A new high-pressure coiled-tubing (CT) drilling system has been developed with support from the DOE that drills 2 to 3 times faster than conventional CT motors. Downhole motors have been developed that utilize diamond thrust bearings, titanium flexshafts, high-pressure rotor/stator sections and other advanced features that allow these motors to operate at high pressures. Laboratory tests have been conducted using the high-pressure system to cut 1 to 2 inch deep helical slots in wellbore walls to remove formation damage (patent pending) and to drill cement out of drillpipe at rates up to 1,400 ft/hr compared to 60 ft/hr for conventional motors. This jet-drilling system can also be used to remove barite scale from tubing, to clean slotted liners, to ream holes at high rates, and to under-ream when drilling with casing. The high-pressure system will be field tested and commercialized during the next phase of this DOE project.
Maurer Engineering Inc. (MEI), under contract to the United States Department of Energy (DOE), has completed laboratory testing of a high-pressure CT drilling system. The system is now ready for field trials.
High-pressure (10,000 psi) jet drilling systems have shown they can drill oil and gas well at high rates,2,3,4 but they have not been commercialized due to problems with leaking drill-pipe tool joints. CT is a continuous reel of tubing that contains no connections and therefore eliminates leakage problems associated with drillpipe. This feature offers the opportunity for successful implementation of high-pressure drilling. MEI developed a special high-pressure motor for use with the high-pressure CT drilling system and has successfully completed laboratory tests of critical components.
With jet assisted drilling, high-pressure jets cut kerfs into rock ahead of the bit and then mechanical cutters break the rock ledges between the kerfs (Figure 1). Jet assisted drilling has the potential to significantly increase penetration rates in many formations. Figure 2 shows the results of laboratory drilling tests conducted for the Air Force in 19861 where jet-assisted bits drilled at 1000 ft/hr compared to 300 ft/hr for conventional drilling motors and 100 ft/hr for rotary drills.
Figure 3 shows the high-pressure CT system. The two critical components of the system are the downhole motor that must operate at 10,000 psi and the CT that must not fatigue at high operating fluid pressures.
MEI developed a Moineau motor that operates at 10,000 psi (Figure 4). This motor uses diamond thrust bearings to absorb the high loads caused by the pressure drop across the bit. A proprietary rotor/stator design is used that allows the motor to operate at high pressures. Additional design details on the high-pressure motors are presented in the ASME paper ETCE2000/Drill-10098.
As coiled tubing (CT) is reeled on and off the reel and over the gooseneck, it yields and eventually fails from fatigue. Fatigue life of the tubing is significantly reduced when CT is operated at high pressures.
Grades of CT readily available today will fail after about 50 to 90 cycles when operating at 10,000 psi pressure. Therefore, improved CT had to be developed for use with this high-pressure drilling system.
Quantity Tubing worked with MEI to develop QT-1200 CT, which provides much longer fatigue life than conventional grades. The new tubing was extensively tested on the fatigue machine shown in Figure 5. The laboratory tests showed that QT-1200 CT will operate for 240 cycles at 10,000 psi before it fails, compared to 90 cycles for QT-1000 tubing (Figure 6).
In addition, other manufacturers have developed composite CT that will operate at 10,000 psi pressure for over 1000 cycles without fatigue failure.