 TAC Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7 Chapter 4

Survey Documentation Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 Final Survey Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 Forms, Reports, and Worksheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 Chapter 5

Gyro Single Shot Surveys Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 GSS Instrument Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 Pre-Job Information Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4 Running a Gyro Single Shot Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4 Rigsite Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4 GSS Survey Running Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6 Applying Orientation and Drift Corrections to GSS Surveys . . . . . . . . . . . . . . . . . 5-7 Chapter 6

High-Angle Gyro Surveying Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 Revised Foresight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 Calculating the Revised Foresight for a 2½" Gyro . . . . . . . . . . . . . . . . . . . . . . . 6-2 Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 Calculating the Revised Foresight for a 1½" Gyro . . . . . . . . . . . . . . . . . . . . . . . 6-5 Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 Inter-Gimbal Error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 Inter-Gimbal Correction for the 1½" Gyro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9 Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 Correcting Observed Drift Readings for Inter-Gimbal Error . . . . . . . . . . . . . . 6-10 Example Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11

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Baker Hughes INTEQ Confidential

750-500-071 Rev. A / April 1997


Table of Contents

Chapter 7

In-Hole Orientation Training Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 Downhole Orientation Using Conventional Multishot System. . . . . . . . . . . . . . . . . 7-1 Necessary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 Running Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2 Establishing Orientation Correction and Total Drift . . . . . . . . . . . . . . . . . . . . . . 7-3 Appendix A

Tool Axis Correction (TAC) Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1 Survey Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1 Rotation Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1 “Yo-Yo” Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2 In-Run Versus Out-Run Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Calculation of the TAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Method A: Rotation Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Method B: “Yo-Yo” Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Method C: In-Run / Out-Run Shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3 Derivation of Formulae Used in the TAC Program. . . . . . . . . . . . . . . . . . . . . . A-4 Appendix B

Exercises Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-2 Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3

Reference Manual 750-500-071 Rev. A / April 1997

iii Confidential

Chapter

1

Gyroscopic Surveys & Equipment Training Objectives Upon completion of this chapter, the trainee should be able to: •

Explain the basic principles of the level rotor gyro.

•

Explain what is meant by gyro “drift”.

•

Explain the terms foresight, foresight direction and orientation correction.

•

List and identify the instruments required for Gyro Multishot Surveys (GMS).

•

List and identify all items of running gear and ancillary equipment required for GMS surveys.

Instrument Overview A conventional gyroscope consists of a rapidly spinning wheel (called the Rotor), which is mounted in a frame (called Gimbals). The term “gyroscope” was first used by a French physicist who, in 1852, designed a 3-frame gyroscope to demonstrate the Earth's rotation. He called his mechanism a gyroscope from the Greek words GUROS (revolution) and SKOPEEIN (to view). See Figure 1-1. The rotor is held in the inner gimbal by rotor bearings. The frame has ball bearings between the inner gimbal and the outer gimbal and also between the outer gimbal and the body of the gyroscope. When the gyro is running, the spin rotor has a lot of angular momentum and resists attempts to change the direction of its spin vector. In plain terms, the spinning wheel wants to point its spin axis in the same direction all the time. The function of the gimbal system is to allow the case of the gyroscope to turn to different orientations without disturbing the spin rotor. (In technical terms, the gimballing system isolates the rotor from base rotation).

Training Manual 750-500-071 Rev. A / May 1997

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Gyroscopic Surveys & Equipment


Figure 1-1 The Baker Hughes INTEQ Gyroscope

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750-500-071 Rev. A / May 1997



The Baker Hughes INTEQ Gyroscope Baker Hughes INTEQ has two sizes of conventional gyroscope, 1½" OD and 2½" OD. These gyroscopes are known as two-degree-of-freedom level-rotor gyros. A compass card is mounted on top of the outer gimbal. In the 1½" gyro, the East-West line on the compass card is aligned with the spin motor axis. In the 2½" gyro, the North-South line on the compass card is aligned with the spin motor axis. When the gyro is running, the spin axis points in a fixed direction (theoretically) so the compass card maintains a fixed orientation. You will be shown a 2½" Baker gyroscope. It will be “run up” in a warmup box and its property of “rigidity” demonstrated. Note: Regardless of how the warm-up box is turned and tilted, the rotor spin axis points in a fixed direction and the North-South line on the compass card always stays in the same vertical plane. The Baker Hughes INTEQ gyros are used simply to provide a compass card which will maintain its initial orientation. Note: In practice, the spin axis gradually precesses from its initial heading (Gyro Drift) which complicates the interpretation and calculation of conventional gyro surveys. Angle units containing either plumb bob or drift arc inclinometers are attached to the top of the gyros. These angle units have a transparent glass base so the inclinometer and angle scale are superimposed on the gyro compass card in the survey picture. The gyro compass replaces the magnetic compass of “magnetic type” angle units, but otherwise the readings of inclination and hole direction are obtained by the same basic method as for the magnetic type surveys. Note: The fundamental point to appreciate is that gyroscopes are not significantly affected by the proximity of magnetized steel, so gyro survey tools are used in situations where magnetic interference prevents the use of any magnetic type survey tools. Gyroscopic surveys are always run on a wireline of some type because the gyro is a delicate instrument.


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Orientation of the Gyro Compass Card These “conventional” gyros have no preferred orientation. They will preserve their initial spin axis orientation. It is, therefore, necessary to go through a standard procedure to determine the relationship between the North of the gyro compass and either True or Grid North. A reference mark (called the foresight) must exist, which can be seen from the rotary table or orientation point on the rig. The direction from the rotary table to that reference mark must be accurately known. On land rigs, this reference might be a landmark whose coordinates could be obtained from a map. On offshore platforms in the North Sea, there are usually metal plates fixed to the structure – one for each line of slots. The direction from the rotary table to the metal plate is determined using a land surveyor's “North Seeking” gyroscope and theodolite. In many parts of the world, foresights may not be as readily available; therefore, help may have to be solicited from the company representative. At the start of the survey, the lower half of the tool is rested on an orienting table using a support arm. The gyro is stabbed onto the control sub and a telescope arrangement is used to physically align the tool and, hence, a particular point on the head of the gyro to the foresight (reference mark). When aligned in this way, a line bisecting the head of the gyro and passing through the zero on the outer Vernier scale should point directly at the foresight. The Vernier scale is fixed to the body of the gyro, and the Vernier 0 is in a fixed orientation relative to the orienting lug and canon connector on the base of the gyro. Now, if the North of the gyro compass were actually pointing to North (True or Grid), then the reading on the compass card next to the Vernier 0 would be the known direction from the rotary table to the reference point (the foresight direction). In practice, there will be a difference which is determined at this stage in the proceedings. By using the Vernier scale, this difference, which is the necessary Orientation Correction, can be accurately determined. Examine the 2½"gyro again. Look at the Vernier scale. Note the relationship between the zero on the Vernier scale and the position of the orienting lug and Canon connector on the bottom of the gyroscope.

Example of Orientation Correction •

Known Reference Direction

N3.5°E (003.5°)

•

Initial Vernier 0 Reading on Gyro

N6.8°E (006.8°)

•

Orientation Correction

–3.3° (3.3° WEST)

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750-500-071 Rev. A / May 1997



This orientation correction is a constant correction for one particular gyro survey. In the case of a gyro multishot, it must be applied to every single survey reading of hole direction.

Gyro Drift We saw earlier that the two-degree-of-freedom level-rotor gyro will try to point its rotor spin axis in the same direction all the time. Unfortunately, in practice the spin axis will slowly precess from its initial heading. Surveyors refer to this as Gyro Drift. There are several causes of gyro drift: 1.

The gyro spin axis tends to maintain a constant direction relative to an inertial frame of reference, e.g., a distant star, and not to the surface of the Earth, which is spinning on its axis. Therefore, a “perfect” gyro of this type would appear to drift because of the rotation of the Earth.

2.

The bearings of the gimbal system are not perfectly frictionless.

3.

It is extremely difficult for a technician to balance the spin motor housing perfectly to achieve zero drift, even at one particular latitude.

In practice, the gyros are balanced so that the rate of drift is below acceptable limits. Technicians work to a limit of 2' drift per hour; surveyors work to a limit of 6' per hour when checking gyros. Gyros tend to drift more at high inclinations because they are in a less stable gimbal configuration. In both gyro single shot and gyro multishot surveys, an attempt is made to correct for gyro drift. In both types of survey, the alignment of the gyro compass is checked at the end of the survey as well as the start. Since the gyro is aligned to the same reference mark in both cases, the difference between the Vernier 0 readings at the start of the survey and the end of a survey is the Total Observed Drift. We shall return to the subjects of orientation and drift corrections later in this manual.

