Ultra high rpm drilling offers faster penetration rates and reduced environmental footprint.
If oilfield drilling technology cannot be used in dentistry, perhaps the technology behind dental drills can be applied to the oil industry. The US Department of Energy (DOE) thinks so. It envisions that ultra high speed drilling will not only result in faster penetration rates at lower weight-on-bits but also reduce environmental footprints.

Although Chinese and Arabic physicians reputedly drilled teeth long before the middle ages, hand held motor-driven drills only appeared in the Western world in 1864. Powered by elegant 'clock-spring' movements, the speed and power of these drills was limited to 100 rpm and several ft-lb/sec. Soon afterwards pneumatic drills - powered by bellows arrived. With the advent of electromagnetic motors, drills became lighter, cheaper and much more powerful.

Mostly 1/16-in. in size, dental bits are made of tungsten carbide or natural diamond and kept cool by an air/water mist that circulates through the bit. The motors require between 30 psi - 40 psi pressure to operate and can rotate miniature turbines at speeds exceeding 400,000 rpm.

Among several wide ranging technology programs is the DOE 'Ultra High Speed Drilling Program' initiated in July 2003. During a successful Ultra-High Speed Diamond Drilling feasibility analysis, which was spun off from the Jet Propulsion Laboratory 'Drilling on Mars' project, National Aeronautics and Space Administration (NASA) and TerraTek demonstrated that ultra-high speed air drilling could achieve higher rates of penetration with lower weight-on-bit in a variety of rock types - sandstone, limestone and basalt. The DOE hopes to take this technology to the oilfield.

In fact, significantly lower loads at the bit and reduced specific energies were sufficient to improve penetration rates that were considered normal in oil and gas drilling. Reduced energy consumption was considered a key success factor by NASA because of the limitations space missions imposed on logistics and available energy for drilling equipment.

Downhole equipment such as turbines and electrical motors are capable of achieving high rotational speeds. However, these are often lower than 2,000 rpm. During the NASA tests, electric brushless DC motors (1.6-in. diameter) were run at speeds of up to 52,000 rpm using a continuous 2,000 watt power source. In this test a 7/8-in. diameter coring bit was used to drill sandstone. 1,250 watts of power were required to run the motor and torque was calculated at approximately 0.25 ft-lb. Results showed that with 9 pounds weight-on-bit at a rotary speed of 40,000 rpm, a 220 ft/hr penetration rate was achieved. Although, the laboratory sample and bit length were limited to a 1-in. stroke, this has encouraged further studies.

Covering a range of drilling fluids and reproduced 'well-bore' pressures, drilling research will use natural diamond coring bits of less than 1-in. diameter. Once connected to commercially available electric brushless DC motors, these bits will be capable of being run at speeds of over 50,000 rpm. Bits and motors or turbines will be scaled up in two phases. First, test bits will be increased to reach 3-in. slim-hole diameters and ultra high speed motors or turbines will be designed to handle increased velocities. The second step encompasses the design of ultra high speed down-hole components for conventional bit sizes up to 171/2-in.

There are plenty of intriguing questions regarding ultra high speed drilling. How will salt, shale or clay based formations react to ultra high speed drilling? How will directional behavior be affected? What characteristics will cutting size and distribution display? Although it is hard to answer these questions without test data, it is fair to say that much will change. An example of this would be the nature and size of cuttings. Cuttings from hard formations (above 20,000 psi compressive strength) are likely to be smaller than 0.5mm as they are ground to a fine powder or dust, rather than being crushed solids. Consequently, this will have implications on the distribution and removal of cuttings within the well-bore and mud return equipment.

At first sight, ultra high rpm drilling seems outlandish, but the concept fits with coiled tubing and under-balanced techniques. If electrical power is required cables can be integrated within the coil, during or after the time of manufacture. Additionally, penetration rates will not be held back by connections. In the case of air driven motors, traditional under-balanced air compressors units should be able to meet the operating pressure requirements while rotating wellheads can provide well control.

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