TM6004

Teknik Pemboran Lanjut

DIRECTIONAL DRILLING

Oleh

PRADINI RAHALINTAR

NIM: 22214003

(Program Studi Magister Teknik Perminyakan)

INSTITUT TEKNOLOGI BANDUNG2015

1

CONTENTS

CONTENTS.............................................................................................................2

LIST OF FIGURES.................................................................................................3

Chapter I Directional Drilling..................................................................................4

I.1 Introduction............................................................................................4

I.2 Definitions and Terminology.................................................................4

I.3 Applications of Directional Drilling......................................................6

I.4 Well Types.............................................................................................9

I.5 Devices.................................................................................................12

I.5.1 Current Directional Drilling Technology Limits...................................14

I.6 Wellbore Survey..................................................................................15

I.7 Advancements......................................................................................15

I.8 Directional drilling limitations.............................................................16

I.9 Surveying (including MWD)...............................................................18

I.10 Buoyancy and Drillstring Weight Calculations...................................18

I.11 BHA Design Considerations................................................................19

I.11.1 Pendulum Principle............................................................................19

I.11.2 Fulcrum Principle...............................................................................20

I.11.3 Packed Hole Stabilisation Principle...................................................20

Chapter III Discussions..........................................................................................23

2

LIST OF FIGURES.

Figure 1. Measurement parameters of a directional well.........................................5

Figure 2. Application of Directional Drilling..........................................................7

Figure 3.. Extended Reach Drilling.........................................................................9

Figure 4. Vertical Type Well.................................................................................10

Figure 5. "J" Type Well.........................................................................................10

Figure 6. “S” Type Well........................................................................................11

Figure 7. Horizontal Wells.....................................................................................12

Figure 8. Fulcrum Principle...................................................................................23

3

Chapter I Directional Drilling

I.1 Introduction

Directional drilling is the science of deviating a well bore aling a planned

course to a subsurface target whose location is a given lateral distance,

depth, and direction from the surface. It is the process of directing a

wellbore along some trajectory to a predetermined target. Basically it refers

to drilling in a non-vertical direction. Even “vertical” hole sometimes

require directional drilling techniques.

Controlled directional drilling is a technique for directing a well along a

predetermined course to a bottom hole target located at a certain distance

and direction from a surface location.

There are many reasons for drilling a directional wells, including:

1. Side-tracking existing

wells (because of hole

problems or fish or

reaching new targets)

2. Restricted surface locations

3. To reach multiple targets

4. To reduce number of

offshore platforms

5. Horizontal Drilling

6. To reach thin reservoirs

(using horizontal or

multilateral drilling)

7. Environmental footprint

8. Salt dome drilling (direct

the well away from the salt

dome to avoid casing

collapse problems)

9. Geological requirements

10. To avoid gas or water

coning problems

11. For intersecting fractures

12. For re-entering existing

wells

I.2 Definitions and Terminology

At it has been explained, directional drilling is the methodology for

directing a wellbore along a predetermined trajectory to a target. Vertical

wells are usually defined as wells with an inclination within 5°. Wells with

an inclination greater than 60° are referred to as highly deviated wells.

Wells with a section having an inclination greater than 85° for a significant

distance are called horizontal wells. The following terminology is used:

4

- Azimuth: The angle (°) between the north direction and the plane

containing the vertical line through the wellhead and the vertical line

through the target.

- Build-up rate: The angle from the kick-off point is steadily built up.

This is the build-up phase. The build-up rate (°/30 m) is the rate at

which the angle is built. The build up rate and drop off rate (in degrees

of inclination) are the rates at which the well deviates from the vertical

(usually measured in degrees per 100 ft drilled). The build-up rate is

chosen on the basis of drilling experience in the location and the tools

available, but rates between 1 degree and 3 degree per 100ft.

- Tangent (or Drift) Angle

The tangent angle (or drift angle) is the inclination (in degrees from the

vertical) of the long straight section of the well after the build up

section of the well. This section of the well is termed the tangent section

because it forms a tangent to the arc formed by the build up section of

the well. The tangent angle will generally be between 10 and 60 degrees

since it is difficult to control the trajectory of the well at angles below

10 degrees and it is difficult to run wireline tools into wells at angles of

greater than 60 degrees.