Gyros Used by Baker Hughes INTEQ Historically, BHI had five different gyros which were used for conventional gyro single shot or gyro multishot surveys. These were: •

Humphrey 2½" and 1½"

•

D.K. 2½"

•

Eastman 2½" and 1½"


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All of these are two-degree-of-freedom level rotor gyros. At present, the Baker Hughes INTEQ gyros are used almost exclusively, but some districts may still have Humphrey or D.K gyros on their inventory. The following tables give some basic information about all of these gyros:

Gyro

Size

Current

Speed

Motor

Torquer

Humphrey

2½"

275 mA

19 - 21 K

Induction

Mercury Switch / Electromagnetic

Humphrey

1½"

315 mA

38 K

Synchronous

Mercury Switch / Electromagnetic

DK

2½"

l50 mA

20 - 21 K

Induction

Electrolytic Switch / Mechanical

Eastman

2½"

l80 mA

21 K

Induction

Electrolytic Switch / Electromagnetic

Eastman

1½"

220 mA

42 K

Synchronous

Electrolytic Switch / Electromagnetic

Gyro

Size

Construction

Temp. Limit

Metals

Caging

Stops

Humphrey

2½"

Non-Symetrical open motor

250°F

Different metals

Y/N

55°- 70°

Humphrey

1½"

Non-Symetrical open motor

250°F

Different metals

Y

28°- 34°

DK

2½"

Symetrical sealed

300°F

Same metals

N

70°

Eastman

2½"

Symetrical sealed

300°F

Same metals

N

70°

Eastman

1½"

Symetrical sealed

300°F

Same metals

N

35°

Notice that all of these gyros have a torquer to keep the spin rotor axis horizontal. The torquer is controlled by either a mercury switch mounted on the underside of the spin motor housing (Humphrey gyros), or by an electrolytic switch on top of the rotor housing. If the spin axis tilts away from the horizontal, fluid moves to one end of the switch and current to the torquer is switched ON. A small torque is then applied to the outer gimbal which causes the spin rotor axis to precess back to the horizontal. In the case of the Baker Hughes INTEQ gyros, the torque is applied via electromagnetic coils. During an actual survey, the gyroscope is powered by 16 D-cells in a battery barrel. The battery voltage will decrease with time, so the gyroscope is connected to a Control Sub which has the battery voltage as its input and supplies current to the gyro at a constant voltage level of 28V. The control sub is screwed into the battery barrel.

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750-500-071 Rev. A / May 1997



Gyro Multishot Surveys Gyro Multishot Surveys (GMS) are taken of sections of eased wellbore. Since the gyroscope is a delicate instrument, GMS surveys are always run on a wireline of some sort. Either the 2" or the 3" system can be used to run GMS surveys. However, the 3" system is normally preferred because the 2½" gyro is more stable and will have less tendency to drift at higher inclinations. The conventional gyro multishot instrument consists of: •

The multishot battery pack (as for DMS).

•

The multishot instrument body incorporating the camera, motor assembly, switch assembly and electronic timer (as for DMS).

•

The gyro angle unit. Available ranges are 0° to 2°, 0° to 5°, 0° to 12°, 0° to 24°, 0° to 34°, 5° to 90°.

•

The gyroscope. Either a 1½" or a 2½" Baker Hughes INTEQ gyro may be used, but as already stated, the 2½" gyro is usually preferred.

Running Gear The running gear for the 3" GMS system consists of: •

Wireline cablehead + crossover or spearpoint + wireline sub or rope socket

•

2 gyro swivels

•

Gyro spacer bar with top centralizer and stop collars

•

3" instrument barrel

•

3" control sub

•

2" or 3" battery barrel

•

Lower spacer bar with centralizer and stop collars

•

Bottom landing shock assembly

On the next page, make a large diagram of a complete GMS tool.


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Diagram of a Complete GMS Tool

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750-500-071 Rev. A / May 1997



Warm-Up Box This is a portable power supply. It can be used to bring the gyro to full speed before running the survey. Stab the gyro into the Gyro Cup (see Figure 1-2) on the warm-up box at least 30 minutes before running the survey. Figure 1-2 Feature and Control Placement on Warm-Up Box

Me ter A djustme nt S crew

M eter Simpson N o. 3323 External Power Conn ector

Cann on Plug Gyro Conn ector

Cag ing Con nector

G yro Cup

E XT. P O W E R C A G IN G

Battery Indicator Diodes B ATT E R Y

External Power Indicator Diode

External Power Sw itch

C HAR GE D

OFF

LO W

ON

B AT TE R Y

GYRO

V O LT S

V O LTS

E XTERNAL VO LT S

GY R O C U R RE N T

$66(7 12

OFF

BA K ER H U G HE S

ON G YRO

IN TE Q

Selector Knob

2-1/2 in. Gyro A lignm ent Pin S ocket

Gyro Pow er Switch

1-1/2 in. Gyro Alignm ent Pin So cke t

5-pin lead.............................110 volts only 7-pin lead.............................110 or 220 volts Note: M is a “modified” box able to take the initial surge of EC gyros. Have the warm-up box switched ON and stab the gyro. External power switch need only be on when you are charging the battery by an external power source. Battery volts should read 11 to 13 volts. The Low light will flash if power is too low. The Charged light will come on when fully charged. Training Manual 750-500-071 Rev. A / May 1997

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Control Sub This maintains a constant voltage of 28 volts to the gyro. Sixteen D-cell batteries in the battery barrel provide 24 volts. This is boosted to 28 volts by the control sub and is maintained at that until the supply is reduced to 8 volts without a load or 14 volts under gyro load. Control Sub pins one (+VE) and three (–VE), shown below, can be used to monitor gyro volts using a multi-meter.

1

3

Monitor Box The Monitor Box is connected to the Control Sub to check electrical functions. The Selector Switch has five positions: OFF . . . . . . . . . . . . . . . . . Power to the gyro is disconnected. BATTERY VOLTS. . . . . Voltage output delivered to the Control Sub from the batteries. GYRO VOLTS . . . . . . . . Stabilized output voltage from the Control Sub, i.e., the voltage of the DC supply to the gyro. (should be 28V). GYRO CURRENT . . . . . Electric current supplied to the gyro in milliamps. NULL . . . . . . . . . . . . . . . . Torquer-motor current consumption in milliamps. This switch position is used only when orienting the EC 1½" gyro. Note: When using an BHI 2½" gyro, do not use the OFF or NULL settings because, in either case, current to the gyro will be switched OFF.

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750-500-071 Rev. A / May 1997

Chapter

2

Gyro Multishot Surveys Training Objectives Upon completion of this chapter, the trainee should be able to: •

List all the pre-job information required for GMS surveys.

•

List the pre-job equipment checks which must be done prior to running a GMS survey.

•

List and explain the steps to be followed in running a GMS survey.

•

Assemble a GMS tool correctly.

Pre-Job Information Required Most of the following information should be available before you leave your base for the rig. However, some of it should be verified with the Company man or the drilling engineer at the rigsite. 1.

The name of the field, the well name, the slot number and the slot coordinates.

2.

The size of casing to be surveyed, OD and ID.

3.

The total depth interval to be surveyed and the interval between surveys.

4.

The type of wireline to be used.

5.

The nature and location of the foresight to be used.

6.

The foresight direction.

7.

The maximum angle (inclination).

8.

For wells of over 10° inclination, the average hole direction.

9.

The maximum downhole temperature.

10.

The fluid weight and condition.

11.

The tie-on coordinates to be used for the final survey calculation.


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Gyro Multishot Surveys


12.

The target direction and the origin to be used for calculating vertical section.

13.

The depth of the casing shoe.

14.

If the survey is to be run just after the casing has been cemented, the depth of the top of the cement.

Running a Gyro Multishot Survey When the kit is made up at the district base, a complete check of every item should be carried out. Obviously, only green-labeled instruments will be used.

Rigsite Preparations 1.

On arrival at the rigsite, go and see the Company man to let him know you have arrived and find out how soon you will be running the survey. Verify all the job information with him.

2.

Ensure all equipment has arrived safely. Make sure you have all the equipment and instruments you need to run the survey. Check for visible signs of damage. Store the running gear in a convenient, safe place. If you are not running on Baker Hughes INTEQ wireline, check with the wireline operator to affirm that you have the correct cross-over.

3.

If the multishot cameras (the back-up instrument included) were not loaded with film in the office, they should both be checked and fully loaded.

Note: Under normal circumstances, you should load the cameras in the office when you are making up the kit. 4.

Ensure there are no bubbles obscuring the scale of the angle units. Load ten AA-cells into a battery pack and test the voltage output with a multimeter (15 volts). Check the current output also, which should exceed 5 amps and is typically 6 to 7 amps. This is done only momentarily, as it is a severe drain on the batteries.

5.

Note the asset numbers of the instruments you will use.

6.

See the drilling engineer to double check any job information for which you are uncertain. It is vital that you are 100% certain what you will use as the foresight and what the foresight direction is. You should also verify the tie-on coordinates to be

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750-500-071 Rev. A / May 1997


Gyro Multishot Surveys used for the final survey calculation. For a high angle GMS, it is important to obtain the MWD surveys (or single shots) of the section of well which you will survey so you can calculate your revised foresight. (see Chapter 6, Revised Foresight).

7.

Go to the rig floor and ensure you can see the foresight.

8.

Make up the lower half of the tool. Put 16 D-cells, tip up, in the battery barrel. Screw in the control sub hand tight, check the battery volts and gyro volts using the monitor box, then tighten the control sub in the battery barrel with pipe wrenches. The centralizer may be fixed either on the battery barrel or on a spacer bar, if one is used. Store the bottom half of the tool somewhere safe, e.g., behind the draw works.

9.

Make up the top half of the tool. The top centralizer should be fitted on the top spacer bar if one is used, otherwise on the instrument barrel. Secure the centralizers fixed end which should face upwards. Adjust the centralizer to the required size and lock the stop collars. Store this section of the tool in a safe position, preferably near the cat walk.

10.

If there is plenty of time before you will run the survey and if there is a safe, dust-free and vibration-free environment (e.g., your cabin on an offshore platform), drift check both gyros.

11.

Plug the warm-up box into a main power source and put it on charge until the CHARGED light comes on.

Note: Ideally, the internal battery of the Warm-Up Box is fully charged before you leave your base. 12.

Prepare your field sheets and other paperwork. Plan the depths at which you will take drift checks.

Steps 1 through 12 should be completed as soon as possible after you have arrived at the rig. Some of these steps may be unnecessary if you made up the kit yourself and loaded the cameras, charged up the warm-up box, etc. at that stage (which you should have done), particularly if the job is on a land rig and you have transported the equipment yourself. However, when equipment is freighted to an offshore platform, there is always the possibility of damage during transit. The remainder of the rig-up procedure should be undertaken two hours before the survey is due to start. 13.

