Rotary Steerable Systems

Rotary steerable systems (RSS) deliver continuous steering while rotating. Compared to drilling with a motor this gives the benefits of:

• steadier deviation control (continuous rather than slide-rotate steering)

• a smoother in-gauge hole (no bent housing to rotate causing hole enlargement)

• better hole cleaning (cuttings are agitated into the mud flow by the rotation)

• extended hole reach (reduced drag as rotating friction is less than static friction)

• overall improvement in the rate of penetration (eliminated time spent orienting)

• continuous rotation of the BHA ensures that azimuthal formation data is acquired over the entire length of the well, which is particularly advantageous for well placement.

There are two main types of RSS:

• Push-the-bit: applies side force to increase the side-cutting action of the bit.

• Point-the-bit: introduces an offset to the drilling trajectory similar to a bent housing but allowing continuous rotation.

A push-the-bit system uses pads on a bias unit to push against the borehole wall and so push the bit in the opposite direction.

Flex Stabilizer Control Unit Bias Unit

Activated Pad

Figure 3-24: A push-the-bit system uses three pads to push against the borehole wall and so deflect the well in the opposite direction.

As the system is rotating, the pads must be activated in sequence to ensure consistent steering in the desired direction. The control unit contains the electronics for control of the tool face and the percentage of time spent steering. The system operates by diverting a small percentage of the mudflow to activate the pads. By sensing the rotation of the BHA relative to the earth’s magnetic field and controlling an electric motor to rotate in the opposite direction, the control unit holds a control valve (blue element in Figure 3-25) geostationary. The port in the control valve is oriented opposite the desired steering direction (tool face). Each of the three holes in the orange element in Figure 3-25 guides mud behind one of the three pads. A small proportion of the mudflow is thus diverted behind each of the pads in sequence as the entry port to the piston behind each pad rotates in front of the geostationary port in the control valve. This causes the pads to open in sequence as they rotate into the “pushing” sector and apply force to the borehole wall opposite the desired steering direction. The pad opens a fraction of an inch, as shown in Figure 3-26 as the diameter of the bias unit is only slightly smaller that the borehole diameter. The drill bit, which must have cutting elements on the side as well at the front (so called side-cutting action), then preferentially cuts the rock on the side opposite the activated pad resulting in a change in the well trajectory.

Figure 3-25: The spindle (blue) is held geostationary by the control unit, thereby diverting a small proportion of the mudflow behind each of the pads in sequence as their respective entry ports rotate in front of the hole in the control valve.

Pad out

Pad out Pad out Pad in Pad in

Pad out Pad in Pad in

Limited travel distance No sharp edges

Figure 3-26: The diameter of the bias unit at the pads is only slightly smaller than bit size so the pads do not have to travel far before contacting and applying force to the borehole wall. Pad travel is limited to approximately ¾”.

Due to the high power requirements of the control motor, the system has its own power generation capabilities through a mud turbine and alternator assembly. In addition to supplying power to the motor to counter-rotate the control valve against the rotation of the collar and so keep it geostationary, the turbo-alternator system also supplies power to the electronics.

Steering is controlled by commanding the tool to spend a certain percentage of the time steering (duty cycle) in a specific direction (tool face) and the remaining time in neutral. While in neutral the BHA may drill ahead straight if it is a hold assembly, but it could have build or drop tendencies depending on the configuration of the three touch points. The driller needs to take the BHA tendency into account when downlinking commands to the tool.

Downlinking is performed by adjusting the mud pumps at surface to deliver a sequence of mud flow rate changes. The tool detects these as changes in downhole turbine RPM. Each point on the command diagram, an example of which is shown in Figure 3-27, can be selected with a specific sequence of flow rate changes. Angles clockwise from the positive y-axis correspond to gravity tool face (GTF). Distance from the center corresponds to proportion of time spent steering.

For example, if the driller wants to build inclination as quickly as possible, the downlink sequence commanding the tool to operate at the point at the top of the diagram would be sent. This corresponds to 100% duty cycle at a GTF of zero.

Downlinking to the tool to operate at the red point in the top right quadrant would result in the tool spending 67% of drilling time steering up and to the right. This should result in the trajectory building and turning to the right, however, depending on the interaction of the RSS with the formation the turn rates vertically and horizontally may be different. If the BHA has a strong drop tendency, for example, this amount of build may be required to maintain the well at horizontal.

0.2

Figure 3-27: A steering command diagram for a rotary steerable system. Various tool face and duty cycle options are programmed into the tool before running in hole. Downlinking to the tool allows the driller to change to any other point of the diagram. The red point is an example discussed in the text.

The downlink command to the tool is the means by which the tool performance is adjusted. The interaction of the entire BHA with the formation will define the trajectory that this delivers. Many RSS tools now incorporate an inclination hold feature. In this mode of operation the driller downlinks the desired borehole inclination, and the tool automatically adjusts the tool face and duty cycle settings to maintain the requested inclination. This can deliver very straight trajectories and is particularly useful for well placement with deep directional measurements such that the well can be drilled parallel to a remotely detected boundary.

Because of the requirement to push off the opposite side of the borehole to cause a change in the trajectory, push-the-bit systems are sensitive to the mechanical properties of the formation.

Kicking off from vertical in an existing well can also be problematic as the pads become unable to contact the borehole wall at the hole enlargement that occurs at the kick-off point. Point-the-bit rotary steerable systems overcome these limitations.

A point-the-bit system delivers all the benefits of a push-the-bit system with reduced sensitivity to the formation, resulting in more consistent steering, and generally higher dogleg capability.

The system is centered on a universal joint that transmits torque and weight on bit, but allows the axis of the bit to be offset with the axis of the tool. The axis of the bit is kept offset by a mandrel that is maintained in a geostationary orientation through the use of a counter rotating electrical motor. Where the push-the-bit system keeps a control valve geostationary to divert mud behind the pads, the point-the-bit system keeps a mandrel geostationary.

Due to the high power requirements the system has its own power generation capabilities through a high power turbine and alternator assembly. The system also contains power electronics to control the motor, and sensors that monitor the rotation of the collar and motor. These sensors provide input and feedback for the control of the system.

Figure 3-28: The PowerDrive Xceed point-the-bit system uses an electric motor to counter rotate a mandrel against the rotation of the collar. This keeps the mandrel, and thus the bit oriented in the same direction while still rotating with the collar.

Figure 3-29: The PowerDrive Xceed point-the-bit system uses a geostationary offset angle between the bit and collar to create a steering tendency.

As with the push the bit system, the directional driller controls the dogleg by downlinking to the tool to change the proportion of steering versus neutral time.

Because there is no rotation provided downhole by either of the rotary steerable systems, the entire drillstring must be continuously rotated from surface. If additional downhole RPM is

desired, or surface rotation must be kept to a minimum (such as when casing wear is a concern), a mud motor (without the bent housing) can be used above the RSS to provide downhole rotation of the RSS assembly.

Radius

Radius