Figure 1. Measurement parameters of a directional well (modified from Gabolde and Nguyen, 1991)

5

- Drop-off point: The depth where the hole angle begins to drop off (i.e.

tending to vertical).

- Displacement: The horizontal distance between the vertical lines

passing through the target and the wellhead.

- Inclination: Angle (°) made by the tangential section of the hole with

the vertical.

- Kick-off point (KOP): The depth at which the well is first deviated from

the vertical. The kick off point is the along hole measured depth at

which a change in inclination of the well is initiated and the well is

orientation in a particular direction (in terms of North, South , East and

West). In general the most distant targets have the shallowest KOPs in

order to reduce the inclination of the tangent section of the well (see

below). It is generally easier to kick off a well the shallow formations

than in deep formations. The kick-off should also be initiated in

formations which are stable and not likely to cause drilling problems,

such as unconsolidated clays.

- Measured depth (MD): Depth (length) of the well along the well path.

I.3 Applications of Directional Drilling

1. Sidetracking

This technique may be employed either to drill around obstructions or to

reposition the bottom of the wellbore for geological reasons. Drilling

around obstructions, such as a lost string of pipe, is usually accomplished

with a blind sidetrack. Oriented sidetrack is required if a certain direction

is critical in locating an anticipated producing formation. It is in fact quite

difficult to control the angle of inclination of any well (vertical or

deviated) and it may be necessary to ‘correct’ the course of the well for

many reasons. For example, it may be necessary in the event of the

drillpipe becoming stuck in the hole to simply drill around the stuckpipe

(or fish), or plug back the well to drill to an alternative target.

6

a). Sidetracking (b). Salt Dome Drilling

Figure 2. Application of Directional Drilling

2. Inaccessible Locations

A well is directionally drilled to reach a producing zone that is otherwise

inaccessible with normal vertical-drilling practices. The location of a

producing formation dictates the remote rig location and directional-well

profile. Applications like this are where “extended-reach” wells are most

commonly drilled. Vertical access to a producing zone is often obstructed

by some obstacle at surface (e.g. river estuary, mountain range, city). In

this case the well may be directionally drilled into the target from a rig site

some distance away from the point vertically above the required point of

entry into the reservoir

3. Salt Dome Drilling

Salt domes (called Diapirs) often form hydrocarbon traps in what were

overlying reservoir rocks. In this form of trap the reservoir is located

directly beneath the flank of the salt dome. To avoid potential drilling

problems in the salt (e.g. severe washouts, moving salt, high pressure

blocks of dolomite) a directional well can be used to drill alongside the

Diapir (not vertically down through it) and then at an angle below the salt

to reach the reservoir.

4. Fault Controlling

If a well is drilled across a fault the casing can be damaged by fault

slippage. The potential for damaging the casing can be minimised by

drilling parallel to a fault and then changing the direction of the well to

cross the fault into the target.

7

5. Relief Well

An uncontrolled (wild) well is intersected near its source. Mud and water

are then pumped into the relief well to kill the wild one. Directional

control is extremely exacting for this type of application. If a blow-out

occurs and the rig is damaged, or destroyed, it may be possible to kill the

“wild” well by drilling another directionally drilled well (relief well) to

intercept or pass to within a few feet of the bottom of the “wild” well. The

“wild” well is killed by circulating high density fluid down the relief well,

into and up the wild well.

6. Platform

Multi-well Platform drilling is widely employed in the North Sea. The

development of these fields is only economically feasible if it is possible

to drill a large number of wells (up to 40 or 60) from one location

(platform). The deviated wells are designed to intercept a reservoir over a

wide aereal extent. Many oilfields (both onshore and offshore) would not

be economically feasible if not for this technique

7. Multilaterals

Directional drilling can also be used to drill multilateral wells.

Multilaterals are additional wells drilled from a parent wellbore.