Take the ancillary items you need up to the rig floor, such as orienting table with legs and support arm, the telescope kit including monitor box, spare gloves, rags, etc.


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Gyro Multishot Surveys 14.


Find a convenient, safe place to “warm up” the gyro, preferably with main power for the warm-up box. Switch ON the gyro and let it run up to speed. The gyro should be warmed up for at least 30 minutes (preferably about 1 hour) before the survey begins.

Note: Remember to align the gyro compass card to the desired orientation before you start up the gyro. 15.

About 15 minutes before the survey is due, take the gyro in the warm-up box to the rig floor and place it in the dog house. Line the warm-up box up with the direction to the foresight and take the Vernier 0 reading. If the gyro has drifted appreciably from the desired orientation, you may at this stage “tweak” it back to the desired orientation.

GMS Survey Running Procedures 16.

As soon as the rig is ready, the wireline operator will rig up the sheave wheels.

17.

Connect the top half of the tool to the wireline. Lift it up to the rig floor and set it to one side, e.g., in the mousehole.

18.

Lift up the lower half of the tool (using the tugger line attached to the control sub’s lifting ring), lower it into the well, and support it on the orienting table with the orienting arm.

19.

Plug in the monitor box and quickly check the battery volts and gyro volts readings. Then, turn the select switch to the Gyro Current position.

20.

In the dog house, assemble the multishot instruments to start the camera running. If you are going to run a long survey and it is important to conserve film, you should not attach the battery pack until you reach step 27.

21.

Take the gyro from the warm-up box and stab it onto the control sub. Watch for the current reading on the monitor box. Secure the gyro to the control sub with one wrap of Kapton tape.

22.

Fit the lower telescope onto the control sub. Attach the upper scope to the head of the gyro. Set the two scribe lines on the glass on either side of Vernier 0.

Note: The Vernier 0 should face towards you and away from the foresight. 23.

Using the orienting arm, turn the tool until you are “sighted” on the foresight through the upper scope. Now adjust the lower

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750-500-071 Rev. A / May 1997


Gyro Multishot Surveys scope until you are sighted on the foresight through it also (using the adjusting thumb screw shown in Figure 2-1). Once both telescopes are aligned on the foresight simultaneously, lock the lower scope with the locking thumb screw. Then check again that both telescopes are still simultaneously aligned on the foresight. Take off the upper scope and store it away safely. Use a plastic bag to protect the gyro once you have removed the top scope.

Figure 2-1 Three-Inch OD Gyro System


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24.

Check the gyro current reading on the monitor box. By now, it should have stabilized. Also check the battery volts reading again. Unplug the monitor box and store it away.

25.

Ensure the lower telescope is still aligned on the foresight. If not, turn the tool using the orienting arm until it is.

26.

Use a single shot reader to take the Vernier 0 reading visually. This is your Visual Start Case.

27.

Fetch the instruments. Attach the battery pack to start the camera, if you have not already done so. Ensure the lights of the camera are working by holding up the instrument assembly and looking in through the base of the angle unit. Do not invert the instruments. This will also allow you to ensure there is no condensation on the glass of the angle unit and the bubble is not obscuring the inclination scale. Now attach the instrument assembly to the head of the gyro.

28.

Simultaneously “key” the multishot timer and start your stopwatch. Listen for the motor winding the film on twice during the first minute.

29.

Ensure the scope is still aligned on the foresight as the camera takes a picture at minute one. This is not used as a start case picture, but is useful information nonetheless.

30.

Have the instrument barrel lowered over the instrument package and gyro, and make it up to the control sub hand tight. Use one pipe wrench and the orienting arm held against your leg to tighten the connection a little further, but do not use undue force.

31.

Now take the Start Case pictures. Line up on the foresight. Take two successive pictures aligned on the foresight. Then, turn the tool about 30° using the arm, and wait until a picture has been taken. Now turn back to the foresight and take a third start case picture.

Note: When the camera is actually taking the start case pictures, you should be looking through the telescope to make certain the tool is oriented exactly on the foresight and that there is no tool movement while the picture is being taken. 32.

Remove the telescope and store it in the alignment kit case.

33.

Signal to the wireline operator to pick up the tool. Remove the orienting arm and table. Lower the tool until the gyro is level with the rotary table, then stop the wireline and zero the wireline depth counter.

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750-500-071 Rev. A / May 1997



34.

Once the driller has opened the rams, run in hole to the first survey depth.

35.

You now proceed to run the survey by stopping the wireline every 100' (or designated depth interval between surveys) to let the camera take a stationary picture. The tool should be stationary from ten seconds before the camera lights come on until ten seconds after.

36.

Periodically, you will keep the tool stationary for several minutes at certain depths. These stationary periods are called Drift Checks. They allow you to measure the gyro drift accurately during each period by taking the change in the compass card reading relative to any of the numbers on the Vernier scale. Since the tool is not moving, the Vernier scale (which is fixed to the case of the gyro) is not moving. Therefore, any change in a Vernier reading (e.g., Vernier 1) must be due to the compass card turning, i.e., to the gyro drifting.

37.

You should either take a five minute drift check every fifteen minutes or a four minute drift check every twelve minutes (25% of downhole survey time).

38.

Take the first drift check quite early in the survey, say five minutes after the last start case picture was taken. When running offshore, take the first drift check as soon as the tool is below sea bed.

39.

Even if you have to traverse a large depth interval before you reach the section you are to survey, you must still take drift checks at regular time intervals while running in.

40.

Plan your drift checks so you reach your deepest survey station midway between the last in-run drift check and the first out-run drift check.

41.

While pulling out of hole, stop and take a survey at every fourth in-run station. Continue to take drift checks at regular time intervals.

42.

When the tool returns to surface, check the wireline depth reading when the tool is back at rotary table level. The reading should be within 10' of zero.

43.

As the tool comes out of the well, hose it down with water or wipe it with rags.

44.

Support the tool with the orienting arm and table and attach the lower scope.


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45.

Turn the tool with the orienting arm until the cross-hairs of the scope are sighted exactly on the foresight. Take the End Case picture.

46.

Carefully break off the instrument barrel and lift it up to reveal the instruments.

47.

Remove the instrument assembly from the gyro, cover the gyro with a plastic bag, and put the instruments in the dog house.

48.

Line up on the foresight and take your Visual End Case.

49.

Plug the monitor box into the control sub and note readings of gyro current, gyro volts, and battery volts. Disconnect and put away the monitor box.

50.

Verify the camera is still working, then disconnect the battery pack, noting the final picture time (the “last flash”).

51.

Unstab the gyro from the control sub, put it in the warm-up box and allow it to run down.

52.

Screw the protective cap onto the control sub and use the tugger to lift the lower half of the tool out of the hole. Remove it to a safe location on the rig floor.

53.

Store the orienting table, telescope kit, etc. out of harm's way.

54.

Remove the gyro and all instruments from the rig floor.

55.

Develop, wash, and dry your survey film. Then proceed to read the film using the projector.

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750-500-071 Rev. A / May 1997

Chapter

3

Processing and Calculating a GMS Survey Training Objectives Upon completion of this chapter, the trainee should be able to: •

Explain what is meant by an orientation correction.

•

Calculate an orientation correction, given a start case reading and a foresight direction.

•

Calculate values of gyro drift from initial and final Vernier readings.

•

Perform the necessary calculation and draw a drift curve, given the required data.

•

Apply both orientation and drift corrections to observed GMS survey data.

•

Explain what a Tool Axis Correction (TAC) is.

•

Explain the running procedures which may be followed to ensure good data sets for the TAC calculation.

•

Use a TACCAL/TACCOR program to correct a GMS survey for “TAC”.

Overview A total of four corrections are applicable to the raw azimuths as read from the film. Depending upon the inclination of the survey, only three will apply at any one time. The corrections must be applied in the following order. •

Inter-gimbal correction (only if inclination is > 10°)

•

Orientation correction

•

Drift correction

•

Tool Axis Correction (only if inclination is < 10°)


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Processing and Calculating a GMS Survey


Orientation Correction This was explained briefly in Chapter 1, Orientation of the Gyro Compass Card. This correction compensates for the fact that the North of the gyro compass card is not perfectly aligned to True (or Grid) North. If the start case reading (Vernier 0) and the foresight direction are expressed in azimuth, then the orientation correction may be simply calculated using: Orientation Correction = Foresight Azimuth – Start Case Azimuth

Orientation Correction Example 147.50 142.70 4.8° (4.8°E)

Foresight Direction: Start Case Vernier 0 reading: Orientation Correction:

If the start case reading and the foresight direction are expressed as quadrant bearings, the magnitude of the orientation correction is just the angular difference between the start case and the foresight bearings. In order to determine whether this is an East or West correction, imagine rotating the start case reading to the foresight bearing. If this rotation is clockwise round the compass card, the orientation correction is EAST; if the rotation is anti-clockwise then the orientation correction is WEST. F o re s ig h t D ire c tio n = S 0 3 5 o E a st S ta rt C a se R e a d in g = S 0 4 7 o W e s t

1

E

W

8 .2 o

S tart C a se S 04 7 o W e s t

F o res igh t S03 5 o East

S D ire c tio n of ro tatio n S ta rt C as e

F o res igh t

Figure 3-1 Orientation Correction

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750-500-071 Rev. A / May 1997



The total angle between the start case bearing and the foresight bearing is 4.7° + 3.5° = 8.2°. If we turn from the start case bearing round to the foresight bearing we are turning anti-clockwise. Hence, this is a WEST orientation correction. Orientation Correction =

8.2°W or – 8.2°.

