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Steerable motors consist of a positive displacement motor (PDM) with a surface adjustable bend housing which allows the bit to be oriented in the desired drilling direction. The system is comprised of six key elements, as shown in Figure 3-15:

• Top Sub - acts as a crossover from the power section to the drill string.

• Power Section - has a rotor and stator which convert mud flow into bit rotation.

• Transmission Section - transmits the rotation from the power section to the drive shaft.

• Surface Adjustable Bent Housing - allows the setting of the bend in the motor housing.

• Bearing Section - bearings which support all the axial and radial loads on the drive shaft.

• Drive Shaft - driven by the power section via the transmission section, it has the drill bit screwed into a bit box at the bottom of the motor.

Top Sub Top Sub

Power Section Power Section

Transmission Assembly Transmission Assembly

Surface Adjustable Bend Housing (SABH) Surface Adjustable Bend Housing (SABH) Bearing Section

Bearing Section Drive Shaft Drive Shaft

Figure 3-15: Steerable motors consist of six key elements.

Figure 3-16: Positive

Displacement Motors (PDM) convert hydraulic power from the mud flow into mechanical power in the form of bit rotation.

The power section, also known as a positive displacement motor, converts hydraulic power from the mud circulation into mechanical power in the form of bit rotation.

This is achieved through a progressing cavity design in which the movement of the mud pushes on a rotor with one less lobe than the number of cavities in the stator in which it is housed.

Cavities

Lobes

Rotor

Stator

Figure 3-17: Cross-section of a motor looking down the length of the motor (left). The rotor has one less lobe than the corresponding stator has cavities, leaving a gap for mud

circulation to push on the spiraled rotor and hence create rotation, as shown in the cut-away section of a motor (right).

The movement of mud from one cavity to the next turns the rotor and thus the bit that is coupled to the rotor through the connecting rod and drive shaft. Note that the key feature of a positive displacement motor is that the bit rotates (when there is mud circulation) even if the drillstring is stationary.

Increasing the number of lobes and cavities increases the torque available but decreases the speed of bit rotation. Consider a unit volume of mud flowing through a motor. If the motor has a 1:2 configuration the rotor progresses 360/2 = 180 degrees. If the motor has a 7:8 configuration then the rotor progresses 360/8 = 45 degrees for the same volume of mud. Hence for a given mud flow rate the RPM of a motor is mainly controlled by the rotor-stator configuration.

Similar to a gearbox in a car, higher gearing ratios give lower RPM and higher torque. As shown in the lower panel of Figure 3-18, torque increases and the RPM decrease with higher motor configurations.

1:2 3:4 4:5 7:8 Rotors

and matching

Stators

Figure 3-18: Increasing the number of lobes and cavities increases the torque available with a corresponding decreases in the bit RPM.

The rotor has a highly polished surface to create a seal with a rubberized elastomer insert that forms the internal profile of the stator. The elastomer is housed in the stator tube or ‘can’. The dimensions of the rotor and stator must be carefully matched to ensure a good seal while also preventing excessive interference between the rotor and stator (Figure 3-19).

Figure 3-19: Correct fitting of the rotor to the stator is vital to motor performance. Negative interference results in loss of power, excessive positive interference results in rapid wear and heat generation.

Pressure differential across two adjacent cavities forces the rotor to turn – opening adjacent cavities and allowing the fluid to progress down the length of the stator as shown in Figure 3-20.

FLOW

STATOR CONTOUR ENTRANCE

REGION (CUTBACK)

STATOR TUBE ELASTOMER

INSERT

ROTOR

CAVITIES

Figure 3-20: A cross-section along the length of the motor shows how the spiraled rotor seals on an elastomer inside the stator tube so that the mud flow must turn the rotor to gain access to the next cavity.

The stage length is defined as the axial length required for one lobe to rotate 360° along its helical path, as shown in Figure 3-21. Stage length is also referred to as the pitch.

Figure 3-21: The stage length is the axial length required for a lobe to create one 360°

spiral on the rotor.

The projected surface areas of the rotor in the radial and axial directions influence the amounts of torque and axial thrust that a power section will create. The longer the stage length, the greater the projected surface area in the radial direction, the larger the force applied to turning the rotor.

Generally, a longer stage length (pitch) is more desirable because it affords more torque and less axial thrust in the power section.

For a given lobe configuration, longer pitch results in lower rotational speed. This characteristic is used in the High Flow motors which have long stage lengths to give long power sections with relatively few stages. This allows higher flow rates which generate lower RPM.

