1

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 by 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 mud loss problems; (2) 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: horizontal directional drilling, hydraulic 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 its‟ right of 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 courses, 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

PREDICTING DRILLING MUD LOSS OCCURRENCE WHILE USING

DIRECTIONAL DRILLING TO INSTALL PIPELINES ACROSS RIVERS

Obumse Chukwuebuka Michael; SPE, Federal University of Technology, Owerri

Email: mcebuka@yahoo.co.uk ; Phone: +234(0)806 478 3775 ; SPE Member ID: 3383020

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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 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,

due to excessive overbalance pressure. This is often referred to as „frac-out‟ or „hydraulic fracturing by mud‟.

Hydrofractures initiate 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 mud hydraulics and efficient drill path design are therefore essential to

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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.

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2. The depth of cover at both the build and the horizontal segments are large enough 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 thus presented.

Fig 1: showing the „5 segments‟ drillpath trajectory in 2-D

Where α and β are the entry and the exit angles respectively, R1 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}.

α

β

h

H

β α α

Datum

Joint length

T

Exit Point

Entry Point

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DERIVATION

= [1]

= [2]

[3]

[4]

[5]

α [6]

[7]

[8]

, , , and follows up similarly, by replacing 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; , where Δy is the „Rise/Drop‟. [10]

Similarly, „Away‟ [11]

Along Segment 1:

10 MDpipelength ; 11 Kn Where n = joint number, [12]

sinint lengthjoy

[13]

cosint lengthjox [14]

The initial point for this segment corresponds to the entry point i.e.

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Along Segment 2:

)( 211 MDMDpipelengthMD ; 211 )1( kknk [15]

])1(cos[])(cos[ 111 knknRy

[16]

])(sin[])1(sin[ 111 knknRx

[17]

The initial point for this segment corresponds to the last point in segment 1, i.e. ),(x

11k ky

Along Segment 3:

)()( 2121 TMDMDpipelengthMDMD ; ,1 32121 kkknkk [18]

0y [19]

lengthjox int

[20]

The initial point for this segment corresponds to the last point in segment 2 i.e. ),(x

212k1k kky

Along Segment 4:

)()( 32121 MDTMDMDpipelengthTMDMD ; 4321321 1 kkkknkkk

]))(1cos[(]))(cos[( 3213212 kkknkkknRy [21]

]))(1sin[(]))(sin[( 3213212 kkknkkknRx

[22]

The initial point for this segment corresponds to the last point in segment 3 i.e. ),(x3213k2k1k kkky

Along Segment 5:

,321 )( engthtotalpipelpipelengthMDTMDMD sototalpipejnkkkk int14321

sinint lengthjoy

[23]

cosint lengthjox

[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.

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2.4.1 Annular Pressure Module

Pressure in the annulus of the borehole includes the hydrostatic fluid pressure and the pressure drop ∆P [4]

.

Pressure drop, + + [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, the 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 [32]

= diameter of hole/bit size, in; drillpipe outside diameter, in; plastic

viscosity of mud, ; = mud yield point, ; 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.

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Kennedy et al [6]

: This equation defines the minimum pressure required for fracturing to occur:

;

[33]

Delft Equation: This equation defines the maximum allowable pressure in the annulus [2, 4]

, thus:

=

[34]

Queen‟s Equation: This was developed after the work of Xia and Moore [2]

Critical mud pressure

[35]

In the equations above; oP = Initial soil compressive stress; oK = coefficient of earth lateral pressure; Bore

Radius, ; = Radius of the Plastic Zone, ( = 0.5* for clay, or 2* /3 for sands); = internal (soil)

friction angle [°], = cohesion, ( = undrained cohesion); G = Shear Modulus , ; = Groundwater Pressure,

; = Effective Stress, .

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 Table 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‟.

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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 additional 210ft.

Drills at tangent by „holding‟ till he exits to the surface, which will require 10 new joints.

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

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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. The 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.

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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

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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/100ft2

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.

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APPENDIX: THREE (3) SEGMENTS DESIGN

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]

,

[A.5]

When

, [A.6]

[A.7]

( , , and follows up similarly, by replacing H and α, with h and β respectively)

T = [A.8]

Exit Point

Entry Point α

β

h

T

H

β α α

Datum

Joint length

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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 1st build, ft 1200

Bottom Elevation of river, ft 50

Radius of 2nd 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]

. pg. 13)

Depth (ft)

ENTRY SIDE EXIT SIDE

(°) (psi) (psi) (psi) (psi) (°) (psi) (psi) (psi) (psi)

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

Soil Type = Sand

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Fig A.1: showing Pilot-hle drilling, Reaming & Pull-back stages (Source: CAPP Publication 2004-0022 [5]

)

Fig A.2: A physical flowing model in the annulus showing induced fractures

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Terminate Program ?

Predict Annular

Pressure or

maximum

allowable mud-

Pressure?

Predict Pressure

annular Enter Bingham Plastic

model data

END

Max. allowable mud pressure

Kennedy

et al. Enter Kennedy

et al. data

data

Delft

Enter Delft

data

Enter queen’s

data

Display Pressure

Plots

Is predict

Pressure Annular

or max. allowable

mud pressure?

Include max.

allowable mud

pressure?

annular

3

Design drill path profile, Determine: no of joints {k1, k2… k5} Total pipe length required etc.

Input drill Path

design data and

Radius 1,Radius 2 Input drill path

design data

5 Input

ProjectInformation

No of

segments?

START

NO Max. allowable mud pressure

YES

YES NO

Fig A.3: The Program Flowchart

Queen’s

Model?

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Fig A.4: showing the „Project Information‟ pane

Fig A.5: „5 segments‟ profile input screen

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Fig A.6: Drill Path Profile, for the example

Fig A.7: Results output for drill path design, for the example

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Fig A.8: Annular Pressure Input, for the example

Fig A.9: Annular Pressure vs. Pipe Length, for the example

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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