We could also calculate the example shown in Figure 3-1 by expressing the start case and foresight directions in azimuth. Orientation Correction

=

Foresight Azimuth – Start Case Azimuth

=

176.5° – 184.7°

=

– 8.2°

Orientation Correction Calculations Calculate the orientation correction required in each of the following cases. Foresight Direction

Start Case Reading

341.0°

338.5° S 63° W

S 64.3° W

N 89.5° W

S 89.5° W

S 83.5° E

S 80.7° E

163.5°

165.2°

270.5°

269.5°

N 17° E

N 17.25° W

S 07.5° W

S 04.2° E

N 03.5° E

N 08.4° E

138.0°

143.9°


An East or West orientation correction is applied to the observed directions in the same way as an East or West magnetic declination correction, although, of course, a gyro orientation correction has nothing whatever to do with a declination correction.

Drift Corrections As explained earlier, the gyro spin axis gradually precesses from its initial heading. This is referred to as Gyro Drift. The rate of precession is sampled periodically by taking regular Drift Checks. Since the case of the tool is stationary during a drift check, the change in any of the Vernier readings between the beginning and the end of the drift check is the drift which has occurred in that time. Training Manual 750-500-071 Rev. A / May 1997

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Example of Drift During a Drift Check Start of Drift Check, Vernier 0 reading: End of Drift Check, Vernier 0 reading: Drift during this Drift Check:

N 81.6° E N 81.1° E 0.5° EAST

Remember that the compass card is actually turning in the opposite sense to that of the apparent rotation of the Vernier (which does not move during a drift check). So, to decide whether the gyro drift is East or West, imagine turning the final Vernier reading to the initial Vernier reading (Last to First). If this rotation is clockwise, the drift is EAST; if it is a counterclockwise rotation, the drift is WEST. When you read your survey film for a GMS survey, look at the Vernier readings for each minute of the drift check. Provided there are no anomalous readings, you will take the difference between the initial and final Vernier readings as the total drift during the drift check. Notice that the individual Vernier readings have no significance – it is only the difference between the initial and final readings that matters. You must also appreciate that for these measurements of drift during the drift checks. We can use any number on the Vernier scale as a reference (Vernier 0, Vernier 1, etc.), but obviously we must use the same Vernier to take both readings for an individual drift check. In order to determine the appropriate drift correction for each survey station of a GMS, we construct a graph of Gyro Drift vs Time. This graph is called a Drift Curve. The steps involved in drawing a drift curve are enumerated below.

Drawing a Drift Curve 1.

Divide the survey into time periods with one drift check per time period. Calculate the mid-points between the finishing time of one drift check and the starting time of the next drift check.

2.

Calculate the drift during each time period using the drift rate in the drift check for that time period. Drift from start to end of Drift during drift check = ---------------------------------------------------------Number of minutes in × Length of period time period drift check

3.

The drift at each mid-point will be the sum of the values of drift for all the preceding time periods. Plot these values of calculated drift at each of the mid-point times and at the end case. The value of drift (in degrees) found by summing the values for all the time

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750-500-071 Rev. A / May 1997


Processing and Calculating a GMS Survey periods (taking account of sign) is the Total Calculated Drift. This is also the value of drift calculated to have occurred by the end case time, i.e., over the whole survey.

4.

Draw the graph of calculated drift by drawing straight lines connecting the value of drift at each mid-point to the value at the next mid-point.

Note: The graph starts from the start case time with “Degrees of Drift” equal to zero. The graph does not pass through zero time.

8

D rift (degrees) Total calculated drift

7 (b)

6 5 4

Closure Calculate d drift curve

b-a

3

(a)

Total observed drift

b-a

2

Closed (corrected) drift curve

1

`

0 -1

Tim e (m ins)

M id point b etwe en d rift che cks

-2 -3

Figure 3-2 Graphing the Drift Curve

5.

Calculate the Total Observed Drift which is the difference between the start case and end case Vernier 0 readings (on film). Plot this value of drift on the graph at the end case time.

6.

Draw a straight line (lightly, in pencil) from the point of zero drift at the start case time to the point defined by total observed drift at the end case time. (Plotted in Step 5). This is line (a) in Figure 3-2, Observed Drift.

7.

Draw a second straight line (lightly, in pencil) from the start case to the point defined by the Total Calculated Drift at the end case time. This is line (b), Calculated Drift.


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8.

At each mid point between drift checks, measure the difference in drift value between lines (b) and (a). (See Figure 3-2).

9.

Subtract the respective differences (b – a) from the calculated drift curve mid-points and plot the new set of “mid-point” drift values.

10.

Join the points plotted in Step 9 to obtain the Closed (i.e., corrected) Drift Curve. Our convention is that this is drawn in red pencil.

11.

The hole direction at each survey station is corrected by applying the closed drift curve value for the time in minutes when the picture was taken.

The example shown in Figure 3-3 shows a proper drift curve drawn for the following data. 08.5° Vernier 0 reading N 11.5° E Vernier 0 reading N 7.4° E

Foresight Azimuth: Start Case at Minute 6: End Case at Minute 66: Drift Check Number

Time (Minutes)

Start Vernier Reading

End Vernier Reading

Drift (Degrees)

1

12 - 17

N 25.4° E

N 25.0° E

0.4° E

2

31 - 36

N 13.7° W

N 13.9° W

0.2° E

3

52 - 57

S 56.8° E

S 57.5° E

0.7° E

D rift (degree s)

`

O rie nta tio n + D rift (de grees) = + 1 .4 4 R

4.0

1.0

3.0

0.0

2.0

-1.0

1.0

-2.0

0.0

= + 2 .2 4 R

D rift curve clo sure = 1.2

R

Total observed drift = 4.1R E

Start C ase

Tim e (m in s)

-3.0 0

R

Total calculated drift = 5.32 E

5.0

6

10

20 24 R

(0.4 /5) x 18 R = 1.44 E

30

40 R

(0.2 /5) x 2 0 R = 0.80 E

44 50

60

70

(0.7 R/5) x 2 2 R = 3.08 E

Figure 3-3 Drift Curve Example

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750-500-071 Rev. A / May 1997



TAC Correction The final correction which may have to be applied to a low angle GMS survey is the so-called TAC Correction which corrects for misalignment of the tool axis. This is explained in detail in Appendix A. This correction is applied at the rigsite using the Baker Hughes INTEQ TAC program for a Hewlett Packard programmable calculator.


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Chapter

4

Survey Documentation Training Objectives Upon completion of this chapter, the trainee should be able to: •

List the standard survey documentation required for GMS surveys.

•

Complete all the standard post-job reports correctly.

Final Survey Calculation You must perform the final survey calculation at the rigsite and give copies to both the Company man and the drilling engineer. However, indicate on the calculation sheet that it is a field calculation only and will be checked and corrected in the office.

Forms, Reports, and Worksheets The survey envelope, which should be clearly labeled, should contain all the documentation listed here plus the original survey film. Remember, this is the permanent record of your survey. Note: Remember to get your job ticket signed before you leave the rigsite. •

Orientation Diagram – polar graph paper with foresight, start case, and orientation correction shown clearly.

•

Survey Report.

•

Equipment Performance Report.

•

Gyro Field Sheets.

•

Field Calculation Sheets (photocopy).

•

Drift Curve.

•

Office Check Field Sheets.


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Survey Documentation


•

Copy of Computer Survey Calculation.

•

Any other documentation required by your district. Note: You should note all asset numbers before you run in hole.

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750-500-071 Rev. A / May 1997

Chapter

5

Gyro Single Shot Surveys Training Objectives Upon completion of this chapter, the trainee should be able to: •

Explain the circumstances in which gyro single shots are taken.

•

List and identify the components of a gyro single shot instrument and explain the function of each component.

•

Operate gyro single shot instruments correctly.

•

List and identify all the items of running gear required for gyro single shot surveys.

•

Assemble a gyro single shot tool correctly.

•

List the pre-job information required for gyro single shot.

•

List the pre-job equipment checks which must be done prior to running a gyro single shot survey.

•

List and explain the steps to be followed in running a gyro single shot survey.

•

Calculate the necessary orientation and drift corrections and apply these to the gyro single shot survey.

GSS Instrument Overview Conventional gyro single shots are rarely taken today because they have been largely replaced by either Seeker or Sigma 175. The gyro single shot is primarily used to orientate deflection tools in areas of magnetic influence. Of course, it will at the same time give the inclination and direction of the wellbore. The tool is run in through the drillstring and seats in an orienting sub in the BHA, with the tool itself lying inside a drill collar. The instrument consists of: 1.

The Single Shot Battery Pack (as for R single shot).


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Gyro Single Shot Surveys 2.


A Single Shot Timing Device - usually a 33 minute mechanical timer or a 99 minute electronic timer.

Note: 33 minute mechanical timers are officially obsolete, but may still be encountered in some regions. 3.

A single shot camera (R type).

4.

A single shot adaptor which screws on to the front of the single shot camera. This adaptor has a lens and also a light gate (shutter) which can be opened or closed manually. There are three bulbs on the front of this adaptor which replace the bulbs of the single shot camera. The front of this adaptor screws onto the gyro angle unit.

5.

A gyro angle unit – usually a 0° to 12° unit.

6.

A 1½" Baker Hughes INTEQ gyroscope.

