Steerable Drilling Systems
A steerable drilling system allows directional changes (azimuth and/or inclination) of the well to be performed without tripping to change the BHA, hence its name. It consists of: a drill bit; a stabilized positive displacement steerable mud motor; a stabilizer; and a directional surveying system which monitors and transmits to surface the hole azimuth, inclination and toolface on a real time basis (See Figure 8).
|
Bit Offset |
Figure 8 Steerable drilling system
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 drillbit 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 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) or to drill in a straight line by both rotating and circulating fluid through the drillstring. 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.
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. When drilling in a straight line, by rotation of the assembly, the term “rotary” drilling is used to describe the drilling operation. The directional tendencies of the system are principally affected by the navigation sub tilt angle and the size and distance between the PDM stabilizer and the first stabilizer above the motor.
The steerable drilling 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) or 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 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.
There are five major components in a Steerable Drilling System (Figure 11). These components are:
(a) Drill Bit
(b) Mud Motor
(c) Navigation Sub
(d) Navigation Stabilizers
(e) Survey System
(a) Drill Bit
Steerable systems are compatible with either tricone or PDC type bits. In most cases, a PDC bit will be used since this eliminates frequent trips to change the bit.
(b) PDM
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.
(c) Navigation Sub
The navigation sub 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 Systems:
• The double tilted universal joint housing or DTU and
• The tilted kick-off sub or TKO.
The DTU and TKO both utilize double tilts to produce the bit tilt required for hole deflection. The DTU’s two opposing tilts reduce bit offset and sideload forces, and thereby maintaining an efficient cutting action. The TKO has two tilts in the same direction that are close to the bit.
(d) Navigation Stabilisers
Two specially designed stabilizers are required for the operation of the system and influence the directional performance of a steerable assembly. The motor stabilizer or Upper Bearing Housing Stabiliser, UBHS is an integral part of the navigation motor, and is slightly undergauge. The upper stabilizer, which defines the third tangency point, is also undergauge 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.
(e) Survey System
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 toolface orientation. In some cases, a wireline steering tool may be used for this purpose.