PREDICTING DRILLING MUD LOSS
OCCURRENCE WHILE USING
DIRECTIONAL DRILLING TO INSTALL PIPELINES ACROSS RIVERS
Obumse Chukwuebuka Michael; SPE, Federal University of Technology, Owerri Email:
[email protected] [email protected] ;; Phone: +234(0)806 +234(0)806 478 478 3775 ; SPE Member Member ID: 3383020
Abstract A paper is presented that studies the use of horizontal directional drilling (HDD) for installing pipelines across obstacles (especially rivers) in general, and its usually associated drilling mud loss problem caused b y the hydraulic fracturing of the formation by drilling mud, in particular. A simulation model was developed from analytical geometry, drilling mud hydraulics and geotechnical studies. The model is capable of : (1) designing efficient drill path profiles- a precursor to avoiding drilling drilling mud loss problems; (2) calculating calculating annular pressures at any measured depth drilled; (3) determining limiting mud pressures; and (4) predicting the possibility of drilling mud loss occurrence by hydraulic fracturing for any such drilling programme.
Keywords: Keywords: horizontal directional drilling, hydraulic fracturing, fracturing, drilling mud, annular pressure, drill path.
1. INTRODUCTION
Moving oil and gas from a field to refining and processing plant, and petroleum products from refineries to consumers require a complex transportation system
[1]
with pipelines playing the major role. Pipelines are
constructed through different terrains and environments along along its‟ right of ways ways (ROWs). Conventionally, pipelines are installed by the open-trench method which involves burying of pipelines into excavated ditches. Although the open-trench method is the simplest, cheapest and fastest way on a smooth topography, it often appears uneconomical or totally infeasible when certain obstacles, e.g water co urses, buildings, railways, etc are encountered along the pipeline route. For this reason, the trenchless methods have evolved.
One of the alternative construction methods, and perhaps the fastest growing technology in the trenchless industry is horizontal directional drilling (HDD)
[2]
. It involves the application of techniques and equipments that are used in 1
horizontal oil well drilling and conventional road boring to install pipelines underground, using a surface-monitored drilling rig that launches and places a drill string at a shallow angle to the surface and has tracking and steering capabilities [3]. The operation involves three (3) main stages
[2,4,5]
:
Pilot-hole Drilling: which involves the drilling of the pilot-hole along a pre-determined drill path, using a drill-rig operating from the ground surface. Periodic readings from a probe situated close to the drill bit are used to determine the horizontal and vertical coordinates along the pilot hole in relation to the initial entry point. The pilot hole may also be tracked using a surface monitoring system that determines the downhole probe location by taking measurements from the surface point (see fig A.1);
Reaming of the Pilot-hole: which involves the replacing of the drill drill bit with a back reamer that is pulled back to enlarge the borehole size up to the desired diameter. Multiple reaming passes may be required depending on the soil type and the required degree of borehole enlargement (see fig A.1); and
Pipe String Pullback: which involves pulling the entire pipeline length in one segment (usually) back through the drilling mud along the reamed hole pathway until the entire pipe string has been pulled into the bore hole (see fig A.1).
One of the greatest challenges faced by the HDD contractor is how to achieve a successful installation without a resultant adverse impact on the surrounding environment. This is usually in the form of inadvertent return of drilling mud to the surface which may eventually contaminate the aquatic or terrestrial environment. This situation may constitute serious problems when chemical additives are used in the drilling mud. Mud loss into the aquatic environment may have severe consequences if the host community depends on the water body for domestic use as evident in some remote parts of Nigeria
[4]
.
Inadvetent return of mud usually results from mud escape through propagated fractures developed in the formation, formation, due to excessive overbalance pressure. This is often referred to as „frac -out‟ or „hydraulic „hydraulic fracturing by mud‟. Hydrofractures initiate initia te when the pressure in the annulus exceeds the „maximum allowable mud pressure‟ that the formation can withstand without fracturing. This phenomenon is not only dependent on the drilling fluid pressure inside the newly created bore, but the properties and stress state of the surrounding soil as well
[6]
. A proper
understanding and application of drilling drilling mud hydraulics and efficient drill path design are therefore essential to
2
avoid or reduce the risk of inadvertent mud returns. Monitoring of the annular pressure against established maximum allowable mud pressure calculated from geotechnical studies enables such a check to be made.
2. MODEL DEVELOPMENT
2.1 Model Introduction
The model developed in this study- HDD PREDICTOR (Beta 1.0) was written in
using the
integrated development environment of Microsoft Visual studio 2008. The model was designed from analytical geometry, drilling mud hydraulics, and standard equations developed from geotechnical studies. It can perform the following:
1.