Multilaterals can be as simple as an open hole sidetrack or it can be more

complicated with a junction that is cased and has pressure isolation and

reentry capabilities. Multilaterals are used where production can be

incrementally increased with less capital costs. Multilaterals can be used

offshore where the number of slots are limited. It is also used to place

additional horizontal wells in a reservoir. A very cost-effective way of

delivering high production rates involves intersecting multiple targets with

a single wellbore. There are certain cases in which the attitudes (bed dips)

of the producing formations are such that the most economical approach is

a directional well for a multiple completion. This is also applicable to

multiple production zones adjacent to a fault plane or beneath a salt dome.

8

8. Extended Reach drilling

Another application of directional drilling is what is commonly termed

extended reach drilling. As illustrated in Figure below, extended reach

drilling is where wells have high inclinations and large horizontal

displacements for the true vertical depth drilled. Extended reach drilling is

used to develop reservoirs with fewer platforms or smaller sections of a

reservoir where an additional platform cannot be economically justified.

Extended reach drilling will become more popular as the cost of platforms

in deeper water and severe environments becomes more expensive.

Figure 3.. Extended Reach Drilling

Advances in technology have allowed operators to drill extended reach

wells with very high HD/TVD ratios (the ratio of the horizontal

displacement to true vertical depth). Wells have been drilled with

HD/TVD ratios in excess of 6/1. In these wells the horizontal departure

was more than six times the true vertical depth with the total measured

depth exceeding 32,800 feet (10,000 m).

I.4 Well Types

1. Vertical

This type of wells are only made up of a vertical section.

9

Figure 4. Vertical Type Well

2. Slant (“J”)

Type Slant wells are made up of a vertical section, a deep kick off and a

build up to target. They are also called Deep Kick off wells or J Profile

wells (as they are J - shaped). They are similar to the Type S well except

the kickoff point is at a deeper depth. The well is deflected at the kickoff

point, and inclination is continually built through the target interval (Build

component). The inclinations are usually high and the horizontal departure

low. This type of well is generally used for multiple sand zones, fault

drilling, salt dome drilling, and stratigraphic tests. It is not used very often.

Figure 5. "J" Type Well

3. “S”

Type S wells are made up of a vertical section, a kick- off point, a build-up

section, a tangent section, a drop-off section and a hold section up to

10

target. They are also called S Profile Wells (as they are S - shaped). Like

Type “J” Wells, the Type “S” wells are drilled vertically from the surface

to the kick-off point at a relatively shallow depth. From the kick off point,

the well is steadily and smoothly deflected until a maximum angle and the

desired direction are achieved (Build component). The angle and direction

are maintained until a specified depth and horizontal departure has been

reached (Tangent Component). Then, the angle is steadily and smoothly

dropped (DROP) until the well is near vertical. Finally the angle and

direction is maintained till we reach the target depth.

Figure 6. “S” Type Well

A disadvantage of the Type “S” is that it will generate more torque and

drag for the same horizontal departure. Usually this method is employed to

hit multiple targets or to avoid faulted region or to minimize the

inclination in the zone which will be fractured during completion or for

sidetracking.

4. Horizontal

Horizontal wells are wells where the reservoir section is drilled at a high

angle, typically with a trajectory to keep the well within a specific

reservoir interval or hydrocarbon zone. In a strict sense, these wells are

rarely perfectly horizontal, but they tend to be near horizontal mostly,

generally at an angle greater than 80° from vertical.

11

Figure 7. Horizontal Wells

Horizontal wells are drilled in a specific configuration. The tangent section

of the well is drilled along a deviated well path to just above the reservoir

section, to what is known as the kick off point. From the kick off point, the

well is drilled at an increasingly higher angle, arcing around toward an

angle close to horizontal. The point at which the well enters (or lands on)

the reservoir is called the entry point. From there on, the well continues at

a near-horizontal orientation with the intention of keeping it substantially

within the reservoir target until the desired length of horizontal penetration

is reached

I.5 Devices

The techniques began with the use of devices such as:

1. Whipstock

The whipstock is a steel wedge, which is run in the hole and set at the

KOP. This equipment is generally used in cased hole when performing

a sidetracking operation for recompletion of an existing well. The

purpose of the wedge is to apply a sideforce and deflect the bit in the

required direction. The whipstock is run in hole to the point at which

the sidetrack is to be initiated and then a series of mills (used to cut

through the casing) are used to make a hole in the casing and initiate

the sidetrack. When the hole in the casing has been created a drilling

string is run in hole and the deviated portion of the well is commenced.