There are special 0° to 1° and 0° to 4°Camera and Plumb Bob assemblies which replaces items 3 and 5. A “clear angle unit”, which does not contain an inclinometer, is then necessary as a cross-over from the single shot adaptor to the head of the gyro. Note: Camera and Plumb Bob assemblies are officially obsolete, but may still be encountered in some regions The gyro single shot instruments fit inside either a 2" OD or a 1.75" OD pressure barrel. The 2" OD system is most often used. The gyro is stabbed onto a voltage control sub which is screwed into a battery barrel containing 16 D-cells in series. The control sub boosts the battery voltage and maintains the voltage supplied to the gyro at a constant 28 volts. The complete downhole tool consists of: • • • • • • • • •

Wireline cablehead + crossover or spearpoint + wireline sub or rope socket 2 Swivels Finger Pin Stabilizer (with finger pins cut to size) 2" Instrument Barrel (containing instruments) 2" Control Sub 2" Battery Barrel Finger Pin Stabilizer Soft-Shock Assembly Adjustable Muleshoe

Draw a diagram of a complete gyro single shot tool and label the diagram carefully. 5-2


750-500-071 Rev. A / May 1997


Gyro Single Shot Surveys Complete Gyro Single Shot Tool


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Gyro Single Shot Surveys


Pre-Job Information Required You will need all the standard information required for any type of survey, viz, well name, slot coordinates, tie-on coordinates, etc. You must also know the nature and location of the foresight and what the foresight direction is. Also you must know what type of wireline will be used; usually it is the rig sandline. Gyro single shots are almost exclusively used to determine tool face orientation of deflection tools in shallow kick-offs, so downhole temperature, fluid condition, etc., should not be a consideration. At the rig, note the bottomhole assembly in your tally book. Calculate the distance from the bit to the muleshoe sleeve. This is necessary so that you know at what depth the tool will “land” in the muleshoe when you are running surveys. Once you have made up the two “halves” of the tool, measure and note lengths of each. The sum of the “bit to muleshoe” distance plus the length of the lower half of the tool (muleshoe to top of control sub) gives the distance which must be subtracted from the bit depth to obtain the survey depth. This calculated survey depth should cross-check with the wireline depth counter reading. The length of the top half of the tool is useful when calibrating a Cavins depthometer prior to running in hole. You should also note the minimum ID through which the tool must pass and the ID of the collar it will be inside when seated downhole.

Running a Gyro Single Shot Survey Rigsite Preparations 1.

At the rigsite, advise the company man of your arrival and find out when the job will start. Also, see the directional driller who will know all the details of the job.

2.

Verify that you have all the equipment you need and look for any visible signs of damage.

3.

Verify you have all the required job information.

4.

Check the sandline/wireline connections.

5.

Ensure that you can see the foresight from the rig floor.

6.

Test the gyro single shot instruments (if time permits).

7.

Make up the two “halves” of the tool: •

Top Section – Wireline crossover, 2 swivels, finger pin stabilizer (with rubber finger pins), 2" gyro instrument barrel.

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750-500-071 Rev. A / May 1997


Gyro Single Shot Surveys •

Lower Section – 2" control sub, 2" battery barrel, finger pin stabilizer (optional), soft-shock assembly, adjustable muleshoe.

Remember to load 16 D-cells into the battery barrel (tip to control sub). Check the “battery volts” and “gyro volts” readings with the monitor box before making up the control sub tight. 8.

Lay the lower half of the tool horizontal on tool stands and attach bubble levels to the bubble level recess on the 2" control sub and to the muleshoe slot. Turn the whole tool until the control sub recess is facing vertically upwards, then adjust the adjustable muleshoe until the muleshoe slot is also facing vertically upwards. Tighten the locking screws on the adjustable muleshoe, then double check that the muleshoe slot is properly aligned to the recess on the control sub.

Note: The above procedure is vital so that correct tool face orientation is obtained. When the 1½" gyro is attached to the control sub, the Vernier 0 will be 180° “out” from the muleshoe slot. Since we effectively orientate the gyro compass card “180° out”, the Vernier 0 reading on our gyro single shot gives the toolface orientation (“gyro toolface”). 9.

Put a lead (“tell tale”) slug in the hole in the muleshoe slot.

10.

Put the inserts for the 2" system in the orienting arm and the lower telescope assembly.

11.

Take both sections of the tool plus instruments and ancillary items to the rig floor. As soon as convenient, connect the top section of the tool to the sandline. Remember, you need developer and fixer fluid and a single shot developing tank. Also have a cup of water in the dog house for washing the developed film disk.

12.

Warm the gyro up for 15 to 30 minutes before the first single shot run. On subsequent runs, a 10 to 15 minute warm-up should suffice, depending on how frequently you are running the surveys. If they are drilling very quickly and you are taking a GSS every single, it is probably not worth running the gyro down between surveys. (This also depends on the total time the job is expected to last). Orientate the gyro compass card as required before you switch on the gyro, following the same procedure as for GMS.

13.

Make up the complete gyro single shot instrument. Close the light gate (shutter) before you load the single shot camera.


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GSS Survey Running Procedures 1.

Once the kelly has been broken off, put the lower half of the tool into the drillstring resting on the orienting table. Remember, the orienting arm fits onto the battery barrel in the 2" gyro system.

2.

Stab the gyro onto the control sub and use the monitor box to check the electrical readings. Use the support sleeve to protect the 1½" gyro.

3.

On the first run only, fit upper and lower telescope assemblies and adjust the lower telescope as described in steps 22-23 of the GMS running procedure enumerated in Chapter 2, GMS Survey Running Procedures

Note: On subsequent runs, you can use the lower scope as a “fixed” scope to save rig time, but you must be certain that it has been firmly locked so that no subsequent adjustment occurs. Wrapping tape around the locking thumb screw after the first run will help immobilize the scope. 4.

After removing the top scope, turn the tool using the orienting arm until you are precisely lined up on the foresight through the lower scope. Use a single shot reader to take your Start Case visually. Note this in your tally book.

5.

Remove the support sleeve and monitor box. Attach the instruments to the gyro and set the required time on the instrument timer. Start your surface stop watch and the instrument watch simultaneously.

6.

Open the light gate (shutter).

7.

Lower the instrument barrel over the instruments and make up to the control sub.

8.

Take the tool weight on the sandline and remove the orienting arm. Zero the wireline depth counter when the angle unit is approximately at rotary table level, or attach the Cavins depthometer to the sandline and set initial reading based on the length of the GSS tool.

9.

Run tool in hole. When the depthometer reading indicates 50' to go, have the sandline slowed so the tool lands gently in the muleshoe sleeve.

10.

Flag the sandline at this point. On the next run, the flag will indicate when the tool has nearly reached bottom.

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750-500-071 Rev. A / May 1997



11.

Wait on bottom until the picture has been taken, then pull the tool out of hole.

12.

Check the lead slug for indentation. Rest the tool on the orienting arm and unscrew the instrument barrel from the control sub to access the instruments.

13.

Close the light gate (shutter).

14.

Remove the instruments from the gyro, place a plastic bag over the gyro, then immediately take the instruments into the dog house and unload the single shot disc into the developing tank.

15.

Attach the lower (“fixed”) telescope, line up on the foresight, and visually take the End Case Vernier 0 Reading. Stop your stopwatch.

16.

Quickly take final monitor box readings. When you are taking a lot of gyro single shots, you should pay particular attention to the “battery volts” reading. You should change out the batteries in the battery barrel when this reading falls to 16 volts.

17.

Unstab the gyro from the control sub and transfer it to the warmup box in the dog house. Unless you expect to run again within 20 minutes, you should switch OFF and run down the gyro, but not until you know you have a good survey

18.

Remove the single shot disc from the developing tank, wash and dry it.

19.

Read the values of inclination, hole direction, and toolface from the disc. Apply the necessary orientation and drift corrections to both the direction and toolface readings.

20.

Store both running gear sections out of the way on the rig floor. Store instruments carefully. Since it may be used many times, have everything accessible.

Applying Orientation and Drift Corrections to GSS Surveys The orientation correction is calculated from the start case reading and the foresight direction, just as for GMS. The total drift from start case to end case is calculated as for GMS. There is no possibility of taking drift checks with conventional single shot surveys, so we calculate the drift which has occurred when the survey is taken, assuming the gyro drifts at a constant rate.


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Example 1 Foresight Direction:

N 34° E

Start Case reading:

N 36½° E (minute 0)

End Case reading:


Survey readings (taken at minute 10):

•

Inclination:

6½°

•

Hole Direction:

N 40° W

•

Toolface:

N 30° W

•

Orientation Correction:

•

Total Gyro Drift (start case to end case): 2° WEST

2.5° WEST

time of downhole survey Drift correction = total drift × ----------------------------------------------------------------------time start case → end case 10 minutes o o ∴drift correction = 2 West × -----------------------------18 minutes = 11 West Total Correction = Orientation Correction + Drift Correction1

In this example, total correction = 2.5° WEST + 1.1° WEST, i.e., approx. 3½° WEST. So the corrected values are: Hole Direction

N 43½° W

Toolface

N 33½° W

1.

Added algebraically

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750-500-071 Rev. A / May 1997



Example 2 Calculate the necessary orientation and drift corrections and apply them to the observed hole direction and tool face readings. Foresight Direction:

N 19° W

Start Case reading:

N 17½° W (minute 0)

End Case reading:

N 20° W (minute 25)


•

Inclination:

4½°

•

Hole Direction:

S 37° E

•

Toolface:

S 45° E


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S 38° E

Start Case reading:

S 44½° E (minute 0)

End Case reading:

N 49¾° E (minute 30)


•

Inclination:

8½°

•

Hole Direction:

N 36° E

•

Toolface:

N 24° E

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750-500-071 Rev. A / May 1997




N 03° E

Start Case reading:


End Case reading:

N 01½° W (minute 16)


•

Inclination:

5½°

•

Hole Direction:

S 47½° W

•

Toolface:

S 62° W


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Chapter

6

High-Angle Gyro Surveying Training Objectives Upon completion of this chapter, the trainee should be able to: •

Explain why a revised foresight is used in high angle GMS.

•

Calculate a revised foresight direction.

•

Explain what is meant by inter-gimbal error.

•

Correct gyro survey data for inter-gimbal error, including observed drift readings.

Overview When gyro surveys are run in wells of over 10' inclination, there are some additional effects and procedures to be followed. These are: 1.