Design drill-path profiles/curves from survey data. The various curves can then be analyzed to select the drillpath that is most technically and economically feasible.
2.
Perform calculations to generate the various pressure profiles: hydrostatic pressure, frictional pressure losses, and hence the downhole annular pressure; based on the selected drill path curve, drilling parameters, and drilling fluid properties. This will be very essential in pre-drilling planning and mud selection decision making.
3.
Perform calculations to generate the „maximum allowable mud pressure‟ [that the formation can withstand without „fracturing‟] profile in a case where geotechnical studies were conducted from formation cores obtained from vertical bores at various depths near zones along the pre-established drill path.
4.
Match (2) and (3) above together to mark out the various zones along the drill path that are likely to have „frac-out‟ potential. This will aid the driller in applying preventive measures such as proper drilling/mud management, and the use of protective casing.
2.2 Model Assumptions
1.
Hydraulic fracturing of the formation by drilling mud occurrence initiates during the pilot-hole drilling stage.
3
2.
The depth of cover at both the build and the horizontal segments are large en ough to prevent hydro-fracture propagation, i.e. propagation of hydraulic fracturing can only be significant at the tangential segments.
3.
Uniform drill-pipe joint-length is used throughout the drilling process, and the drillbit-monel assembly is equivalent to a pipe-joint.
4.
There is no significant deviation away from the centre-line, i.e.
. As such, a two dimensional
analysis (x, y) suffices to describe the bore trajectory. 5.
The designed drill path trajectory is adhered to, i.e. deviation is insignificant.
6.
Laminar flow regime prevails in the annulus.
7.
The Annulus is nearly concentric throughout the pilot hole, i.e. eccentricity ≈ 1.
As such, the model has two modules: the „Drill-Path Planner‟, and the „Pressure Predictor‟(see fig A.3).
2.3 Drill-Path Planner
This considers the two standard and most common borehole profiles: the „5 segments‟ and the „3 segments‟ designs.
The „5 segments‟ design development is t hus presented.
Entry Point
α
H
T
α α
Fig 1: showing the „5 segments‟ drillpath trajectory in 2-D
β
Datum
β Exit Point
h
Joint length
Where α and β are the entry and the exit angles respectively, R 1 and R2 are the radius of curvatures of the first and the second build segments respectively.
The data input required: { α, β, R1, R2, entry elevation, exit elevation, entry station, exit station, elevation of river bottom, depth of cover}.
4
DERIVATION
=
[1]
=
[2]
[3]
[4]
[5]
α
,
,
,
and
follows up similarly, by replacing
[6]
[7]
[8]
and α, with
and β respectively in the above
set of equations.
=
[9]
During the drilling process, the pipe-length (measured length drilled) is the actual displacement parameter known. Determination of the x and y coordinates per joint length is obtained thus:
At any position/segment;
Similarly,
, where Δy is the „Rise/Drop‟.
[10]
„Away‟
[11]
Along Segment 1:
0 pipelength MD1 ; 1 n K 1 Where n = joint number,
y jo int length sin
[12]
[13]
x jo int length cos
[14]
The initial point for this segment corresponds to the entry p oint i.e.
5
Along Segment 2: MD1
pipelength
( MD1
MD2 ) ; (k 1
1)
n k 1
k 2
[15]
y R1
cos[ (n k 1 ) ] cos[ (n 1 k 1 ) ]
[16]
x R1
sin[ (n 1 k 1 ) ] sin[ (n k 1 ) ]
[17]
The initial point for this segment corresponds to the last point in segment 1, i.e. (x k 1 , y k 1 ) Along Segment 3: ( MD1 MD2 ) pipelength
( MD1 MD2
T ) ;
k 1 k 2 1 n k 1 k 2 k 3 ,
y 0
[18] [19]
x jo int length
[20]
The initial point for this segment corresponds to the last point in segment 2 i.e. (x k 1 k 2 , y k 1 k 2 ) Along Segment 4:
( MD1
MD2 T ) pipelength ( MD1 MD2 T MD3 )
; k 1 k 2 k 3 1 n k 1 k 2 k 3 k 4
y R2
cos[( n (k 1 k 2 k 3 )) ] cos[( n 1 (k 1 k 2 k 3 )) ]
x R2
sin[(n (k 1 k 2 k 3 )) ] sin[(n 1 (k 1 k 2 k 3 )) ]
[21] [22]
The initial point for this segment corresponds to the last point in segment 3 i.e. (x k 1 k 2 k 3 , y k 1k 2 k 3 ) Along Segment 5: ( MD1
MD2 T MD3 ) pipelength totalpipel ength,
k 1 k 2 k 3 k 4 1 n totalpipejo int s
y jo int length sin
[23]
x jo int length cos
[24]
The initial point for this segment corresponds to the last point in segment 4, while its last point corresponds to the exit point (exit station, exit elevation). 2.4 Pressure Predictor
This consists of the „Annular Pressure‟ module and the „Limiting Mud Pressure‟ (Maximum Allowable Mud Pressure) module. 6
2.4.1 Annular Pressure Module
Pressure in the annulus of the borehole
Pressure drop,
includes the hydrostatic fluid pressure and the pressure drop ∆P
+
+
[4]
.