12

2. Jetting

Jetting bits can be used to change the trajectory of a borehole, with the

hydraulic energy of the drilling fluid used to erode a pocket out of the

bottom of the borehole. The tricone bit with one large nozzle is

oriented to the desired hole direction to create a pocket. The drilling

assembly is forced into the jetted pocket for a short distance. This

procedure continues until the desired trajectory change is achieved.

Jetting is seldom used today because of its slow penetration rate and its

limitations in soft formations.

3. Motor and bent sub

The most commonly used technique for changing the trajectory of the

wellbore uses a piece of equipment known as a “bent sub”. and a

Positive Displacement (mud) motor. A bent sub is a short length of

pipe with a diameter which is approximately the same as the

drillcollars and with threaded connections on either end. It is

manufactured in such a way that the axis of the lower connection is

slightly offset (less than 3 degrees) from the axis of the upper

connection. When made up into the BHA it introduces a “tilt angle” to

the elements of the BHA below it and therefore to the axis of the

drillbit. However, the introduction of a bent sub into the BHA means

that the centre of the bit is also offset from the centre line of the

drillstring above the bent sub and it is not possible therefore to rotate

the drillbit by rotating the drillstring from surface. Even if this were

possible, the effect of the tilt angle would of course be eliminated since

there would be no preferential direction for the bit to drill in.

4. BHA to control inclination in tangent section

Adjustable-gauge stabilizers (AGS) are used to control inclination in

tangent section. Running AGSs with the steerable motor assemblies

makes it possible to control inclination with the stabilizer while

drilling in the rotary mode. If the wellbore requires a change in

azimuth, one would have to revert to a sliding mode.

13

5. Wireline steering tool to orient and survey

Although technically wireline steering tools are also MWD systems,

the term MWD is commonly used in the industry to mean systems with

non-wireline data transmission systems. The disadvantage of the mud

pulse MWD systems is their relatively slow data rate and hence update

of the downhole measurements. In a number of applications, e.g. with

deep kick-offs or high torque motors, the MWD data rate is

insufficient for obtaining a consistent orientation, and a wireline

system has to be used.

Wireline steering tools can only be used when the drillstring is not

being rotated. Wireline steering tools are applied when:

• kicking-off;

• side-tracking;

• making correction runs.

Wireline steering tools save substantial rig time and are cost effective

in comparison with single-shot surveys, despite higher direct survey

cost.

6. Rotary steerable systems (RSS)

The RSS is an evolution in directional-drilling technology that

overcomes the drawbacks in steerable motors and in conventional

rotary assemblies. To initiate a change in the wellbore trajectory with

steerable motors, the drilling rotation is halted in such a position that

the bend in the motor points in the direction of the new trajectory. In

extreme extended reach drilling (ERD), the frictional force builds to

the point at which no axial weight is available to overcome the drag of

the drillstring against the wellbore, and, thus, further drilling is not

possible.

I.5.1 Current Directional Drilling Technology Limits

1. L Longest horizontal section (>86°) in excess of 26,700 feet (8150

mtr)

2. Motor run in excess of 610 hours

14

3. Longest Extended Reach well 29,796 feet (9082m) measured depth.

Lateral reach 24,911 feet (7593m). (SPE 98945-MS)

4. Shortest measured length from vertical to horizontal 35 feet TVD.

(SPE 35244-PA)

I.6 Wellbore Survey

Surveys are required to:

1. Satisfy regulatory agencies

2. Stay within lease boundaries or limits

3. Construct accurate subsurface maps

4. Determine location and control wellbore path

5. Reach a target by steering

I.7 Advancements

Current technologies are:

1. Steerable mud motors

This steerable can drill directionally or straight ahead, as required. This

enables the driller to control the well’s trajectory without making time

consuming trips to change bottomhole assemblies. To steer the hole during

kickoffs or course corrections the system is oriented using MWD readings

so the bit will drill in the direction of the navigation sub’s offset angle.

