Drilling Engineering 2 Course (2nd Ed.)

1. the Survey of a Well

2. Calculating the Survey of a Well

3. Deflection Tools and Techniques

4. Hydraulic Method (Jetting)

5. Mechanical Methods

1. Whipstock Running Procedures

2. Adjustable bent sub above motor

3. Motor housing with one or two bends

4. Offset Stabilizer on Motor

5. While Drilling TechniquesA. Data Transfer

Open Hole Whipstock Running ProceduresThe procedures for running the whipstock

can be summarized as follows:A whipstock is to be selected according to

the wedge needed to effect the desired deflection.

A bit that is small enough to fit in the hole with the chosen whipstock is selected.

The whipstock is attached to the bottom of the drillstring by means of a shear pin.

Having run into the hole, the drillstring is rotated according to the survey information,

until the tool-face of the whipstock is oriented in the desired direction.

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Open Hole Whipstock Running Procedures (Cont.)

By applying enough weight, the chisel point is set firmly into the formation or cement plug

• to prevent the whipstock from rotating.

Additional weight is applied to shear the pin that holds the drill collar to the wedge.

Rotation can then begin.A small diameter pilot hole is drilled

to a depth of about 15 [ft] (4.5m) below the toe of the whipstockat which point the whipstock-stop reaches the top collar

of the whipstock.The pilot hole is then surveyed

to make sure that it has been drilled in the right direction.After the pilot hole has been surveyed,

the bit and the whipstock are tripped out.A hole opener is then run

to ream out the pilot hole to the full size hole.

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Casing Whipstock Running Procedures

The running steps for a casing whipstock can be summarized as:The casing whipstock packer with anchor device

is run to the kick-off point.

The alignment key is oriented using a gyro survey, so that the whipstock will land in a unique position, where the side track is needed.

The casing packer is set to provide a base for the whipstock.

The whipstock is attached to a starting mill by means of a shear pin and run in hole.

The whipstock is landed in the pre-oriented packer by means of a lock-sub (mule-shoe stinger), and thereby oriented in the desired direction.

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Casing Whipstock Running Procedures (Cont.)

Weight is applied to break the shear pin thereby freeing the starting mill off the whipstock.

The string is then rotated to mill the casing to create the window.

Once the window has been cut, the mill is replaced by a smaller sidetracking bit which is forced

by the whipstock through the window outside the casing.

A pilot hole can then be drilled.

After drilling the pilot hole, the bottom hole assembly is pulled out and

replaced by an assembly of string and watermelon mills • to make the window large enough

to accommodate a conventional bottom hole assembly.

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Cons and pros of whipstock technique

The whipstock’s biggest advantage is that it provides a controlled hole curvature at the onset, while distributing the side force

over the length of the whipstock body.

Whipstocks can also be run at any depth in any kind of rock although they are best suited for use in very hard rock

where jetting and mud motor deflecting techniques are generally ineffective.

The main disadvantage of the whipstock is the necessity to drill the pilot hole and then trip out

to change the smaller bit to one of the wellbore diameter.

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Downhole Motor With Bending Device

The most common deflection technique currently in use involves running a downhole motor

which drives the bit without rotating the drill string.

Two different types of downhole motors have been developed, the positive displacement

mud motor and the mud turbine.

To create a change in the trajectory, downhole motors require a

deflection device.

The deflection is provided either by a special sub

placed above the motor, called a bent sub, or

by introducing a deflection at the bottom section or

below the motor. (steerable bottom hole assembly)

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Bent Sub

A bent sub is about two feet long having the axis of the lower pin

connection machined slightly off vertical. The amount of this so called

“offset angle” • varies between 0.5 and 3.0◦.

The direction in which the tool is deflected, called “tool face”,

is marked by a reference line on the outer surface of the sub.

The bent sub itself is connected to a motor below it and to an orienting sub above it.

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Making the Deflection

Once the assembly is run to the bottom of the hole, the bent sub is oriented using the orienting sub and a survey tool. After orientation, mud circulation is started

which initiates the operation of the mud motor and drives the bit without rotating the drill string.

The amount of deflection produced is mainly a function of the offset, the length and stiffness of

the motor, and the hardness of the formation.