[25]
Neglecting the pressure drop due to gravity and acceleration since HDD is a near horizontal scenario, and same bit size is usually maintained during drilling.
+
[26]
+
[27]
The average velocity of drilling fluid in the borehole annulus, Va in ft/sec, is given as:
[28]
For a laminar flow condition, t he frictional pressure drop ∆P in psi, in the concentric annulus, using Bingham Plastic fluid model is then given as:
[29]
Where
[30]
Hydrostatic Pressure
[31]
Elevation relative to the entry point, ft
= diameter of hole/bit size, in;
viscosity of mud,
;
= mud yield point,
drillpipe outside diameter, in;
[32]
plastic
; ρ = density of [returning] mud, ppg.
2.4.2 Limiting Mud Pressure (Maximum Allowable Mud Pressure) Module
This module is built on three (3) proven geotechnical equations developed for the determination of the maximum allowable mud pressure, that the formation around the annulus can withstand before fracturing.
7
Kennedy et al [6]: This equation defines the minimum pressure required for fracturing to occur:
;
Delft Equation: This equation defines the maximum allowable pressure in the annulus
[33]
[2, 4]
, thus:
=
[34]
Queen‟s Equation: This was developed after the work of Xia and Moore [2]
Critical mud pressure In the equations above; Po = Initial soil compressive stress; K o = coefficient of earth lateral pressure;
Radius,
;
friction angle [°],
;
= Radius of the Plastic Zone,
= cohesion,
= Effective Stress,
(
(
= 0.5* for clay, or 2* /3 for sands);
= undrained cohesion); G = Shear Modulus ,
;
[35]
Bore
= internal (soil)
= Groundwater Pressure,
.
3. DRILLING-MUD LOSS PREDICTION WITH ACTUAL EXAMPLE
A project consists of installing an 18 inch diameter steel gas pipeline across River-X. A survey referenced to the entry point was carried out and recorded as shown in Table 1. Suppose a geotechnical study was conducted at intervals near the entry and the exit sides with results recorded in T able 2. It is therefore expected to:
Design the drill-path and a suitable drilling programme to avoid hydraulic fracturing occurrence.
Using the program- HDD PREDICTOR, the user launches the program, and then clicks on „Project Information‟ to input the job information (no part of this goes into the calculation). This is shown in fig A.4. The „Drill-Path Planner‟ module is activated after clicking „Done‟.
8
Drill-Path Profile Design
The user selects either the „5 segments‟ design or the „3 segments‟ design (For this case study, the „5 segments‟ design was selected) and then input the survey (entry point and exit point), pipe and geometry datas as shown in fig A.5. The drill-path profile is automatically generated as the „Done‟ button is clicked. By clicking on the „Results‟ botton, a table pops out indicating the length of various segments, required number of pipe joints for each, build/drop angles, etc. Fig A.6 and fig A.7, show the „profile‟ and the „results‟ respectively. The user exits this module by clicking on „Predict Pressure‟ to lauch the module.
Interpretation of Drill-Path Profile Result from the model for case example – The „results‟ displayed suggests that the driller/steering-hand should do the following:
Initiates drilling at 12° entry angle in a straight course (tangential segment) until 7 pipejoints
total
measured distance of 210.56ft is drilled. This includes the drillbit-monel assembly and 6 drillpipes.
Kicks off at buildup rate of 1.5° per joint, until another 251ft (equivalent to 8 new drillpipes and further 11ft are drilled).
Sustains/holds to drill a horizontal segment „blindly‟ at constant zero inclination for another 4204ft (equivalent to 140 new drillpipes and additional 4ft).
Builds up once again at 1.71° per joint for 7 new joints
Drills at tangent by „holding‟ till he exits to the surface, which will require 10 new joints.
additional 210ft.
A total measured depth of 5173.4ft is therefore estimated for the project, requiring about 172-173 drillpipes.