When drilling in this way the system is said to be drilling in the oriented or

sliding (since the drillstring is not rotating) mode. The bit is driven by the

downhole motor, and the rotary table is locked in place, as it is when

conventional motor drilling. As mentioned previously, the system’s two

stabilizers and bit serve as the tangency points that define the curve to be

drilled by the oriented assembly. The dogleg rate produced can be

controlled by varying the placement and size of the stabilizers, by using a

DTU with a different offset angle, or by alternating drilling with oriented

and rotary intervals.

2. MWD

MWD or Measurement tools provide real-time or immediate recording and

transmission to the surface of downhole data related to bit operating

conditions and directional information. Advantage of MWD tools over

15

other methods of acquiring similar data is time, more frequent

measurement, and reduction in risk of pipe sticking while the drillstring is

motionless

A variety of MWD services are available - the most common is the

steering tool application, which provides a continuous or near continuous

reading of drift angle, azimuth and tool face for directional drilling.

3. LWD

LWD (Logging-While-Drilling) is similar to MWD in that it is designed to

provide a real-time or immediate recording and sometimes transmission to

the surface of downhole formation evaluation data

A variety of LWD services are available - the most common is the

resistivity application, which provides qualitative formation evaluation

information about the formation penetrated. Sonic, gamma ray, neutron

density, caliper, annular pressure and other parameters may be measured

4. Rotary steerable systems

A Rotary Steerable System (RSS) is a combination of motor and stabilizer

arrangement that allows surface rotation of the drill pipe while keeping the

bit orientation fixed. May use specialized stabilizer (push to steer). May

use specialized bit directing technology (point to steer)

These tools, in combination, can be used to direct or redirect well profiles without

changing the BHA and have enabled the drilling of extended reach, horizontal and

multilateral wells.

I.8 Directional drilling limitations

1. Torque and Drag

Drag is the force difference between free rotating weight and the force

required to move the pipe up or down in the hole. Pick-up drag force is

usually higher than free rotating weight. While slack-off drag force is

usually lower than free rotating weight. Drag force is used to overcome the

axial friction in the well. Torque or moment is generally a force multiplied

with a lever arm. Torque is the moment required to rotate the pipe. The

moment is used to overcome the rotational friction in the well and on the

16

bit. Torque is lost from the rotating string so that less torque is available at

the bit for destroying rock. High drag forces and high torque normally

occur together. In a perfect vertical well the torque loss would be zero,

except for a small loss due to viscous force from the mud. In a deviated

well the torque loss could be great, especially in long complex or extended

reach well, where torque loss is a major limiting factor to how long we can

drill, as it eventually will overcome the rig or drill strings limitation

2. Hole cleaning

Hole cleaning during drilling of directional wells is a major concern, that

should be monitored and controlled. Cuttings accumulations may cause

costly problems such as stuck pipe and excessive torque and drag. Cuttings

transport in a wellbore depends largely on the inclination, annular flow

velocity, viscosity and rotation of the pipe. Generally high rotational speed

at above 120 RPM increases hole cleaning, small annuluses have the best

effect of rotation. Annular flow velocity and thus flow regime depends on

hole size, drill string size and pump rates. Generally hole cleaning

increases with annular velocity, up to a certain maximum where the

benefit diminishes. Viscosity of the mud is important as a too high

viscosity would lead to poor hole cleaning in a horizontal section since the

low velocity area would be larger, and pump pressure and ECD would also

increase. A too low viscosity would decrease the distance the fluid can

carry a particle and reduce the viscous coupling that agitates the cuttings

and thereby decreasing the hole cleaning. An increase in rate of

penetration ROP increases the hole cleaning requirement. The way

cuttings behave in different inclination ranges are as follows (Nazari and

Hareland 2010).

