A steerable downhole drilling mud motor system allows directional changes (azimuth and/or inclination) of the well to be performed without tripping pipe to change the Bottom hole Assembly BHA, hence its name. This article is one of the Directional Drilling guide articles Which I recommend to visit to understand other deflection tools mechanisms.
It consists of: a drill bit; a stabilized steerable PDM / Mud Motor; a stabilizer; and a directional surveying system which monitors and transmits to surface the hole azimuth, inclination and toolface (read also Surveying Calculations Methods) on a real time basis.
The Major Components Of Steerable Downhole Mud Motor
There are five major components in a Steerable Drilling System. These components are:
- Drill Bit
- Mud Motor
- Navigation Sub
- Navigation Stabilizers
- Survey System
Steerable systems are compatible with either Roller cone Drill Bits or PDC Bit type bits. In most cases, a PDC bit will be used since this eliminates frequent trips to change the bit ( Read Also Bit Dull Grading Guide).
The motor section of the system causes the bit to rotate when mud is circulated through the string. This makes oriented drilling possible. The motors may also have the navigation sub and a bearing housing stabilizer attached to complete the navigation motor configuration.
Navigation Sub In Steerable Drilling Mud Motor
The navigation sub (Drilling Subs) converts a standard Mud motor into a steerable motor by tilting the bit at a predetermined angle. The bit tilt angle and the location of the sub at a minimal distance from the bit allows both oriented and rotary drilling without excessive loads and wear on the bit and motor. The design of the navigation sub ensures that the deflecting forces are primarily applied to the bit face (rather than the gauge) thereby maximizing cutting efficiency.
Two types of subs are presently available for steerable downhole drilling mud motor:
- The double tilted universal joint housing or DTU : has two opposing tilts reduce bit offset and side load forces, and thereby maintaining an efficient cutting action
- The tilted kick-off sub or TKO : has two tilts in the same direction that are close to the bit.
The DTU and TKO both utilize double tilts to produce the bit tilt required for hole deflection in the steerable downhole drilling mud motor system.
Two specially designed stabilizers are required for the operation of the system and influence the directional performance of a steerable downhole drilling mud motor.
The steerable downhole mud motor stabilizer or Upper Bearing Housing Stabilizer, UBHS is an integral part of the navigation motor, and is slightly under-gauge. The upper stabilizer, which defines the third tangency point, is also under-gauge and is similar to a string stabilizer.
The size and spacing of the stabilizers also can be varied to fine-tune assembly reactions in both the oriented and rotary modes.
A real time downhole survey system is required to provide continuous directional information. A measurement while drilling, MWD system is typically used for this purpose. An MWD tool will produce fast, accurate data of the hole inclination, azimuth, and the navigation sub tool face orientation. In some cases, a wireline steering tool may be used for this purpose.
Steerable Downhole Mud Motor Drilling Mechanism
The capability to change direction at will is made possible by placing the tilt angle very close to the bit, using a navigation sub on a standard PDM. This tilt angle can be used to drill in a specific direction, in the same way as the tilt angle generated by a bent sub with the the drill bit being rotated by the mud motor when circulating.
However, since the tilt angle is much closer to the bit than a conventional bent sub assembly, it produces a much lower bit offset and this means that the drill bit can also be rotated by rotating the entire string at surface (in the same way as when using a conventional assembly).
Hence the steerable drilling mud motor assembly can be used to drill in a specific direction by
- Orienting the bent sub in the required direction and simply circulating the fluid to rotate the bit (as in the bent sub assembly)
- Drilling in a straight line by both rotating and circulating fluid through the drill string. When rotating from surface we will of course be circulating fluid also and therefore the rotation of the bit generated by the mud motor will be super-imposed on the rotation from surface.
This does not alter the fact that the effect of the bit tilt angle will be eliminated by the rotation of the entire assembly.
What is Sliding Drilling Mode
When using the navigation sub and mud motor to drill a deviated section of hole (such as build up or drop off section of hole) the term “oriented or sliding” drilling is used to describe the drilling operation.
What is Rotary Drilling mode
When drilling in a straight line, by rotation of the assembly, the term “rotary” drilling is used to describe the drilling operation.
How To Control Directional Work
The directional tendencies of the steerable downhole drilling mud motor system are principally affected by the following:
- Navigation sub tilt angle
- Size of stabilizers
- Distance between the PDM stabilizer and the first stabilizer above the motor.
The Main Applications Of steerable Drilling Mud Motor
The steerable Downhole Mud Motor systems are particularly valuable where:
Changes in the direction of the borehole are difficult to achieve; where directional control is difficult to maintain in the tangent sections of the well (such as in formations with dipping beds) where frequent changes may be required.
The steerable systems are used in conjunction with MWD tools which contain petrophysical and directional sensors. These types of MWD tools are often called Logging Whilst Drilling, LWD tools.
The petrophysical sensors are used to detect changes in the properties of the formations (lithology, resistivity or porosity) whilst drilling and therefore determine if a change in direction is required.
Effectively the steerable downhole mud motor assembly is being used to track desirable formation properties and place the wellbore in the most desirable location from a reservoir engineering perspective.
The term “Geosteering” is often used when the steerable system is used to drill a directional well in this way.
Operation of a Steerable Downhole Drilling Mud Motor System
As described above, the steerable drilling mud motor system can drill directionally or straight ahead, as required. This enables the driller to control the well’s trajectory ( learn more about how Well trajectory calculations) without making time consuming trips to change bottom hole assemblies.
Steering while Kickoffs
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 drilling (since the drill string is not rotating) mode. The bit is driven by the down hole motor, and the rotary table is locked in place, as it is when conventional motor drilling.
Controlling Dogleg With Steerable Downhole Mud Motor System
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.
Dogleg Produced by a Steerable Drilling Mud Motor System
When oriented drilling, the theoretical geometric dogleg severity or TGDS produced by the system is defined by three points on a drilled arc . The three points required to establish the arc are:
- The Bit
- The PDM stabilizer or Upper Bearing Housing Stabilizer.
- The first stabilizer above the mud motor (upper stabilizer).
The radius of the arc is further determined by the tilt of the navigation sub, as seen in the Figure: 2. The following basic relationship is produced by mathematical derivation.
Drilling straight with steerable downhole Mud Motor system
The system can also be used to drill straight ahead by simple string rotation. The rotary table is typically turned at 50-80 RPM while the motor continues to run. When drilling in this way the system is said to be drilling in the rotating mode.
Following The Directional Plan
Through careful well planning and bottom hole assembly design, oriented sections are minimized and the assembly is rotated as much as possible. This maximizes penetration rates while keeping the well on course. Survey readings from an MWD tool enable efficient monitoring of directional data so the driller can maintain the well path close to the desired path. Slight deviations can be detected and corrected with minor oriented drilling intervals before they become major problems.