Pressure Prediction
At this module, the user:
1.
Selects „Annular Pressure‟ and then „Bingham Plastic Model‟ to enter its required input (see fig A.8). This calculates the annular velocity, and generates the hydrostatic pressure, pressure drop and annular pressure profiles/curves over the measured distance drilled or pipelength (see fig A.9).
2.
Exits the „Annular Pressure‟ pane and then select „Limiting Mud Pressure‟ to choose one of Kennedy et al, Delft equation, or Queen‟s equation (Delft equation was selected for this example). Entry-side (see fig
9
A.10) and exit-side datas are inputed in succession, to generate the overall combine curves namely: hydrostatic pressure, pressure drop, annular pressure, exit-side limiting mud pressure and entry-side limiting mud pressure profiles/curves over the measured distance drill or pipelength. Fig A.11, shows the combined curves for the example presented.
Interpretation of Pressure Prediction Result from the model for case example – The calculations performed and the charts generated suggest the following:
Average annular velocity of 3.307ft/s under the current mud/drilling plan.
A maximum annular pressure of less than 60psi near the exit point.
A hydrofracture risk-free entry side, but an exit side with hydraulic fracturing risk potential. This is indicated by the intersecting of the „annular pressure‟ curve with the „exit side limiting pressure‟ curve in fig A.9. Preventive measures in such a case may entail: (1) revisiting of the drilling/mud plan, or (2) the use of intersecting drill with conductor casing at the exit side (which is usually very expensive!).
Since the drillpath profile design appears satisfactory, the drilling mud plan will therefore be adjusted. T he mud pump rate and mud weight may have to be lowered slightly, especially while drilling the exit-side tangential segment of the profile.
4. CONCLUSION
The method of using horizontal directional drilling (HDD) to install pipelines across obstacles, especially water courses, and its usually associated problem (mud loss by hydraulic fracturing) have been discussed. A simulation model was therefore developed from analytical geometry, drilling hydraulics and proven geotechnical equations. The model has been shown to be capable of designing, analyzing and predicting against hydraulic fracturing occurrence and the consequent inadvertent mud loss while using HDD. The usual practice of real-time monitoring of readings from down-hole pressure guages lack the ability to make predictions, as it can only indicate propagating fractures which may not be easily combated due to “rig-downhole time-lag”. It should be used as backup after predictions and analysis have been made with the model presented. The model is therefore recommended for use to HDD contractors, pipeline owners and regulatory agencies. 10
ACKNOWLEDGEMENTS
The author wishes to express his gratitude to his friend Akinboboye Shina, and the entire HDD team of Enikkom Investment Services Nigeria Limited (where he had his industrial training in horizontal directional drilling), especially John Okechukwu, Chris Frisch and Michael Snook, for all their assistance.
NOMENCLATURE
entry angle, degree exit angle, degree
build angle per joint at the exitside build segment, degree plastic viscosity of drilling mud, cp
density of mud, ppg effective soil stress, psi
build angle per joint at the entryside build segment, degree internal (soil) friction angle, degree
soil cohesion, psi diameter of borehole (bit size), in
soil shear modulus, psi coefficient of earth lateral pressure measured depth, ft drillpipe outside diameter, in pressure drop due to acceleration, psi
pressure drop in the annulus, psi pressure drop due to friction, psi
pressure drop due to gravity, psi
pressure in the borehole annulus, psi
fracturing pressure, psi
critical mud pressure, psi maximum allowable pressure in the annulus, psi
initial soil compressive stress, psi
11
pump output, gpm
radius of curvature of build segment, ft bore radius, ft maximum radius of plastic zone, ft
groundwater pressure, psi average velocity in the annulus, ft/s
station, ft elevation, ft mud yield point, lb/100ft 2
REFERENCES [1] Kate Van Dyke. (1997). Fundamentals of Petroleum, 4th ed. Petroleum Extension Service, University of Texas, Austin, USA. [2] Xia Hongwei. (2009). Investigation of maximum mud pressure within sand and clay during horizontal directional drilling , PhD dissertation, Queen‟s University, Kingston, Ontario, Canada.
[3] ASTM International, Designation: F 1962 – 99, Standard guide for use of maxi-horizontal directional drilling for
placement of polyethylene pipe or conduit under obstacles, including river crossings, 1-7
[4] Obumse, Chukwuebuka M. (2011). Overcoming Drilling Mud Loss Problems while using Horizontal Directional Drilling to Install Pipelines across Rivers , Bachelor‟s thesis, Federal University of Technology,
Owerri, Nigeria. Unpublished. [5] Entec Consulting Limited, et al. (2004). Guideline: Planning Horizontal Directional Drilling for Pipeline
Construction. Canadian Association of Petroleum Producers, CAPP Publication 2004-022.
