Drillling Geothermal Well with Laser

laser

From fossil fuels to geothermal heat, accessing the planet’s energy riches usually involves boring deep into the Earth with giant metal drills. But could lasers do the same job?

Foro Energy, a start-up company in Littleton, Colorado, has developed what it claims is an inexpensive system of high-powered lasers that can rip through rock, potentially revolutionising drilling and hastening the adoption of greener forms of power.

Foro announced last month that a test system had sent a beam from a 20-kilowatt commercial laser through 1.5 kilometres of optical fibre. Development has been funded by the US Department of Energy’s research arm, ARPA-E. Borehole drilling trials are planned for next year.

Mechanical drills can easily grind through soft rocks like sandstone to tap petroleum reserves, but they wear out quickly in hard crystalline rocks such as granite and basalt. It is these harder rocks that often hide the best sources of geothermal energy. Foro’s intense laser beam heats hard rock surfaces so fast that thermal shock fractures the upper few millimetres, leaving a crumbled layer that a mechanical drill can scrape away quickly and with little wear. This approach could increase drilling rates, a major component in well cost, by up to a factor of 10, says ARPA-E.

But a flashy prototype is far from proof that the rig will hold up in the brutal environment found in the bottom of a borehole, which is filled with rock chips and churning water that lubricates the drill bit. The optics must deliver the beam directly to the rock, says Jared Potter of Potter Drilling in Redwood City, California, who is developing a drilling process that shatters rock with extremely hot water. If the beam hits fluid, it will heat the liquid instead of the rock face. “It’s amazing how much energy it takes to boil water,” Potter says, and even a powerful laser couldn’t hope to zap both water and rock at once. He thinks Foro “has a long way to go to have a tool they can deploy in a geothermal or oil well”.

Still, the cost of drilling has been a roadblock to expanding the adoption of geothermal energy. If Foro can prove its technology is ready for the grunt work of punching hundreds of holes through the hard igneous rocks, it would change the mathematics of low-carbon energy. In the meantime, Potter says Foro’s lasers could be used instead of explosives to make holes in the steel casings of oil and gas wells, which are needed to drain fluid from the surrounding rock.

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Geothermal Drilling with Kelly Rig

To make the hole or drilling well with kelly rig, energy must be transmitted from the surface to the rock face at the end of the wellbore. Power supply for drilling has evolved from the early days of steam-driven,mechanically coupled rigs to the current standard of diesel-electric drive. In this configuration, two to four diesel engines (up to 2,000 horsepower each) drive electric generators, which supply power to individual electric motors driving the rotary table, drawworks, mua pumps, and other equipment. The rotary table is a mechanism, usually inset into the rig floor, which turns the drill string to break rock and advance the hole. (A "drill string" comprises the drill pipe plus the bottom-hole-assembly, or BHA. The BHA includes drill collars, stabilizers, bit, and any other specialized tools below the drill pipe).

Hole diameters in oil and gas drilling usually range fiom 4 to 26 inches, while geothermal holes generally have a minimum production size of 8-112 inches. To drill these holes, torque is applied to the kelly, which is at the top of the drill string. The kelly is a section of pipe with a square or hexagonal outside cross-section which engages a matching bushing in the rotary table. This bushing lets the rotary table continuously turn the kelly and drill string while they slide downward as the hole advances.

The upper end of the kelly is attached to a 'hvivel", which is a rotating pressure fitting that allows the drilling fluid to flow fiom the mud pumps, up the standpipe, through the kelly hose, into the swivel, and finally down the drill pipe as it rotates. The swivel is carried by the hook on the traveling block and it suspends most of the weight of the drill string while drilling.

Moving the drill string or the casing into and out of the hole is called tripping. Trips are usually required because the bit or some other piece of downhole equipment must be replaced, or because of some activity such as logging, testing, or running casing, and of course trips take longer as the hole grows deeper. Raising or lowering the drill string for a trip is done by the drawworks, which is basically a large winch. (The swivel and kelly are almost always handled as a unit, and are set aside in the "rat hole" while tripping.) The drawworks reels in or pays out a wire rope (drilling line) which passes over the crown block at the top of the rig's mast and then down to the traveling block which carries the hook, which in turn suspends the drill string or casing. Depending on what mechanical advantage is required, the drilling line is reeved several times between the crown and traveling blocks, as in a block and tackle.

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