3. Depth with heat for motors and MWD/LWD equipment

4. Not enough near bit technology available

The limitations of directional drilling are primarily dependent upon

maximum hole angle, rate of angle change, and torque or friction

considerations. In directional drilling, it is now common for the horizontal

displacement of the bottom hole location to be twice the total vertical

17

depth (TVD) of the well. However, in a shallower well, such as one in

which a potential target is two miles away from the drill site but only one

mile deep, directional drilling would be much more difficult, risky, and

costly (Schmidt 1994)

5. Air drilling with motors and MWD/LWD

6. Steering in less than 5ft TVD thick producing zones

I.9 Surveying (including MWD)

The standard surveying technique of today is to use downhole measurement while

drilling MWD directional sensor tools, which measure the direction of the earth

gravity by using 3 orthogonally mounted sensitive accelerometers. Hole

inclination is found by doing simple trigonometry to measured values. The

azimuth direction is measured likewise with 3 orthogonally mounted

magnetometers, which measures earth’s magnetic field. The measured magnetic

direction must be corrected for the magnetic fields declination angle and grid

convergence in order to achieve the true north direction.

The MWD tool transmitted the survey reading to surface through the mud stream

in the drill pipe. The drilling process was stopped for few minutes and survey

readings were obtained in pump off condition. This saved times to greater extent

compared to wireline logging. MWD was considered a better option for survey

data transmission compared to wireline procedure.

Initially the system delivered three basic information: Inclination, Azimuth and

Toolface. These three parameters helped the directional driller to position the well

correctly to the desired target. MWD tools can also provide information about the

conditions at the drill bit. This may include:

- Rotational speed of the drillstring

- Smoothness of that rotation

- Type and severity of any vibration downhole

- Downhole temperature

- Torque and Weight on Bit, measured near the drill bit

- Mud flow volume

18

I.10 Buoyancy and Drillstring Weight Calculations

The weight of a drill string in a well filled with mud is the weight in air minus the

weight of mud that the steel in the string displaces, this is also known as the

Archimedes principle. For convenience we can define a buoyancy factor as:

Buoyancy factor multiplied with weight in air gives the weight of a pipe

immersed I mud. If there is a density difference between the fluid in the inside and

the outside of the pipe, like during tripping in, during displacing to a different

mud weight and cementing. Then the buoyancy factor becomes:

Subscript o means outside the pipe and subscript i means inside the pipe. If the

fluid density inside and outside the pipe is equal the buoyancy factor equation

becomes like first equation. A heavy mud will decrease the effective weight of the

drill string, and thus decrease side force and the loads from friction and torque.

However a heavy mud has more weighing particles which could lead to less

lubricity and therefore higher friction.

I.11 BHA Design Considerations

The bottom hole assembly refers to the drillcollars, HWDP, stabilisers and other

accessorie used in the drillstring. All wells whether vertical or deviated require

careful design of th bottom hole assembly (BHA) to control the direction of the

well in order to achieve th target objectives. Stabilisers and drillcollars are the

main components used to control hol inclination.

There are three ways in which the BHA may be used for directional control:

I.11.1 Pendulum Principle

The pendulum technique is used to drop angle especially on high

angle wells where it is usually very easy to drop angle. The pendulum

technique relies on the principle that the force of gravity can be used to

deflect the hole back to vertical. The force of gravity is related to the

19

length of drillcollars between the drill bit and the first point of tangency

between the drillcollars and hole. This length is called the active length of

drillcollars and can be resolved into two forces: one perpendicular to the

axis of the wellbore and is called the side force and one acts along the

hole.

Increasing the active length of drillcollars causes the side force to

increase more rapidly then the along hole component. The side force is the

force that brings about the deflection of the hole back to the vertical. Some

pendulum assemblies may also use an under gauge near-bit stabilizer to

moderate the drop rate.

High WOB’s used with a pendulum assembly may bend the BHA

and cause the hole angle to build instead of drop. Also pendulum

assemblies have a tendency to walk to the right depending on the type of

bit used and since they are flexible they will follow the natural walk of the

drill bit.

I.11.2 Fulcrum Principle

This is used to build angle (or increase hole inclination) by

utilising a near bit stabiliser to act as a pivot or a fulcrum of a lever. The

lever is the length of the drillcollars from their point of contact with the

low side of the hole and top of the stabiliser. The drillbit is pressed to the

high side of the hole causing angle to be built as drilling ahead progresses.

Since the drillcollars bend more as more WOB is applied, the rate of angle

build will also increase with WOB.