[6] Kennedy, M.J.et al. (2006). Limiting Slurry Pressure to Control Hydraulic Fracturing in Directional Drilling Operations in Purely Cohesive Soil, Proceedings of 2004, proceedings of the North American
Society for Trenchless Technology (NASTT),.No-Dig Conference,2004a. [7] Conroy, P. J. et al. (2002). Guidelines for installation of utilities beneath Corps of Engineers levees using horizontal directional drilling , US Army Corps of Engineers Research and Development Center,
ERDC/GSL TR-02-9.
12
APPENDIX: THREE (3) SEGMENTS DESIGN
Entry Point
H
α α
α
β
β
Datum Exit Point
h
T Joint length
The data input required: {α, β, entry elevation, exit elevation, entry station, exit station, elevation of river bottom, depth of cover}.
DERIVATIONS:
=
[A.1]
=
[A.2]
[A.3]
[A.4]
,
When
(
,
, and
[A.5]
,
[A.6]
[A.7]
follows up similarly, by replacing H and α, with h and β respectively)
T=
13
[A.8]
Table 1: Survey and Drilling Data for example presented Geometry Parameters
Entry angle ,degrees
12
Entry elevation, ft
0.00
Entry station, ft
0+00
Exit angle , degrees
10
Exit elevation, ft
0.00
Exit station, ft
51+61.4
Radius for 1 st build, ft
1200
Bottom Elevation of river, ft
50
Radius of 2 nd build, ft
1200
Required cover under bottom, ft
20
Drilling Parameters
Plastic Viscosity, cp
7
Yield Point, lb/100sq ft
9
Mud Density, ppg
11
Mud flow rate, gpm
300
Diameter of Drillpipe, in
5 (OD of pipe)
Diameter of Bit, in
7.875
Table 2: Geotechnical Data Obtained From Laboratory Analysis (modified from Conroy et al
[7]
Soil Type = Sand ENTRY SIDE Depth (ft)
5
22
0
694.4
4.34
0
0.5
6.94
173.5
4.17
0
10
22
0
694.4
8.68
0
0.5
6.94
173.5
8.33
0
15
22
0
173.6
13.02
0
0.5
3.47
173.5
12.5
0
20
0.5
6.94
694.4
17.36
0
0.5
3.47
173.5
16.67
0
25
29
0
694.4
21.70
0
0.5
3.47
173.5
20.83
0
30
29
0
694.4
26.04
0
30
0
694.4
22.83
2.17
35
34
0
694.4
30.36
0
30
0
694.4
24.83
4.33
40
33
0
694.4
34.29
0.433
30
0
694.4
26.83
6.5
45
33
0
694.4
36.46
2.6
30
0
694.4
28.83
8.67
(°)
(psi)
(psi)
EXIT SIDE (psi)
(psi)
(°)
14
(psi)
(psi)
(psi)
(psi)
. pg. 13)
Fig A.1: showing Pilot-hle drilling, Reaming & Pull-back stages (Source: CAPP Publication 2004-0022
Fig A.2: A physical flowing model in the annulus showing induced fractures 15
[5]
)
START
No of Input
5
segments?
ProjectInformation
3 Input drill Path design data and Radius 1,Radius 2
Input drill path design data
Design drill path profile, Determine: no of joints {k1, k2 Total pipe length required etc.
…
YES
k5}
NO
Terminate Program ?
Predict Annular Pressure or maximum allowable mudPressure?
annular Enter Bingham Plastic model data
Max. allowable mud pressure
Kennedy et al.
Enter Kennedy
Predict Pressure
Model?
Delft
et al. data
Queen’s Display Pressure
Enter queen’s
Plots
data
Enter Delft data Is predict Pressure Annular
Include max.
annular
or max. allowable
allowable mud
YES
pressure?
mud pressure?
Max. allowable mud pressure
END
NO
Fig 16 A.3: The Program Flowchart
Fig A.4: showing the „Project Information‟ pane
Fig A.5: „5 segments‟ profile input screen 17
Fig A.6: Drill Path Profile, for the example
Fig A.7: Results output for drill path design, for the example
18
Fig A.8: Annular Pressure Input, for the example
Fig A.9: Annular Pressure vs. Pipe Length, for the example
19
Fig A.10: showing the „Delft‟s entry-side input and calculation results, for the example
Fig A.11: showing the various pressure curves vs pipe-length, for the example
20