In order to run the gyro in a more favorable gimbal configuration and to minimize inter-gimbal error (see below), we orientate the gyro so that the spin rotor axis points approximately in average hole direction. This is done by calculating a Revised Foresight.

2.

When the survey tool (and hence, the case of the gyro) is tilted away from the vertical, the outer gimbal tilts about the inner gimbal axis, and the compass card, which is mounted on the outer gimbal, is tilted from the horizontal. The hole direction observed on the plane of the compass card must be projected onto the horizontal plane to obtain the true hole direction. This effect is referred to as inter-cardinal gimbal error or Inter-Gimbal Error. All the observed hole directions must be corrected for this effect.

3.

The observed drift readings must also be corrected for inter-gimbal error.

4.

The internal Q.C. specifications are stricter and more clearly defined for GMS surveys at inclinations over 10'. This is because uncertainties in inclination and azimuth measurements have a


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High-Angle Gyro Surveying


much larger effect on bottomhole position uncertainty as the wellbore inclination increases. 5.

Under certain circumstances, a high angle GMS survey of a deep hole section can be in hole oriented to a previous survey. This may be used instead of, or in addition to, normal surface orientation procedures. However, a revised foresight would still be used to put gyro spin axis in average hole direction.

Revised Foresight For wells of inclination over 10°, it is preferable to orientate the gyro so that the spin motor axis points in average hole direction because this is the most stable gimbal configuration. For the 2½" gyro, the North-South line on the compass card is aligned with the spin axis. Hence, for the 2½" gyro, we want the North (or South) of the gyro compass card to point in average hole direction. We achieve this as follows: 1.

Study existing survey data, e.g., MWD surveys of the hole section you are going to survey. Estimate the average hole direction of this section.

2.

Note the normal foresight direction.

3.

Calculate the revised foresight as explained below.

4.

When you start up the gyro in the warm-up box, have the Vernier 0 facing 180° out from the foresight direction. Set the revised foresight direction on the compass card at Vernier 0. Keep your finger on the compass card as you switch ON the gyro and for about 20 seconds thereafter.

5.

At the beginning of the survey, when you align the lower telescope to the foresight as per normal procedures, check that the Vernier 0 reading is approximately the revised foresight (within 5°, preferably).

Calculating the Revised Foresight for a 2½" Gyro Example 1 Foresight Direction:

N 15° E

Average Hole Direction:

N 50° E

We want the North of the gyro compass card to point in average hole direction. Therefore, it will point to N50E in this case.

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750-500-071 Rev. A / May 1997



If the gyro North faces to N50E, as shown in Figure 6-1, then the direction to the foresight will be N35W on the gyro compass card. So, N35W is the revised foresight. If bearings are expressed in azimuth, then the revised foresight azimuth required to orient North in hole direction is found simply from: Revised Foresight = Foresight Azimuth – Average Hole Azimuth.

If a negative number obtained, add 360°. The previous example becomes: Revised Foresight = 015° – 050° = –35°

Adding 360° gives 325°. Revised Foresight = 325o Azimuth.

N

F.S . 1RUWK RI FRPSDVV FDUG

15

R

R .F.S .

35 50

DYHUDJH KROH GLUHFWLRQ

R

R

W

E

S Figure 6-1 Revised Foresight: Example 1

In this case, we decided to run South in hole direction so we add or subtract 180° (to obtain an answer in the range 0° to 360°). In the example shown in Figure 6-2, revised foresight = 201°– 180° = 021°.


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Example 2 Foresight Direction:

N 19 W (341°)

Average Hole Direction:

S 40 E (140°)

Putting South in hole direction gives a smaller orientation correction. Imagine rotating gyro South to S40E (hole direction) and hence gyro North to N40W. On the gyro compass card now the foresight direction would be N21E. So the revised foresight = N21E. Revised Foresight = Foresight Azimuth – Average Hole Direction = 341° – 140°

This would put North in hole direction = 201°

N orth o f com pass card

N

F.S .

R .F. S.

R

21

19 R 40 R

W

E

40 R So uth of co m pa ss card (average hole dire ction )

S

Figure 6-2 Revised Foresight: Example 2

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750-500-071 Rev. A / May 1997



Example Calculations Calculate the revised foresight in each of the following cases assuming a 2½" gyro is being used.

Foresight

Hole Direction

a.

077°

264°

b.

N 25° W

N 65° E

c.

N 19° E

S 80° W

d.

South

N 33° E

e.

N 27° W

South

Revised Foresight

Calculating the Revised Foresight for a 1½" Gyro In the 1½" gyro, the East-West line on the compass card is aligned with the spin motor axis. This means that the North-South line on the compass card is parallel to the inner gimbal axis. If we wish to run the gyro with the spin axis in hole direction, that means pointing the East or West of the compass card in hole direction. When the wellbore inclination is in the range of 10° to 30°, this is a perfectly viable option. However, if the inclination is over 30°, then there is a strong risk that the spin motor of a 1½" gyro will hit the stops and spin out. In this case, you must run the gyro with the inner gimbal axis aligned to the hole direction, i.e., with North or South in hole direction. When the gyro is orientated with the gimbal axis in hole direction, the spin axis is perpendicular to the outer gimbal axis and the spin motor, inner gimbal, and outer gimbal axes are orthogonal (perpendicular to each other). This is also a stable gimbal configuration. As previously stated for the 1½" gyro, this is achieved by running North or South in hole direction.

Note: For simplicity, it is recommended that whenever a revised foresight is calculated for a 1½"gyro, North or South on the compass card is pointed in average hole direction. The revised foresight for a 1½"gyro is then calculated in exactly the same way as for a 2½"gyro.


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Example Calculations Foresight

Hole Direction

a.

S 40° W

N 10° E

b.

N 19° W

N 55° E

c.

165°

025°

d.

South

010°

Revised Foresight

Inter-Gimbal Error As stated previously, when our gyros are used in non-vertical wells, the compass card will no longer lie in a horizontal plane. In fact, the angle between the plane of the compass card and the horizontal is equal to the wellbore inclination. Since the compass card is not horizontal, the observed hole direction on the plane of the compass card is not equal to the angle between spin axis direction and hole direction on the horizontal plane. See Figure 6-3, Figure 6-4, and Figure 6-5. For a 2½" gyro, the North-South line on the compass card is aligned with spin axis direction. So, if there were no orientation or drift corrections, then the angle on the horizontal plane between the spin axis direction and the borehole plane (see Figure 6-3) would be the true hole direction. Hence, we correct for inter-gimbal effect by mathematically projecting the observed hole direction onto the horizontal plane. This is done using the following formula for the 2½" gyro.

tan(True Bearing) = tan(Observed Bearing) × cos(Inclination)

True Bearing = arctan[tan(Observed Bearing) × cos(Inclination)]

Example Inclination

Observed Direction

Direction Corrected for Inter-Gimbal Error

60.5°

N 45.0° E

N 26.2° E

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750-500-071 Rev. A / May 1997



Figure 6-3 Inter-Gimbal Error (IGE)


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Figure 6-4 Inter-Gimbal Error: Horizontal Plane

R V

H orizontal plane

$ UHDO

R = H igh-side projected onto the horizontal pla ne Gyro wheel s = Direction of gyro sp in axis

Figure 6-5 Inter-Gimbal Error: Plane of Compass Card R' P lane of com pass card s' R ' = D ire ction of the hig h side

$ REVHUYHG

s' = S pin axis projected onto the plane of the co m pa ss card

G yro w heel

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750-500-071 Rev. A / May 1997



Example Calculations Use the formula to correct the survey readings (below) for inter-gimbal error (2½" gyro). Inclination

Observed Direction

10.0°

N 12.5° E

15.0°

N 12.5° E

20.0°

N 12.5° E

20.0°

N 35.0° E

30.0°

N 35.0° E

45.0°

S 35.0° E

45.0°

145.0°

60.0°

S 35.0° E


Of course, if you calculate your revised foresight correctly and set up the gyro orientation accordingly on surface, then your observed hole directions should all be close to North or South. This reduces the inter-gimbal effect, which is one reason for using a revised foresight. Below 10° inclination, the inter-gimbal correction makes little difference to the values of hole direction so we do not apply it.

Inter-Gimbal Correction for the 1½" Gyro For the 1½" gyro, the gimbal configuration is the same, but the East-West line on the compass card is aligned with spin axis direction. The intergimbal formula for a 1½" gyro is: tan ( ObserveBrearing ) tan ( True Bearing ) = ---------------------------------------------------------cos ( Inclination ) tan ( ObserveBrearing ) So True Bearing = arc tan ----------------------------------------------------------cos ( Inclination ) Example Inclination

Observed Direction


30.0°

N 60° E

N 63.4° E


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Example Calculations Use the formula to correct the earlier IGE survey readings (1½" gyro). Inclination

Observed Direction

10°

N 45° E

15°

N 75° E

20°

N 75° E

20°

S 75° E

20°

105°

30°

N 80.5° E


Note: For the 2½" gyro, correcting for IGE reduces the bearing (angle from North or South), but for the 1½" gyro, correcting for IGE increases the bearing. Note: The inter-gimbal error correction is the first correction which must be applied to a high angle gyro survey (over 10°).

