Hobbing helps out: technique improves drill motor performance in search for 'crude'.

Position:Oil, gas

Oil and gas discovery poses some tough challenges when it comes to drilling. In the automotive world, it's where the rubber hits the road. However, in the oil and gas industry, it's where the drill motor bites the earth--and that's where better performance is in demand.

Leistritz Corp., of Allendale, NJ, studied the challenges with several well drilling and tooling companies, hoping to find a solution to the machining problems long associated with power section rotors. Excessive tool wear was one of the major challenges. Replacing and indexing inserts was another.

Steerable drill motor

Power section rotors connect directly to the drill bit in directional drilling for oil and gas discovery deep below the earth's surface. The power section is a "positive displacement mud motor" combined with directional technology to steer the drill motor using tilting joints and sophisticated sensors while drilling for oil or gas.

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The motor converts the drilling fluid's hydraulic energy into mechanical energy in the form of torque and rotational speed. This conversion is accomplished through the use of a spiral rotor that fits into a stator of similar shape, to form a tight seal. These systems are now capable of drilling within [+ or -]0.15[degrees] to depths to 18,000'.

The rotors within the mud motors are called power section rotors, which are typically five-, seven-, or nine-lobed with smooth spiral profiles in lengths as long as 8m with long leads of up to 40". The rotors fit snugly into an elastomer stator having one additional lobe contour with the same lead, creating a moineau-style progressive cavity positive displacement rotary drive unit.

When the water-based drilling fluid or "mud" is pressurized at the surface and pumped down the hole to the power section, a high-torque, low rpm drive is created to rotate the rock-crushing drill bits.

These rotors have traditionally been milled using a side milling operation to interpolate the profile using CNC control in a connected series of steps. Excessive tool wear, long cycle times, and profile inaccuracies plagued drill tooling manufacturers striving for better drill motor performance.

Balance needed

The milling process produces a snakeskin-like surface that requires many hours of polishing prior to chrome plating. This polishing has been, for the most part, a hand operation. A balance has been needed to weigh shorter cycle times with more scalloping to polish versus a better surface...

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