Borehole Seismic Survey 1
Borehole Seismic Introduction
2
Borehole Seismic Tool and Acquisition
3
VSP Processing
4
Sonic Calibration and Synthetic Seismogram
5
VSP Examples
Kieu Nguyen Binh HCMC-2010
Borehole Seismic Survey 1
Borehole Seismic Introduction
2
Borehole Seismic Tool and Acquisition
3
VSP Processing
4
Sonic Calibration and Synthetic Seismogram
5
VSP Examples
Kieu Nguyen Binh HCMC-2010
#2 Borehole Seismic Tool and Acquisition
Downhole Tools
Downhole Seismic Tool
40 level VSI tool
Downhole Seismic Tool
40 level VSI tool
Gimbaled and Non-Gimbaled Geophones Logging tool
Gimbaled Gimbaled
Z geophone is always vertical
Easier to interpret Non-gimbaled
Geophone is fixed to the tool
Requires omni-tilt geophones
Mechanically simpler to implement
Non-gimbaled
Borehole seismic tool considerations •
X, Y and Z geophones
• Number of shuttles and inter-shuttle spacing. •
Clamping mechanism and force.
•
Data transfer rate on logging cable, and time between shots. •
•
This is a big factor when using large array tools.
Tool diameter •
For thru tubing or thru drillpipe operations
•
Temperature rating
•
Interface with seismic source
15 metres
4 shuttle configuration
Tool Conveyance Methods
Tool conveyance method for standard logging tools
In a highly deviated well, may need drill pipe conveyed logging (TLC). Difficult to use an array seismic tool with Pipe conveyed technique. Need a “stiff bridle”.
VSP in High Deviation Wells Thru-Drill-Pipe VSP (open hole or cased hole) VSI tool ID is 2.5” VSI tool can be pumped down inside the drill pipe
Example from Malaysia
Downhole Tractor MaxTrac Downhole tractor to pull the VSI tool Only for cased hole. This is a better option for data quality, than thru-drill-pipe. MaxTrac is routinely used with 4-level and 8-level VSI tools in the Middle East.
Sources & Source control
Examples of Good and Bad VSP data The 3 main factors that influence VSP data quality: 1. Downhole environment 2. VSP tool 3. Surface energy source
In this example the downhole enviroment is noisy, and the airgun array has problems
Examples of Good and Bad VSP data Same as previous slide but with increased trace overlap
A good quality VSP requires A perfectly repeatable signature
G and GI gun configurations G-gun
150 cu in (only used for CS surveys)
Double G-gun
300 cu in
Triple G-gun
450 cu in
GI gun
255 cu in (105+150)
G-GI gun
405 cu in
3 Gun Array
3 gun array for offshore
Top right shows a 3 gun array with an “in-sea” Trisor gun controller
Airgun pits on land
Typically recommend a pit 4-5 metres deep 4-5 metre wide
Land Airgun pit and gun deployment
Source type: G-GI air gun cluster Azimuth： 80 deg Offset ：17 m Source Elevation ：2.0 m below GL Hydrophone elevation : 1.0 m below GL
Offset and Walkaway VSP surveys Additional hardware required
A boat from which to deploy the airgun.
A crane on the boat.
A navigation system on the boat interfaced to the wireline logging unit.
A radio controlled airgun firing system. – Fire the airgun – Record navigation for every shot
A rig tender for a deepwater semi-sub in Malaysia. The crane boom can extend to about 10 or 12 metres Safe distance of airgun from the boat hull 3 gun array = 8m 2 gun array = 6m Single gun = 4m
Vertical Incident V.S.P or Walkabove VSP Drilling Rig Source Vessel
Require: - Remote radio control of airgun - Navigation system Well Bore Downhole Seismic Receiver
Navigation System •
Differential Global Positioning System
•
For accuracy positioning the Seismic Source
•
Mainly used for offshore applications using supply boats
Deviated well VI-VSP - Target Circles Target Circles
Receiver Positions
WALKABOVE
Well Track Rig
Typical circle diameter 1/40 or 1/80 TVD
At 2000 m depth, 1/40 TVD = 50 metres At 500 m depth, 1/40 TVD = 12.5 metres Typically would tell boat 10 metres tolerance for entire survey, if weather conditions poor, can got to 20 metres
Walkaway VSP - Line Example
Ideal Shot positions
WALKAWAY
Actual shot positions
Off Track error lines (Tolerance)
25m (Typical) Air Gun Source
Boat goes at constant speed = 4 knots = 2 metres/sec At this speed, can fire the gun every 12.5 seconds gives 25m shot spacing
VI-VSP using single level tool Example is a single level tool, doing 12 levels per hour The time for 4-level VSI survey will be about 3 times quicker. Vertical axis is the borehole depth Horizontal axis is in hours
1 hour grid
VI-VSP Acquisition Efficiency Using a VSI-4 tool Each point corresponds to one setting of VSI-4 tool Survey speed is about 9 settings per hour, or 36 levels per hour Vertical axis is the depth of the 4 level VSI tool Horizontal axis is in hours
Choice between a rig source and VI source in a deviated well Rig Source VSP Compute vertical time-depth. However, this may be approximate as it will be effected by refraction, which are difficult to correct for.
Provide VSP image under the well trajectory. For a rig source VSP, this will require migration.
VSP image to identify structure under the well trajectory.
Rig Source & Deviated well
Choice between a rig source and VI source in a deviated well VI-VSP
Compute vertical time-depth. Since the ray-path is vertical, then better measure of time depth is obtained. If lateral velocity variations are present in the upper sections, the time-depth will still be correct at each shot point, but the computed interval velocity, may not be representative, since this computed from two adjacent shot points. Provide a VSP under the well trajectory. For flat formations, migration is not required. If formations are dipping, migration can be considered. VSP image to identify structure under the well trajectory. Generally, both the time-depth and P-wave VSP image are considerably better for the VI-VSP configuration. VI-VSP will provide an image that follows directly below the curved well path.
VI-source & Deviated well
Error in OWT from a Rig Source VSP This is an example from ray trace modeling Left plot shows the well trajectory. Right plot is the difference in the “vertically corrected transit time”, computed from a rig source VSP and a VI-VSP. This curve shows the OWT error that will result from a rig source VSP. Error is 3 msec OWT (6 msec TWT) at 3000 metres. At TD, the straight line direct raypath angle is 15 deg from vertical. The error occurs, since the raypath is not straight line, but follows a curved refracted path. This error can increase or decrease, depending on the average formation structural dip.
VSP data behind casing
This example is deviated well so we can expect better results for the VSP data if poor cement.
VSP behind casing A single cemented casing string is OK. Often well cemented single casing is better quality than open hole. Un-cemented surface casing generally give poor quality data
20 inch shoe
Double casing with one uncemented will generally give poor quality data. If well is deviated, then casing ring will be more attenuated, than in a vertical well. Top of cement ?
Cemented 9.5/8” casing 13.3/8 inch shoe
VSP behind casing #2 Free pipe
Poor cement / bad hole behind casing
Casing shoe General rule: - VSP works well in open hole - VSP works well in cemented cased hole - Results cannot be guaranteed in casing with no cement (particularly large diameter surface casings) - Deviated well with casing and no cement may work OK
