High-Pressure Drilling

A series of full-scale field drilling tests has shown that high-pressure bits operating at pressures of 10,000 to 15,000 psi can drill many formations two to three times faster than can conventional bits. High-pressure drills can effect significant savings where drilling costs are high, such as in deep land wells, in arctic areas, and offshore. Introduction A survey has been made of more than 25 novel drills, and drilling rates were predicted for each of these drills in oil wells on the basis of available laboratory drilling data. Novel drills that thermally spall rock were found to have limited potential for drilling oil wells because most sedimentary rocks will not spall. Devices such as lasers and electron beams had extrapolated drilling rates of less than 1 ft/hr because of the high energy requirement for fusing rock. Extrapolated laboratory data indicated that high-pressure erosion bits operating at pressures of 10,000 to 20,000 psi should drill medium-strength rocks at rates of 70 to 280 ft/hr compared with 30 to 170 ft/hr for conventional roller bits. As a result of this study, research was begun to further evaluate high-pressure bits for oil-well drilling. Earlier Work A series of laboratory tests was conducted on high-pressure drilling before the full-scale field tests were begun. In all tests, water was used as the drilling fluid. Pulsed-Jet Experiments Pulsed-Jet Experiments After the literature was surveyed, an An-ny surplus cannon (Fig. 1) was modified to fire water pulses at pressures up to 25,000 psi. The diameters of holes pressures up to 25,000 psi. The diameters of holes eroded using 0.2 to 1-in.-diameter nozzles were three to four times larger than the nozzle diameters (Fig. 2). At 25,000 psi, a 1.45-gal water pulse was fired through a 1.0-in.-diameter nozzle in 0.02 second. This water jet drilled a 3.9-in.-diameter X 3/4 -in. deep hole in Indiana limestone, which corresponds to an instantaneous drilling rate of 1 1,000 ft/hr and a hydraulic power output of 70,000 hp. These tests demonstrated that a threshold pressure must be exceeded before fluid jets will drill; Fig. 3 shows that this threshold pressure was about 6,200 psi for Indiana limestone. psi for Indiana limestone. The amount of energy required to remove rock was determined by measuring the amount of energy expended and the amount of rock removed during each water pulse. The data from these tests indicated that a 3,000-hp jet bit would drill an 8-in.-diameter hole in Indiana limestone at rates of 280 to 370 ft/hr. Laboratory High-Pressure Drilling Test After the pulsed-jet tests were completed, a fracturing truck was used to pump water through a 2-in. diameter high-pressure drill (Fig. 4) at pressures up to 13,500 psi. The first bit had two 0.15-in.-diameter nozzles (nozzle area = 0.035 sq in.) and was designed to operate at pressures up to 10,000 psi. As the pressure across this bit was increased to 10,000 psi, the flow rate increased to 133 gal/min and the power output across the bit increased to 780 hydraulic horsepower (hhp). JPT P. 851