Recent field tests showed that a reliable high-power slim-hole motor developed for the U.S. Department of Energy (DOE) drills twice as fast as conventional systems. This slim-hole drilling motor has the potential to significantly reduce drilling costs by reducing drilling time, mud, cementing and casing costs and minimizing environmental impact. The DOE slim-hole drilling system consists of a double-length, high-power PDM motor that delivers twice the torque and power of conventional motors, and hybrid PDC/TSP bits that can operate at these high power levels. Laboratory dynamometer and drilling tests confirmed the high-power output of the motor while field tests conducted at the GRI Catoosa test facility in Oklahoma showed that these motors drill twice as fast as conventional motors. These motors should stimulate increased use of slim-hole drilling while taking advantage of the resulting 30 to 50 percent cost savings associated with these smaller diameter wells.
Background
In the late 1950s and 1960s, more than 3,000 slim holes were drilled worldwide (Shook et al., 1995). These slim holes reduced costs 27 to 75 percent by reducing rig time, mud costs, cementing costs, and tubular costs. Despite significant cost savings, slim holes did not find wide acceptance because of problems with slim-hole equipment (e.g., roller bits, logging tools, etc.), and the misconception that large wellbores were needed to effectively complete, workover and produce oil and gas wells. Advances in PDC drag bits, down-hole motors, and slim-hole logging tools have resulted in renewed interest in slim-hole drilling.
High-Power Motor Design
Positive-displacement mud motors (PDMs) utilize eccentric rotors in rubber stators to convert hydraulic power to mechanical power to rotate drill bits. Motor speed is proportional to flow rate and motor torque is proportional to the pressure drop across the motor.
A high-power slim-hole motor has been developed with a double length power section (i.e., double the number of motor stages) that doubles the motor torque and power output, and allows these motors to drill twice as fast as conventional motors (Figure 1).
Increasing the motor power output required increasing the strength of 1) the universal joint transmitting the torque from the power section to the drill bit, 2) the thrust bearings, and 3) the drive shaft in the motor (Cohen et al., 2000).
Figure 2 shows that the 3 3/8;-inch high-power DOE slim-hole motor delivers 73 hp compared to 28 hp for a conventional oilfield motor. The high power output will increase drilling rates 2- to 3-fold when operating on drilling rigs with pumps that can deliver the higher circulating pressures required with these high-power motors.
Hybrid Bit
Thermally stable diamond (TSP) cutters, which have the cobalt binder removed to allow them to operate at higher temperatures, are seven times more wear resistant than PDC cutters (Figures 3and 4). The major limitation of TSP cutters is that they cannot drill softer formations as fast as PDC cutters because of their smaller size.
To overcome this problem, and to develop a high-power bit for use on the high-power motor, hybrid bits were developed that utilize both PDC and TSP cutters (Figure 5). The PDC cutters are placed in rows on the bit to drill soft formations at high rates. TSP cutters are placed in rows behind the PDCs to drill through hard streaks that would normally damage the PDC cutters.
Catoosa Field Test No. 1
Two field tests were conducted on the DOE high-power slim-hole motors and bits at the GRI Catoosa test facility in Oklahoma (Figure 6). The first "shake down" test was run to identify weak points in the design of the DOE slim-hole drilling system, and the second test was run to compare conventional and DOE motor drilling rates.