The build rate also increases with:

• Distance from near bit stabiliser to first stabiliser in the BHA

• Reduction in RPM

• Increase in hole angle

• Reduction in drillcollar diameter

20

I.11.3 Packed Hole Stabilisation Principle

This is used to hold or maintain hole inclination and direction and

are typically used to drill the tangent section of a well. The packed BHA

relies on the principle that two points will contact and follow a sharp

curve, while three points will follow a straight line. Packed BHA have

several full gauge stabilizers in the lowest portion of the BHA, typically

three or four stabilisers. This makes the BHA stiff and hence it tends to

maintain hole angle and direction.

By using those principles of BHA control discussed above, there are five basic

types of BHA’s which may be used to control the direction of the well.

1. Pendulum Assembly

The pendulum assembly makes use of the gravitational effects acting on the

bit and lower portion of the BHA to maintain vertical hole or drop angle back

to the vertical. In this assembly, the first string stabiliser is placed

approximately 30, 40 or 60 feet above the bit. The assembly is commonly

used as an angle reducing assembly on deviated wells but is difficult to

control.

2. Packed Bottom Hole Assembly

A packed assembly typically uses a near-bit stabiliser and string stabilisers a

further 30 and 60 feet from the bit. A tightly packed assembly incorporates a

further string stabiliser normally located 15 feet from the bit. This type of

assembly is often run where formation dip cause angle building tendency and

is also used to maintain vertical hole when higher weights (WOB) are used.

This BHA is typically used in 12¼" and 8½" hole sections on vertical well and

in tangent sections of deviated wells to maintain the hole inclination.

3. Rotary build assembly

A rotary build assembly is based on the fulcrum principle and is used to build

hole angle after initial steering runs on deviated wells. Rotary build assemblies

are usually used after the initial kick-off to eliminate the need for further use

of a mud motor.

21

The BHA consists of: near bit stabiliser, two drillcollars, a first string

stabiliser located a further 60 feet from the bit, DC and a further string

stabiliser 30 feet above. During drilling operations, application of WOB

causes the two drillcollars above the near bit stabiliser to be bent and

consequently cause the drillbit to loaded on the high side of the hole thereby

causing increases in hole angle as the hole is drilled.

4. Steerable assembly

Steerable assemblies include the use of the following:

• Bent motor housing tool and MWD tool

• Double tilted U-joint housing (DTU) and MWD tool

The above BHA’s are run stabilised and can be used to drill the build and

tangent sections of a hole. When used in steering mode, a steerable system can

be used to correct both hole angle and direction. In rotary mode, a steerable

system is used to maintain hole direction.

5. Mud motor and bent sub assembly

This assembly is typically run for performing the initial kick-off and build up

sections of deviated wells. It is then pulled prior to running a packed BHA for

drilling the tangent sections. This BHA may also be used for correction runs

22

Chapter III Discussions

1. Effect of moment of inertia on directional drilling

The moment of inertia of a system about some rotational point is the measure

of an object's resistance to a change in the object's angular acceleration due to

the action of a torque. The moment of inertia depends upon how an object's

mass is distributed relative to it pivot point. Torque is the moment required to

rotate the pipe. The moment inertia is used to overcome the rotational friction

in the well and on the bit. Torque is lost from the rotating string so that less

torque is available at the bit for destroying rock. In a perfect vertical well the

torque loss would be zero, except for a small loss due to viscous force from

the mud.

2. Fulcrum effect

When a stabilizer is run below the point of tangency, it has a fulcrum effect,

which causes the hole to pick up angle. The angle formed by this point of

contact and the clearance between the drill collars and wellbore approximate

the change in deflection that would accompany the next increment of drilled

hole.