The hydraulic power extracted from the mud flow by the motor is given by:

• Hydraulic Power (Hp) = Flow rate (GPM) x Pressure drop (psi) / 1714 The mechanical power output by a motor to the bit for drilling is given by:

• Mechanical Power (Hp) = RPM x Torque (ft.lbs) / 5252

The mud flow rate is controlled at surface by adjusting the rig mud pump output.

The pressure drop is controlled by the number of stages.

The RPM and torque are controlled by the configuration of the motor. The greater the number of lobes and cavities the higher the torque and the lower the RPM.

The surface adjustable bent housing allows the angle between the bit shaft and the axis of the motor to be changed, generally in the range from zero and 3 degrees, as outlined in Figure 3-22.

This adjustment is performed only on surface where the stator housing can be unscrewed from the adjustment ring (1 and 2). The ring is then lifted so that the alignment teeth disengage and the ring can then be rotated to give the required bent housing angle (3). The adjustment ring is then slotted into position (4) and the stator adaptor screwed back into place to lock the ring (5).

Figure 3-22: The surface adjustable bent housing allows the angle between the bit shaft and the axis of the motor to be changed.

The greater the bent housing angle the greater the dogleg that can be achieved with the motor.

However, high bent housing angles also cause significant stresses in the housing when rotary drilling so the bend is kept as small as possible.

Slide-Rotate Sequences

By orienting the surface adjustable bent housing in the desired drilling direction and rotating the bit by pumping mud through the positive displacement motor it is possible to drill in a desired direction. The process of keeping the drillstring oriented in a desired direction while drilling is called “sliding”. The orientation, or tool face, is referenced to the high side of the hole in deviated and horizontal wells. A tool face of 0° indicates b uilding angle, while a tool face of 90° indicates drilling right. A tool face of 180° indicates drop ping well angle and a tool face of 270° indicates drilling left. In near vertical holes the tool face is oriented relative to North. In this case it is called a Magnetic Tool Face (MTF) to distinguish it from the Gravity Tool Face (GTF) used in high angle wells.

Because the angle on the bent housing is small (typically less than 3°) the steerable motor can also be rotated. This negates the effect of the bend and gives a relatively straight borehole, which is slightly over-gauged (slightly larger than bit size). By alternating sliding and rotating intervals the directional driller can control the rate at which the borehole angle is changed. The rate of change in borehole angle is normally given in degrees per 100ft (or 30 m.), the so-called dog leg severity (DLS).

For example, if a motor gives 5°/100 ft. when slidi ng and holds angle while rotating, then to achieve a dogleg of 2°/100 ft, the driller will nee d to slide for 2/5 = 40% of the interval and rotate for the remaining 60%. If the interval to be drilled with a 2°/100 ft dogleg is 60 ft, then the drille r will slide for 40% x 60 ft = 24 ft. and rotate for the remaining 36 ft. Depending on the intervals involved the driller may chose to break the slide-rotate sequence in to shorter intervals of sliding interlaced with shorter intervals of rotating so the overall change in well angle is the same but distributed more evenly along the 60 ft than if all the sliding were performed at the beginning.

Figure 3-23 shows a directional performances plot for a directional motor over numerous slide – rotate sequences. The darker background shading indicates slide sequences. The gravity tool face (pink dots) is visible during the slide sections but is not seen during the rotate sections as tool face is meaningless when the bent housing is rotating. The continuous azimuth (green dots) overlay the static survey azimuth (yellow squares connected by dark green lines). The static survey inclination (yellow squares connected by blue lines) have been taken every 90 ft and indicate a relatively smooth increase in inclination up to about 59 degrees after which the inclination is held stable. The continuous inclination (red dots) shows the well to be far more tortuous. During the slide sections the driller is building inclination, but the BHA drops inclination when rotating so the slide-rotate sequence results in an undulating borehole. The difference between the static surveys and the continuous inclination and azimuth data is not unusual. The averaging caused by the long distance between static survey stations results in a calculated trajectory that seems smoother than is really the case.

Figure 3-23: Directional performance plot for a directional motor over numerous slide-rotate sequences. The oscillations in inclination (red dots) caused by the slide-slide-rotate sequence results in an undulating well trajectory.

A major difficulty with slide-rotate sequences is that orienting prior to each slide section is time consuming, and hence reduces the overall rate of penetration achieved for the well. In addition, because the mud is not agitated during sliding hole cleaning efficiency is reduced. Finally, the overall length of the well may be limited because static friction during sliding, which is greater than dynamic friction while rotating, may prevent effective weight transfer to the bit. To overcome these limitations, rotary steerable systems were developed.