1. Field of the Disclosure
The subject disclosure relates generally to oilfield drilling, and more particularly to bottom hole assemblies and tools for orienting a bottom hole assembly (BHA).
2. Background of the Related Art
In conventional drilling, the BHA is lowered into the wellbore using jointed drill pipes or coiled tubing. Often the BHA includes a mud motor, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices. A simple BHA having a drill bit, various crossovers, and drill collars is relatively inexpensive, costing a few hundred thousand US dollars, while a complex BHA costs ten times or more than that amount.
Many drilling operations require directional control so as to position the well along a particular trajectory into a formation. Directional control, also referred to as “directional drilling,” is accomplished using special BHA configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors, and special BHA components and drill bits. The directional driller can use drilling parameters such as weight-on-bit and rotary speed to deflect the bit away from the axis of the existing wellbore. In some cases, e.g. when drilling into steeply dipping formations or when experiencing an unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically.
Direction control is most commonly accomplished through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drill string is not rotating. By pumping mud through the mud motor the bit rotates (though the drill string itself does not), allowing the bit alone to drill in the direction to which it points. When a particular wellbore direction is achieved, the new direction may be maintained by then rotating the entire drill string, including the bent section, so that the drill bit does not drill in a direction away from the intended wellbore axis, but instead sweeps around, bringing its direction in line with the existing wellbore. As it is well known by those skilled in the art, a drill bit has a tendency to stray from its intended drilling direction, a phenomenon known as “drill bit walk”. A device for addressing drill bit walk is shown in U.S. Pat. No. 7,610,970 to Sihler et al. issued Nov. 3, 2009, which is incorporated herein by reference.
The use of coiled tubing with downhole mud motors to turn the drill bit to deepen a wellbore is another form of drilling, one which proceeds quickly compared to using a jointed pipe drilling rig. By using coiled tubing, the connection time required with rotary drilling is eliminated. Coiled tube drilling is economical in several applications, such as drilling narrow wells, working in areas where a small rig footprint is essential, or when reentering wells for work-over operations.
In coiled tubing drilling, a BHA with a mud motor is attached to the end of a coiled tubing string. Typically, the mud motor has a fixed or adjustable bend housing to drill deviated holes. Because the coiled tubing is unable to rotate from surface, a so called orienter tool is used as part of the BHA to “orient” the bend of the mud motor into the desired direction. There exists a multitude of different designs for the drive systems of such tools. Some designs support continuous rotation such as electric motor and gearbox drives, while others only permit rotation by a certain limited angle.