Correcting Observed Drift Readings for Inter-Gimbal Error On a high angle GMS survey, the drift observed in the drift checks should also be corrected for IGE, since this drift is observed on the plane of the compass card, not on the horizontal plane. However, the Vernier readings are arbitrary, being determined by the way the tool rotates in the casing when it is moving. We use the difference between the start and end Vernier readings of a drift check to accurately measure the rotation of the compass card. But, in order to calculate how much the spin axis of the gyro has precessed in the horizontal plane, we apply the observed drift to the observed hole direction. We then inter-gimbal the observed hole direction and the observed hole direction plus the observed gyro drift. The difference between these two inter-gimballed values is the actual gyro drift, i.e., the actual number of degrees which the spin axis has precessed during the drift check. An example should make this clearer. Example (2½" Gyro) Vernier Readings Inclination

Hole Direction

Start

End

47.0°

N 26.0° E

S 86.2° W

S 87.3° W

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750-500-071 Rev. A / May 1997



The observed drift is 1.1°W. Now because the Vernier readings are more precise than the hole direction readings, we do not read the hole direction at the end of the drift check, but calculate what it should be using the observed drift. Observed hole direction at the first minute of the drift check is N26.0E. If the gyro drifts 1.1°W, then at the end of the drift check the observed hole direction will be N27.lE. Now we apply the 2½" gyro IGE to both these hole direction readings. N26.0E Inter-gimballed → N18 • 40E N27.1E Inter-gimballed

→ N19 • 24E

The difference between the inter-gimballed values is the true value of gyro drift: 19.24° – 18.4° = 0.84°. So the inter-gimballed drift is 0.8°W. (This correction will not alter the direction of the gyro drift). Example Calculations In the examples below, calculate the drift in the drift check, corrected for IGE. Assume a 2½" gyro is being used. Vernier Readings

Example a. Inclination

Hole Direction

Start

End

40.0°

N 25.0° E

N 45.3° E

N 44.4° E

Vernier Readings

Example b. Inclination

Hole Direction

Start

End

61.5°

N 14.5° E

S 38.4° E

S 39.2° E


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Gyroscopic Surveying Vernier Readings

Example c. Inclination

Hole Direction

Start

End

10.0°

N 39.0° E

S 17.7° W

S 18.2° W

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750-500-071 Rev. A / May 1997

Chapter

7

In-Hole Orientation Training Objectives Upon completion of this chapter, the trainee should be able to: •

Explain what is meant by in-hole orientation.

•

State the conditions which are necessary before a survey may be in-hole oriented.

•

State the procedure for running an in-hole oriented survey.

•

Calculate the correct orientation correction for an in-hole oriented survey.

•

Calculate the total observed drift correctly for an in-hole oriented survey.

Downhole Orientation Using Conventional Multishot System If you are running a gyro multishot of a deep section of hole (95⁄8" casing or 7" liner), then instead of establishing orientation by sighting on surface, you can overlap with the bottom stations of the definitive survey of the previous hole section. This can be done both when running a conventional gyro multishot with a camera, an angle unit, etc., or when running a Sigma 300 gyro multishot. The following notes apply specifically to in-hole orienting with the conventional gyro multishot system. In-hole orientation is only possible if the survey of the previous section satisfies the following conditions:

Necessary Conditions 1.

The survey must have been certified as a good survey and accepted as such by the customer. Normally, it should be the definitive survey of that section.


7-1 Confidential

In-Hole Orientation


2.

There must be a high enough inclination at the bottom of the previous survey so that directions are accurately established. It is hard to set a definite limit on this, since it depends on the angleunit used if the previous survey was a conventional gyro multishot. Certainly, the inclinations at the stations where your survey will “overlap” with the previous survey must be over 3° in any circumstances, even when your survey will be run using a 5° angle unit. If you will be using a higher range of angle-unit particularly a 5° to 90° unit, then it is desirable to have inclinations of over 10° at least. Really, the technique of in-hole orientation is best used on high-angle wells. Under no circumstances should you ever in-hole orient in a near-vertical portion of any well (i.e., under 3°).

3.

The previous survey should be accurate as far as can be determined. Obviously, if it is a Ferranti Survey, there should not be a problem, but if it was a gyro multishot, whether Sigma or conventional, then the in-run/out-run comparison and drift curve closure should be good. You ought to look carefully at the previous survey and satisfy yourself that you can validly in-hole orientate to it.

Running Procedure On surface, sight on the foresight and take a visual start case (to ensure your revised foresight is okay). It is good practice to take a picture on the foresight, although it is not absolutely essential. Now run in hole and proceed without stopping to a point just above the first “overlap” station. (You may, if you wish, take some drift checks while running in to the tie-on point, but this is not really necessary if you are definitely going to in-hole orient). Keep the tool stationary for 10 minutes to allow the gyro to “settle down” (in particular, to permit the torquer motor to erect the spin axis to the horizontal). Do not use this as a drift check. You now proceed with the survey starting by taking pictures at the “overlap” stations. Normally, you should have either 4 or 5 of these overlap stations. Four is the recommended minimum, while more than 5 may give you extra problem when it comes to deciding on the orientation correction. The bottom station of the previous survey must often be considered suspect, and in the case of a FINDS, the bottom 3 surveys should be ignored. Another possible technique is to take “overlap” surveys at 50-foot intervals and interpolate between the appropriate stations of the previous survey. The survey is carried out normally with regular drift checks, but take your first drift check quite soon after the overlap stations (say 5 minutes), or it will be applied over too long a period. 7-2


750-500-071 Rev. A / May 1997


In-Hole Orientation

On the out-run, when you come back to the tie-on point, again take a survey at each of the 4 or 5 “overlap” stations. Your survey is now finished, so you pull out of hole without stopping. [Do not exceed 300 ft/ minute.] However, if you took drift checks while running in, you ought also to take them at appropriate times during the out-run.

Establishing Orientation Correction and Total Drift After you have read your film, apply inter-gimbal correction to your raw readings of hole direction. Compare your inter-gimballed hole directions at the “overlap” stations with the final corrected directions from the previous definitive survey. For each of the stations, calculate the orientation correction needed to correct from your inter-gimballed directions to the true hole directions. If the values show small random variations, use the average value. If one (or 2) value(s) is (are) significantly different from the others, ignore it (them) and use the average of the remaining 3 or 4. Then choose the deepest of the “overlap” stations where this orientation correction approximately corrects your inter-gimballed direction to the true direction. This is your start case. If there is a consistent increment in orientation correction from each “overlap” station to the next, this is due to the gyro drifting. Check this by working out the drift rate per minute from your first drift check and see if this accounts for the difference between each OC value. Use the bottom “overlap” station as your start case, and the corresponding orientation correction. As an example of the normal case, suppose the values were as follows:

“Correct" Values from Previous Survey

Your Inter-Gimballed Values

Depth

Inclination

Direction

Inclination

Direction

10100'

53.5°

N 35.0° E

53.25°

N 14.0° E

10200'

53.5°

N 35.5° E

53.5°

N 14.8° E

10300'

54.0°

N 36.0° E

53.75°

N 15.0° E

10400'

54.25°

N 36.25° E

54.0°

N 16.0° E

Note: First check that your inclinations agree, within acceptable limits, with the previous survey. Then compute the orientation correction in each case.


7-3 Confidential

In-Hole Orientation


Depth


10100'

21.0° E

10200'

20.7° E

10300'

21.0° E

10400'

20.25° E

The value obtained at 10400' is somewhat low, so first check that you have read your film and inter-gimballed correctly. If you still get the same as before, ignore the station at 10400'. (The previous survey could be in error at that depth). In this example, you could either use the average of the other 3, i.e., 20.9°E, or settle for 21°E as being accurate enough, given that this is the exact value from 2 of the remaining 3 survey stations. In either case, the picture at 10300' would be your start case. This means that the picture on the out-run at 10300' would be your end case. Whatever depth you choose as your start case, the same depth on the outrun is your end case. Now the difference between your inter-gimballed hole direction at 10300' on the in-run and out-run (i.e., between your start case and end case), is the total observed drift on film. You use this value to “close” your drift curve as for a normal survey. Finally, notice that you will not use your readings for any of the 4 “overlap” stations in your final calculation. The first station of your survey which you would calculate would be at 10500'. This means that you could have chosen 20.9°E as your orientation correction (OC), and 10300' as your start case depth even though 20.9°E does not correct exactly from your survey to the previous survey at 10300'. 20.9°E is still the most accurate value of OC for you to use, and 10300' would be the best choice of start case depth.

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750-500-071 Rev. A / May 1997

Appendix

A

Tool Axis Correction (TAC) Introduction This is sometimes know as AEC (axial error correction). When using gyro multishot equipment in eased holes inclined at low angles (less than 5°), the effects of imperfect tool centralization are significant in terms of repeatability and absolute accuracy. Although every effort should be made to ensure accurate centralization, biases in centralizers and angle units cannot be completely eliminated. The error in survey data created by such biases, variously described as offset center, or axial error, can be measured and recorded data approximately corrected. The correction method assumes the following: •

The error arises from the misalignment of the tool axis with the true hole axis at any point.

•

The error is systematic for any specific tool configuration.

•

Tool weight and wireline tension do not create a distorting effect, and the centralizers are rigidly attached so that they cannot rotate about the tool pressure barrels.

Survey Procedure To determine the difference between tool axis and true hole axis, it is necessary to have two or more sets of survey data recorded by the same tool configuration at the same depth. In practice, because of errors in recorded survey data due to reading, depth control, and gyro drift corrections, several “pairs” of data are necessary so that these random errors can be averaged out. There are three methods of obtaining pairs of measurements.

Rotation Shots For this method, mark one blade of a centralizer with string or tape and note its approximate orientation on surface. The survey tool is then lowered to a steady point in the casing and a picture recorded. The tool is retrieved and rotated approximately 60° at surface. As our centralizers generally Training Manual 750-500-071 Rev. A / April 1997

A-1 Confidential

Tool Axis Correction (TAC)


have six blades, the simplest method of estimating this turn is to rotate the tool until the adjacent blade has the same approximate orientation as the marked blade had for the first shot. The tool is again lowered to the same depth and a further picture recorded. This procedure is repeated until the tool is back at the initial orientation and six independent measurements have been recorded. The recommended procedure is thus: 1.

