Borehole geophysics encompasses a wide range of parametric borehole measurements. Included are measurements of chemical and physical properties of earth formations penetrated by the borehole, as well as properties of the borehole and material therein. Measurements are also made to determine the path of the borehole. These measurements can be made during drilling and used to steer the drilling operation, or after drilling for use in planning additional well locations.
Borehole instruments or “tools” comprise one or more sensors that are used to measure “logs” of parameters of interest as a function of depth within the borehole. These tools and their corresponding sensors typically fall into two categories. The first category is “wireline” tools wherein a “logging” tool is conveyed along a borehole after the borehole has been drilled. Conveyance is provided by a wireline with one end attached to the tool and a second end attached to a winch assembly at the surface of the earth. The second category is logging-while-drilling (LWD) or measurement-while-drilling (MWD) tools, wherein the logging tool is an element of a bottom hole assembly. The bottom hole assembly is conveyed along the borehole by a drill string, and measurements are made with the tool while the borehole is being drilled.
A drill string typically comprises a tubular which is terminated at a lower end by a drill bit, and terminated at an upper end at the surface of the earth by a “drilling rig” which comprises draw works and other apparatus used to control the drill string in advancing the borehole. The drilling rig also comprises pumps that circulate drilling fluid or drilling “mud” downward through the tubular drill string. The drilling mud exits through opening in the drill bit, and returns to the surface of the earth via the annulus defined by the wall of the borehole and the outer surface of the drill string. A mud motor is often disposed above the drill bit. Mud flowing through a rotor-stator element of the mud motor imparts torque to the bit thereby rotating the bit and advancing the borehole. The circulating drilling mud performs other functions that are known in the art. These functions including providing a means for removing drill bit cutting from the borehole, controlling pressure within the borehole, and cooling the drill bit.
In LWD/MWD systems, it is typically advantageous to place the one or more sensors, which are responsive to parameters of interest, as near to the drill bit as possible. Close proximity to the drill bit provides measurements that most closely represent the environment in which the drill bit resides. Sensor responses are transferred to a downhole telemetry unit, which is typically disposed within a drill collar. Sensor responses are then telemetered uphole and typically to the surface of the earth via a variety of telemetry systems such as mud pulse, electromagnetic and acoustic systems. Conversely, information can be transferred from the surface through an uphole telemetry unit and received by the downhole telemetry unit. This “down-link” information can be used to control the sensors, or to control the direction in which the borehole is being advanced.
If a mud motor is not disposed within the bottom hole assembly of the drill string, sensors and other borehole equipment are typically “hard wired” to the downhole telemetry unit using one or more electrical conductors. If a mud motor is disposed in the bottom hole assembly, the rotational nature of the mud motor presents obstacles to sensor hard wiring, since the sensors rotate with respect to the downhole telemetry unit. Several technical and operational options are, however, available.
A first option is to dispose the sensors and related power supplies above the mud motor. The major advantage is that the sensors do not rotate and can be hard wired to the downhole telemetry unit without interference of the mud motor. A major disadvantage is, however, that the sensors are displaces a significant axial distance from the drill bit thereby yielding responses not representative of the current position of the drill bit. This can be especially detrimental in geosteering systems, as discussed later herein.
A second option is to dispose the sensors immediately above the drill bit and below the mud motor. The major advantage is that sensors are disposed near the drill bit. A major disadvantage is that communication between the non rotating downhole telemetry unit and the rotating sensors and other equipment must span the mud motor. The issue of power to the sensors and other related equipment must also be addressed. Short range electromagnetic telemetry systems, known as “short-hop” systems in the art, are used to telemeter data across the mud motor and between the downhole telemetry unit and the one or more sensors. Sensor power supplies must be located below the mud motor. This methodology adds cost and operational complexity to the bottom hole assembly, increases power consumption, and can be adversely affected by electromagnetic properties of the borehole and the formation in the vicinity of the bottom hole assembly.
A third option is to dispose the one or more sensors below the mud motor and to hard wire the sensors to the top of the mud motor using one or more conductors disposed within rotating elements of the mud motor. A preferably two-way transmission link is then established between the top of the mud motor and the downhole telemetry unit. U.S. Pat. No. 5,725,061 discloses a plurality of conductors disposed within rotating elements of a mud motor, wherein the conductors are used to connect sensors below the mud motor to a downhole telemetry unit above the motor. In one embodiment, electrical connection between rotating and non rotating elements is obtained by axially aligned contact connectors at the top of the mud motor. This type of connector is known in the art as a “wet connector” and is used to establish a direct contact electrical communication link. In another embodiment, an electrical communication link is obtained using an axially aligned, non-contacting split transformer. The rotating and non rotating elements are magnetically coupled using this embodiment thereby providing the desired communication link.