Figure 8. Fulcrum Principle

23

3. Non-magnetic drillcollar

The readings from a magnetic compass will be incorrect if the compass is

close to a magnetised piece of steel. Since both the drillstring and casing will

be magnetised, as they are run through the earths magnetic field, the magnetic

surveying tools cannot be used unless some measure is taken to ensure that the

well direction according to the earths magnetic field is accurately recorded on

the compass. In the case of the drillstring this is done by using non-magnetic

drillcollars in the BHA. These collars are made from Monel and the Earths

magnetic field is undisturbed by their presence

Magnetic surveys suffer from the following sources of error:

• Drillstring magnetisation

• Magnetic effects from casing

strings or BHA

• Geological structures

containing magnetic materials

• Magnetic storm effects

• Wireline magnetization

• Magnetic declination

• Tool misalignment

• Depth measurement

Drill string magnetisation causes the largest errors in magnetic surveys. These

errors can be reduced by housing the survey instrument in non-magnetic

drillcollars (e.g. K-Monel).

4. Whipstiock

The whipstock is widely used as a deflecting medium for drilling multilateral

wells. It consists of a long inverted steel wedge (shute) which is concave on

one side to hold and guide a deflecting drilling or milling assembly. It is also

provided with a chisel point at the bottom to prevent the tool from turning, and

a heavy collar at the top to withdraw the tool from the hole. There are two

main types of Whipstocks:

• The standard removable Whipstock which is used to kick off

wells and for sidetracking. The Whipstock is used with a

drilling assembly consisting of a bit, a spiral stabilizer, and an

orientation sub, rigidly attached to the Whipstock by means of

a shear pin. To deflect the well, the whipstock and kick off

assembly is run in hole and oriented in the required direction.

24

Weight is then applied to shear the pin and allow the drilling bit

to slide down the shute and drill in the set direction.

• The Permanent Casing Whipstock is designed to remain permanently

in the well.

• Thru tubing whipstock

5. Directional Survey

The method used to obtain the measurements needed to calculate and plot the

3D well path is called directional survey. Three parmeters are measured at

multiple locations along the well path—MD, inclination, and hole direction.

MD is the actual depth of the hole drilled to any point along the wellbore or to

total depth, as measured from the surface location. Inclination is the angle,

measured in degrees, by which the wellbore or survey-instrument axis varies

from a true vertical line. An inclination of 0° would be true vertical, and an

inclination of 90° would be horizontal. Hole direction is the angle, measured

in degrees, of the horizontal component of the borehole or survey-instrument

axis from a known north reference. This reference is true north, magnetic

north, or grid north, and is measured clockwise by convention. Hole direction

is measured in degrees and is expressed in either azimuth (0 to 360°) or

quadrant (NE, SE, SW, NW) form.

Each recording of MD, inclination, and hole direction is taken at a survey

station, and many survey stations are obtained along the well path. The

measurements are used together to calculate the 3D coordinates, which can

then be presented as a table of numbers called a survey report. Surveying can

be performed while drilling occurs or after it has been completed.

The purposes of directional survey are to:

• Determine the exact bottomhole location to monitor reservoir

performance.

• Monitor the actual well path to ensure the target will be reached.

• Orient deflection tools for navigating well paths.

• Ensure that the well does not intersect nearby wells.

25

• Calculate the TVD of the various formations to allow geological

mapping.

• Evaluate the DLS, which is the total angular inclination and azimuth in

the wellbore, calculated over a standard length (100 ft or 30 m).

6. Geosteering

The term “Geosteering” is often used when the steerable system is used to

drill a directional well. Geosteering in general is drilling a horizontal wellbore

that ideally is located within or near preferred rock layers. As interpretive

analysis performed while drilling or after drilling, geosteering determines and

communicates a wellbore's stratigraphic depth location in part by estimating

local geometric bedding structure. Early geosteering was performed mostly

with interpretation from cuttings samples, paper well logs and maps, and

rough sketches. Modern geosteering normally incorporates multiple

dimensions of information, including insight from quantitative correlation

methods. Ultimately, today's geosteering methods provide explicit

approximation to the location of nearby geologic beds in relation to a wellbore

or coordinate system, and as such, help to explain rock/wellbore completion

and subsequent oil/gas/water/frac fluid-flow observations from or into rock.

26

REFERENCES

Bourgoyne, Adam T. et.al., 1986, Applied Drilling Engineering, Society of

Petroleum Engineers, Richardson TX, United State of America.

Heriot-Watt University, Drilling Engineering Handbook.

Rabia, Hussain, Well Engineering & Construction.

27