Run the survey in the natural manner.

2.

Record end case shots.

3.

Mark a centralizer blade and make the six rotation shot runs.

4.

Record a final end case shot.

This method may be impractical, especially on offshore locations where steady readings cannot be obtained near the rotary table.

“Yo-Yo” Shots Run the survey tool to a convenient steady point with a centralizer blade marked as for the rotation shot method. Record a picture and lower the tool a further 10 feet. Repeat this procedure until six pictures are recorded (at 600, 610, 620, 630, 640 and 650 feet, for example). Retrieve the tool from the casing and rotate it so it re-enters the casing approximately 180° from its original orientation. Again, record pictures at 600, 610, 620, 630, 640 and 650 feet while proceeding with the survey. The recommended procedure is thus: 1.

Rig up to run the survey in the normal manner.

2.

Record start case shots.

3.

Mark a centralizer blade and make the six point "yo-yo" run.

4.

Record start case shots again and run the survey in the normal manner, ensuring that the tool assembly enters the casing oriented approximately 180° from the initial entry.

5.

Re-survey the six "yo-yo" points during the course of the full survey.

This procedure should provide six pairs of usable measurements without taking up as much rig time as would six rotation shots at 600 feet. Note: The “yo-yo” points need not necessarily be at the very start of the full survey, but should be recorded at a depth where steady pictures can be reasonably expected.

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750-500-071 Rev. A / April 1997



In-Run Versus Out-Run Shots This method relies upon the tool naturally rotating between in-run shots and the out-run shots at the corresponding depths. There is no qualitative difference in the normal survey procedure except that out-run check shots should be taken more frequently than would be normally required. It is recommended that at least 10 are recorded at depths where steady pictures can be reasonably expected. This method can also be used to check the validity of the results obtained from the previous two methods.

Calculation of the TAC Methodology Data preparation for the above methods is done in the following manner. Method A: Rotation Shots 1.

Read and record the survey. Note Vernier 0 at each station.

2.

Draw the drift curve in the conventional manner using the first of the two recorded end case shots to determine overall drift.

3.

“Straight line” the drift between the survey end case shot and that obtained after the rotation shots.

4.

Apply orientation and drift corrections to the observed hole direction and Vernier 0 azimuths.

5.

Transfer this data to the TAC calculation sheet.

Method B: “Yo-Yo” Shots 1.

Read the initial start case shot, the “yo-yo” shots and the start case shot of the actual survey.

2.

“Straight line” the drift between the two start case shots and apply orientation and drift corrections to the observed hole direction and Vernier 0 azimuths.

3.


4.

Transfer the corrected data from the two sets of “yo-yo” shots to the TAC calculation sheet.

Method C: In-Run / Out-Run Shots 1.


2.

Transfer the corrected out-run data and the data from the corresponding in-run stations to the TAC calculation sheet.

Training Manual 750-500-071 Rev. A / April 1997

A-3 Confidential



The calculation of TAC (i.e., the error vector), is performed by a programmable calculator, such as the Hewlett Packard 67, 97, or 41C, or by a computer. Basically, two surveys taken at exactly the same depth should give the same result. In practice, if the tool has rotated to be suspended in a different attitude from before, the surveys will yield different results. The two are taken together and a true hole axis inclination and direction is determined. Now, given the true position of the hole and two apparent positions, an error vector can be determined.

Derivation of Formulae Used in the TAC Program The misalignment of the tool axis relative to the well bore axis is assumed to be a vector quantity, referred to as the error vector. The well has, of course, only one true value of inclination and direction at a particular depth. This value is referred to as the true hole vector. The observed values of inclination and direction are referred to as the observed hole vector. Remember, a vector quantity has both magnitude and direction. In this context, the inclinations are the magnitudes of the vectors while the directions of the inclinations are the directions of the vectors. In the case of the observed and true hole vectors, the directions are simply the hole directions. The direction of the error vector is assumed to be fixed in relation to the case of the gyro and therefore fixed in relation to the Vernier 0 position.

Let H denote the true hole vector, O the observed hole vector and E the error vector. Then O = H + E (vector addition) ∴E = O – H Note that the direction of the observed hole vector is the hole direction obtained from the gyro survey corrected for orientation and drift but, of course, not corrected for TAC. Suppose we have two surveys, taken at the same depth, the case of the tool having turned to a different position for the second survey. Suppose the two surveys give different values of inclination and hole direction (see Figure A-1).

A-4


750-500-071 Rev. A / April 1997



Let IE be the magnitude of E , the error vector. Let I01 and I02 be the inclinations of O 1 and O 2 the observed hole vectors. Let A01 and A02 be their respective azimuths. L is the line segment between (I0l, A01) and (IO2, A02), the tips of O 1 and O 2 .

N

N

N AL

AL

P ar al lel to L AE 9

IE

IE

AL (I 0 1 , A 01 )

(I 02 , A 02 ) L

O1 O2

E V0 1

9

$(

,(

V02

Figure A-1

We draw the perpendicular bisector of L and construct an isosceles triangle whose apex is on the perpendicular bisector of L and whose apex angle is equal to ∆V, the angle between the two Vernier 0 directions. The two equal sides of the triangle represent IE, the inclination (magnitude) of the error vector, while the third side is the line segment L. Other constructions are as shown in Figure A-1.


A-5 Confidential



Now we have: L --2-  ∆ V  = ---sin  -----2  IE   

∴I E

L --2= -------------------∆V  sin  -----2 

Applying elementary geometry to the figure gives: ∆V A E = AL + 90 0 – -------  2 The difference in azimuth between the direction of the error vector and the corresponding Vernier 0, ∆AE = V02 – AE. The length of the line segment L is found by applying the cosine rule which gives: 2

2

2

L = I01 + I02 – 2I01 I02 cos ( A 02 – A 01 )

The angle between O 1 and L is also found from the cosine rule. If we call this angle ∠Z then: 2

2

2

L + I 01 – I 02 cos ∠Z = ------------------------------2LI01 Finally, ∠A L is determined from: o

o

∠A L = 360 – ∠Z – ( 180 – A 01 ) After L has been calculated, IE can be determined. Then the program calculates the angle Z, followed by ∠A L , then ∠AE and hence ∆AE. Thus the magnitude and direction of the error vector have been determined. The second half of the TAC program adds the negative of the error vector to the observed hole vector to obtain the true hole vector: H =O –E This is done by simple application of the cosine rule.

A-6


750-500-071 Rev. A / April 1997

Appendix

B

Exercises Example 1 1.

What is the amount and direction of the following orienting corrections?

Foresight

Start Case

S 63° W

S 64° W

N 89° W

N 88.25° W

S 83.25° E

S 80.75° E

163.5°

165.25°

270.25°

273.50°

N 17.75° E

N 17.75° W

S 86.75° E

N 89.25° E

S 88.25° W

N 87.17° W

2.

Orienting Correction

What is the amount and direction of the following drift checks? Start

End

D1

S 86.25° W

S 86.75° W

D2

N 80.25° W

N 78.75° W

D3

N 76.75° E

N 76.5° E

D4

N 89.25° E

S 89.50°

D5

N 89.0° W

S 88.25° W

D6

S 01.25° W

S 01.25° E


B-1 Confidential

Exercises


Example 2 1.

Correct the following polar coordinates to rectangular.

a.

30' @ N 63 E

b.

65' @ N 76 W

c.

986' @ S 83 1/2 E

d.

392' @ S 40 1/2 W

e.

76' @ 163'

f.

98' @ 192.5'

2.

Convert the following rectangular coordinates to polar giving distance and direction.

a.

N 65'

E 79'

b.

N 73'

W 192'

c.

S 83'

E 186

d.

S 98'

W 72'

e.

S 836'

W 793'

f.

N 90'

W 900'

B-2


750-500-071 Rev. A / April 1997


Exercises

Example 3 1.

What would your revised foresight be in order to put spin axis in hole direction with a 2½" Gyro in the following cases: Revised Foresight

a.

Foresight Hole Direction

N 1O° W N 42° E

b.


N 16¼° W N 64° W

c.


S 18½° E S 83° E

d.


S 33¼° W S 46½° E

e.


South N 18° W

f.


N 62° E S 20° W

2.

For a 1½" Gyro, calculate the revised foresight to put gimbal axis in hole direction: Revised Foresight

a.


N 10° W N 42° E

b.


S 33¼° W S 46½° E

c.


N 17½° E N 80° W


B-3 Confidential

Exercises


FIELD CALCULATION SHEET Company: Surveyor:

Declination Applied: MEASURED DEPTH

Objective:

STUDENT

INCLINATION

YES OBSERVED DIRECTION

°E/W TVD

Block:

SCHOOL CO Well:

N 85 E

1

UPPER/LOWER Camera

VERTICAL SECTION

DLS

Page:

1 of 1

DENMORE 1 / 1 Date:__________________

Magnetic Multishot Survey

NORTH

SOUTH

EAST

WEST

TIE-ON AT SURFACE 100'

1.0°

N 63 E

200'

2.0°

N 69 E

300'

3.0°

N 85 E

400'

4.0°

N 89 E

500'

5.0°

N 87 E

600'

6.0°

N 80 E

TARGET: N 87° W 2700'

36.0°

S 82 W

2432 78

788 02

86 76

793 65

TIE-ON CONDITIONS 2800'

36.5°

S 84 W

2900'

37.5°

S 86 W

3000'

38.25°

S 89 W

3100'

39.0°

WEST

3200'

39.5°

WEST

3300'

40.0°

N 88 W

3410'

41.0°

N 87 W

3520'

42.0°

N 87 W

3610”

42.5°

N 85 W

CLOSURE IS:

B-4


750-500-071 Rev. A / April 1997

GYROSCOPIC SURVEYING.pdf

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