Typically, this type of assembly is engaged in drilling until the hole inclination reaches about 20◦. At this point the motor and the bent sub are

pulled out of the hole, and the building rotary assembly is engaged to

complete the building section of the hole.

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Steerable Bottom Hole Assembly

The increased application of downhole motors and turbines as deflection tools has led to the concept of having an adjustable component

with the bottom hole assembly that is capable of altering the well path

without having to pull out of the hole in order to change the bottom hole assembly.

Such a steerable drilling system is comprised of a bit, a steerable motor, MWD tools and stabilizing unit(s).

The three categories of commercially available steerable systems are: adjustable bent sub above the motor, motor housing with one or two bends, and offset stabilizer on motor.

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conventional bent subs vs. multi-angle bent-sub The conventional bent subs with fixed angle

have the disadvantage that they cannot be run in the hole in a straight position coaxial

to the string axis and therefore, cannot be used in rotary drilling.

Thus, the advantage of a down hole adjustable deflection device is that it can be run in the hole coaxially and the required

amount of deflection can be controlled from the surface. This makes directional drilling more efficient and less time consuming.

The multi-angle bent-sub associated with a downhole motor allows for drilling of the complete build up zone and of the constant angle zone with the same bottom hole assembly.

Here the bent sub angle is controlled from the surface.

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The adjustable bent sub

The adjustable bent sub consists of an upper and a lower sub that are connected

by an offset conical swiveling joint.

The axis of the conical joint is tilted with respect to the main axis of the tool.

The lower sub is constructed so that it is able to rotate at an angle that is

slightly offset from the vertical axis.

Initially the tool is made up so that the upper and the lower subs are aligned.

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Bent housing vs. adjustable bent sub

when the deflecting device is placed on the top of a downhole motor, it introduces the deflection

at a distance far enough from the bit to create a considerable bit offset. The amount of

bit offset introduced by the bent subs prohibits rotating of the drill-string.

Under this circumstance drilling proceeds in sliding or orienting mode only.

Building a bent house at the lower end of a positive displacement motor itself

introduces a deflection which is much closer to the bit and

therefore, more effective than what is possible with the bent sub on the top of the motor.

This means that a bent housing will provide a larger turn than a bent sub

of similar size and deflection.

The bent housing motor assembly can be used in steering mode

as well as in rotary mode

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double-tilted universal joint housing

The bit offset in the bent house assembly can be reduced further without affecting

the bit tilt angle by introducing a second tilt

in the opposite direction to the first one. Here the body of the motor

is brought back into a position aligning with the borehole axis.

When the rotary table is engaged while the downhole motor is in the hole, bit offset is negated and the assembly

drills straight ahead

to maintain inclination and direction.

Such a deflecting unit is known as double-tilted universal joint housing (DTU). The DTU joint housing

develops a minimum bit offset to give the navigation drilling system a full steering capability.

A bit angle of 0.25 to 0.78◦is adequate to provide directional control using the DTU.

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straight hole drilling

The rotation of the bending motor housing for straight hole drilling causes a slightly over

gauged hole and that creates a “step” when

the drilling switches from rotary to orient mode or vice versa.

Therefore, the smaller the bit offset, the less the bit will cut with

its side, and the smaller the size of the step.

Cutting with the bit face extends bit life and

optimizes the rate of penetration.

Similarly, by keeping the motor concentric to the hole, rotary drilling

proceeds smoothly without excessive rotational bending to the assembly.

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the step problem

A limitation with all steerable systems is that the stabilizers hang on

to the hole wall at a step, and hence reduce the weight on bit.

Although this step can happen with a conventional rotary assembly, it is more common with steerable systems because the diameter of

the hole drilled in rotary mode is slightly larger than the part drilled in orienting mode.

The magnitude of this step depends on

the formation hardness, stability and the build rate of the system.

To minimize the step problem, the near bit stabilizer

should be under-gauged and should have

shallow nose heel angles.

The step size can be further reduced by minimizing

the build rate of the system.

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The deflection below the turbine

The positive displacement motor can use either a bent sub above the motor or have the housing (bend housing) below it.

The deflection below the turbine is provided by a special stabilizer with an under-gauge blade, known as offset stabilizer, and is located on the turbine near the bit.

The under-gauge blade is considered to be the tool face. It is oriented in the same way as

the bent sub and the bent house.

