A high-speed drilling system has been developed which drills medium strength rocks at rates up to 1000 ft/hr (328 m/hr). This system uses special motors to operate high-pressure PDC bits at pressures up to 10,000 psi (69 MPa) and at speeds of 800 to 1000 RPM. High drilling rates were achieved due to the high rotary speeds and the use of PDC cutters to remove the rock ledges left on the hole bottom by the high-pressure jets.
Introduction
In the late 1960's and early 1970's, Exxon and Shell conducted field tests with high-pressure diamond, drag and roller bits operating at pressures of 10,000 to 18,000 psi (69 to 124 MPa). These tests were conducted with conventional drill muds and modified rotary rigs operating at rotary speeds of 50 to 100 RPM. These high-pressure bits drilled many formations 2 to 3 times faster than rotary bits, but they were not commercialized due to difficulties in pumping the high-pressure fluids.
In the late 1960's, Gulf conducted extensive field tests with abrasive jet drills which circulated viscous muds containing 6-percent steel shot (20/40 mesh) at pressures of 8,000 to 15,000 psi (55 to 103 MPa). The abrasive pressures of 8,000 to 15,000 psi (55 to 103 MPa). The abrasive jet bits drilled hard formations in West Texas 4 to 20 times faster and 3 to 7 times further than conventional roller bits. They were not economical due to 1) severe erosion problems with the abrasive fluids, and 2) excessive pump problems with the abrasive fluids, and 2) excessive pump horsepower requirements (3,000 to 10,000 hp).
Although these systems were not economical, the tests demonstrated that high-pressure jet bits could drill oil-field rocks at high rates.
Another important development relating to high-speed drilling during the 1970's was the development and implementation of man-made polycrystalline diamond cutter (PDC) bits. These bits drill many formations 2 to 3 times faster than roller bits when used on high-speed drilling motors (400 to 1000 RPM).
In 1984, Maurer Engineering undertook the development of a high-speed drilling system to drill 8-inch (203 mm) diameter holes in medium strength rocks (5,000 to 10,000 psi; 34.5 to 69 MPa) at rates of 500 to 1000 ft/hr (164 to 328 psi; 34.5 to 69 MPa) at rates of 500 to 1000 ft/hr (164 to 328 m/hr). Conventional rotary rigs can drill these rocks at 50 to 100 ft/hr (16 to 33 m/hr) and downhole motors at 100 to 300 f t/hr (33 to 98 m/hr), so a new drilling system had to be developed to drill these rocks at the very high penetration rates.
This goal was accomplished by developing a new down-hole motor which operates at high pressure (10,000 psi) and at high speeds (400 to 1000 RPM). Extensive laboratory tests confirmed that drilling rates of 500 to 1000 ft/hr can be achieved in medium-strength rocks with 8-inch diameter high-pressure (10,000 psi) PDC bits operating at 800 RPM.
2.5 INCH BIT TEST'S
The initial tests were carried out in 1984 using the 2.5-inch (63.5 mm) diameter high-pressure PDC bit shown in Figure 1. This bit contained six 0.07-inch (1.78 mm) diameter nozzles.
Figure 2 shows the 2.5 inch high-pressure bit operating at 10,000 psi in the Drilling Research Center, Inc. drilling test facility in Houston, Texas.
This bit drilled at rates in excess of 1000 ft/hr in Berea sandstone as shown in Table 1 and Figure 3.
A conventional PDC bit (80 psi; 0.55 MPa) operating on a rotary rig at 100 RPM drilled Berea sandstone at 90 ft/hr (29.5) m/hr). Increasing the rotary speed of this bit to 1000 RPM with a downhole motor increased the drilling rate to 260 ft/hr (85.3 m/hr). Equipping the same bit with high- pressure nozzles (10,000 psi) and operating it at 1000 RPM with a new high-pressure motor increased the drilling rate to over 1000 ft/hr (328 m/hr).
