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Vol. 231 No.3 |
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| DAVID MICHAEL COHEN, MANAGING EDITOR |
Oilfield solutions to geothermal challenges
Because both sectors involve the extraction of high-energy fluids from deep beneath the earth’s surface, it’s only natural that the oil and gas industry should share many technologies with its little cousin, geothermal. And, in fact, conventional rotary drilling rigs, drillbits, mud, casing and cement developed for oilfield use have all been standard equipment in the geothermal industry for years. Yet, while these technologies have made oil and natural gas the dominant forms of energy on the planet, geothermal has remained a niche resource, until recently located almost exclusively on the edges of tectonic plates, where high-temperature water and steam are available in relatively shallow, highly permeable formations. The United States leads the world in geothermal development, with 30% of the global online capacity, yet even in the US geothermal represents less than 1% of total electrical supply.
That may change dramatically in the near future, as new technologies pave the way for a rapid expansion of geothermal energy from a niche market to a major energy resource. Not surprisingly, these technologies come once again from the oil patch; in fact, they are some of the same new technologies that are currently pushing the E&P envelope by enabling development of previously unavailable or uneconomic oil and natural gas deposits.
Hot fractured rock. There are practically limitless amounts of heat below the earth’s surface at depths of less than 30,000 ft, and these hot zones are widely distributed geographically, but most of the heat is contained in low-permeability, often dry rocks. Since water and steam cannot circulate, geothermal wells drilled into such rock formations would not produce hot fluids for long enough to be viable as a productive energy source.
Fortunately, the oil and gas industry has developed an effective means of circulating fluids through low-permeability rock: hydraulic fracturing. Having made possible the development of gas shales, hydrofracing is capable of similarly enabling commercial energy production from low-permeability geothermal wells by creating the fluid circulation necessary to sustain heat production to the surface. This new geothermal production concept, called hot fractured rock or hot dry rock geothermal, is one form of a growing array of enhanced (or engineered) geothermal systems (EGS). A 2006 Massachusetts Institute of Technology study estimated that, based on current technologies alone, EGS in the US could be producing more than 100 gigawatts of affordable electricity by 2050, equivalent to about 10% of current US capacity.
EGS technology has been successfully demonstrated by a European research project in Soultz sous Forêts, France. After 20 years of research, electricity production of the world’s first hot dry rock plant started in June 2008.
On the other side of the world, Australian companies are pushing forward with major investments in EGS, with commercial plants expected soon. Geodynamics Ltd. is building the first multi-well hot fractured rock power project, a 25-megawatt demonstration plant, in the Cooper Basin of South Australia. Meanwhile, Petratherm has recently confirmed temperature measurements within its targets at its Paralana geothermal project. The Paralana project, also in South Australia, uses a unique “heat exchanger within insulator” (HEWI) model involving injection of water into naturally porous and permeable insulating rock just above an impermeable granite heat source. The injected fluid is heated by the underlying granite and is circulated via controlled rock fractures to a production well, where the hot fluid returns to surface to power a binary-cycle electrical plant.
Positive results coming from these and other EGS projects are turning heads at the US Department of Energy, such that Energy Secretary Stephen Chu announced awards totaling $338 million last year for 123 geothermal projects across the country as part of the DOE’s Geothermal Technologies Program. Fifty-nine of those projects are related to EGS development, with a commitment to demonstrate its technical feasibility by 2015.
HPHT components. The DOE program is also funding R&D of new downhole technologies able to withstand the extreme temperatures and pressures of deeper geothermal reservoirs where the most promising resources are located. Like hydrofracing, these HPHT technologies are primarily developed for oilfield use, as operators seek to tap oil and gas from increasingly harsh formations.
“Unlike in oil and gas wells, where higher temperature is a nuisance you need to overcome in order to reach the resources, in geothermal applications, the higher the temperature, the better,” said Ahmed Sabet, marketing manager for General Electric’s Oilfield Technology business. GE received an $11 million grant in October under the DOE program, in part to develop high-temperature circuitry for sensors and communication devices in deeper, hotter geothermal wells.
The effort is based on the application of silicon carbide technology to extend the top operating temperature of downhole electronics to about 300°C. Success would represent a step change over current HPHT electronics, which typically have operating temperatures up to a range of 175°C–200°C.
GE has concluded bench testing of analog circuit components at high temperature, and found no degradation from room temperature to 300°C, Sabet said. Extended cycling was also conducted at high temperature, under conditions that simulated the downhole environment. Sabet said the next phase will be to manufacture enough silicon carbide to build actual downhole tools.
Reaping the rewards. “We feel that whatever we do in the geothermal area, which is probably the most extreme conditions, will also serve us in oil and gas, because the trend is going to continue to be toward higher-temperature wells,” said Sabet of GE’s geothermal R&D effort.
What’s true for HPHT tools is also true for reservoir stimulation and any manner of other oilfield methods adapted for use in geothermal applications. Even as they help to bring geothermal energy into the mainstream, advances made in these technologies for geothermal applications will certainly rebound to the benefit of oil and gas engineers. 
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