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Procedure

When the drill-string is not rotated, the turbine drives the bit along a predefined course which is given by the under-gauge blade orientation.

The greater the stabilizer offset (higher under-gauge blade), the greater the rate of build, but the amount of offset is limited.

Once the wellbore is brought back onto the planned trajectory, the drillstring can be rotated.

Rotating the offset stabilizer results in a slightly over-gauge hole.

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While Drilling Techniques

With “while drilling techniques”, the direction of

the wellbore, condition of the drillstring

as well as the formations that have been penetrated can be measured and the measurements

transferred to the rig-site while drilling.

While drilling sensors are typically mounted at the BHA as

close as possible to the bit.

Depending on the drillstring configuration, the distance between

the bit and the measuring devices can be as little as 10 [ft] (3m).In this way,

the measurements taken are somewhat behind the bit and depending on the penetration rate, are recorded with some lag-time.

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While drilling systems

While drilling systems generally consist of a power system,

measuring sensors and

a telemetry system for data transfer.

The power system can be either based on a battery, a turbine or a combination of them.

Batteries have the advantage that no circulation is needed to carry out measurements,

thus while tripping, control logs can be run.

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Measurement While Drilling

The term “measurement while drilling” (MWD) refers to the while drilling measurement of directional parameters (MD, inclination, azimuth) as well as certain drilling parameters like

WOB, downhole torque, temperature, etc.

The sensor to perform these measurements are three orthogonal fluxgate magnetometers and three accelerometers.

The use of gyroscope navigated MWD offers significant benefits over navigation sensors.

They offer greater accuracy and are not susceptible to inference from magnetic fields.

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Operating temperature of MWD

The drilling parameters measured with MWD tools are aimed to increase the drilling efficiency (stick-slip),

be applied to detect abnormal formation pressures or any kind of hole problems.

Most MWD tools can operate at tool-temperatures up to 150 ◦C, some sensor work up to 175 ◦C.

It should be noted that the tool-temperatures are generally about 20 ◦C

less than the formation temperatures, measured by wireline logs which is

caused by the cooling effect of mud circulation.

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Operating pressure and shock load of MWDDownhole pressures create less problems for MWD

tools than downhole temperatures. Most MWD tools are designed

to withstand up to 20,000 [psi] which is rarely encountered.

MWD tools are most sensitive to shock and vibrations. Torsional shock, created by stick-slip

have been found to be able to cause tool failure, lateral shocks which can be magnitudes higher than axial

shocks, can be reduced by the use of jars.

Normally sensors measure MWD shock loads constantly and transmit them to the rick. There the driller can manipulate the drilling parameters

to keep them in acceptable limits.

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Logging While Drilling

The term “logging while drilling” (LWD) refers to the while drilling measurements

of wireline equivalent parameters like resistivity,

porosity,

density and

sonic logs.

When these parameters are known, “geosteering” can be performed

where the trajectory of the well is “re-designed” according to the actual formation’s position and shape.

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Introduction

Since the amount of data measured by while drilling techniques can be large, mostly not all measurements are continuously

transferred to the rig.

Data that are not transferred are commonly stored and retrieved at the following trip.

Several different systems have been developed to transfer the measured data to the surface, the “mud pulse telemetry”

is the by far most often applied on.

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different mud pulse systems

Three different mud pulse systems are commercially available today:Positive pulse system:

creates a momentary flow restriction (higher pressure than the drilling mud volume) in the drillpipe.

It is the most often applied one (since it is easiest to achieve even for extended reach wells.)

Negative pulse system: creates a pressure pulse

lower than that of the mud volume by venting a small amount of high pressure drillstring mud from the drillpipe of the annulus.

Continuous wave system: creates a carrier

frequency that is transmitted through the mud and encoded data using phase shifts of the carrier.

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signal to noise range

When the signals reach the surface, they are retrieved by transducers

that are located on the standpipe and

send to computers at the site for further evaluation.

The data transmitted are overlayed with noise where the mud pumps are the main source.

Other parameters that influence the “signal to noise range” are: what mud type and bit type are in use,

the length of the well and the drilling parameters applied itself.

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1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling Engineering.” Master of Petroleum Engineering. Curtin University of Technology, 2001. Chapter 9