Geology, Technology Mix In Geothermal Systems

American Association of Petroleum Geologists (AAPG)

Scott Miller
Scott Miller
The Department of Energy (DOE) envisions that recent advances in subsurface mapping, data collection, data dissemination and leveraging of oil and gas industry techniques can translate into widespread commercial adoption of enhanced geothermal systems (EGS).

Integrating this knowledge into EGS research and development also could be beneficial in addressing challenges currently faced by the oil and gas industry.

But first: What is EGS, and how does it work?

Closed Loop Approach

EGS uses low-pressure hydraulic or cold-water injection to produce a network of shear fractures in hot, low-permeability rock whose temperatures can exceed 200 degrees Celsius.

Geothermal fluid, consisting mostly of water, is pumped from an injection well through the network of fractures in hot rock, and the fluid is heated by conduction during travel.

The fluid exits via the production well as a source of thermal energy.

Conventional geothermal power production utilizes hot fluid circulating in naturally permeable systems at accessible depths, which currently restricts U.S. development to tectonically active regions found mostly in the West. EGS’s theoretical advantage stems from its ability to access the inexhaustible supply of heat available in the subsurface beyond these naturally favorable settings.

While oil and gas extraction methods require expensive pumping and disposal of water that has been contaminated with lubricating fluids and mineral deposits during reservoir stimulation, geothermal operations recycle water in a “closed loop” system.

Recycling water keeps reservoir pressure constant, allowing for more accurate maintenance of fractures. Plant operators can alter the size of these fractures to increase or decrease fluid flow volumes per unit time throughout the reservoir.

Low flow rates are critical to EGS, because of the long time-period needed for adequate heat transfer from the reservoir rock to the cold, injected water.

The 2008 U.S. Geological Survey’s Geothermal Resources Circular stated that EGS has the potential to produce on the order of 100 Gigawatts of electricity over a 30-year span, an order of magnitude greater than the potential of any other geothermal source, and a tenth of the current total electric generating capacity in the United States.

DOE is increasing funding for its Geothermal Technologies Office (GTO), which has sponsored full-scale EGS demonstration projects in locations such as Geysers Field, Calif.

In 2012, the Geysers Field project achieved steam production of 0.005 Gigawatts from a well depth of about four kilometers and a maximum reservoir temperature of 400 degrees Celsius, boosting confidence for possible national-scale development.

Geothermal energy extraction requires broadly similar technologies to those employed in the oil and gas industry, including use of integrated multi-dimension, multi-variable models to more accurately quantify estimates of both hydrocarbon and EGS resources.

GTO projects factor in variables such as thermal conductivity and fracture spacing of rock formations to create 3-D seismic, thermal and flow visualizations of highest-yield EGS areas. These models provide a basis for researchers to manipulate reservoir pressure, flow and temperature conditions for optimal EGS output.

This research also could provide insight on the feasibility of carbon sequestration and the oil/gas industry’s current quandary on disposal of wastewater.

FORGE-ing Ahead

In February, GTO announced plans for a subsurface laboratory studying thermo-mechanical-chemical-hydrologic processes in deep rock, called the Frontier Observatory for Research in Geothermal Energy (FORGE).

FORGE’s research on hot, deep rock properties could provide valuable insights toward advancing EGS as a commercial resource.

FORGE developments also could also benefit groups interested in examining deep crystalline rocks for information on supercritical gas reservoirs, isolation of radioactive waste and geologic storage of CO2.

While oil and gas industry horizontal drilling practices capitalize on the horizontal nature of hydrocarbon-rich bedding by providing extended contact between wellbores and reservoirs, EGS wells could employ horizontal drilling for maximized contact with hot rock, allowing more efficient fracture generation due to the sub-vertical nature of most fractures at depth.

GTO-contracted Baker Hughes is presently testing a directional drilling system designed to withstand temperatures up to 300 degrees Celsius in granite basement rock at the company’s testing site in Tulsa.

Oil, gas and geothermal capabilities are bounded by equipment failure at depth due to high pressures and temperatures (P/T).

The electronics that drive most subsurface tools fail at temperatures greater than 150 degrees Celsius – well below target temperatures for EGS. This poses a significant challenge to EGS, so GTO’s research includes copious testing on high P/T well drilling, monitoring and pumping equipment.

In 2011, Oak Ridge National Laboratory successfully demonstrated operation of a tool for measuring porosity, lithology and density of rock as a function of depth at 350 degrees Celsius. With high temperature-tolerant monitoring equipment, researchers will be able to study reservoir evolution in real time.

Studies such as the 2014 JASON report on Enhanced Geothermal Systems say “micro-drilling” small holes 2-2.5 inches in diameter could improve reservoir characterization, engineering and micro-seismic interpretations for EGS and oil/gas operations. With advances in this technology, geophones, accelerometers and seismometers could be emplaced in an abundant network of micro down-hole arrays, providing for critical reservoir permeability characterization with minimal drilling costs.

This research is coupled with induced seismicity studies related to EGS development, and has broad applications related to the current hot topic of induced seismicity, which is under scrutiny because of events triggered by wastewater disposal from oil and gas activities.

An Integrative Approach

GTO explores other geothermal energy methods in addition to EGS.

Demonstration projects such as the University of North Dakota test at Cedar Creek Oil Field, for example, are examining whether co-producing geothermal fluids from existing fossil fuel infrastructure can be economic.

Also, higher costs of production per kilowatt-hour compared to conventional energy still plague geothermal endeavors in many settings, so GTO has dedicated a portion of its Systems Analysis research program to finding geothermal advantages through energy market and energy policy analysis.

Moving forward, federal investments in big and open data, public/private partnerships and cross-cutting initiatives provide an integrative approach to energy generation with benefits going beyond EGS and geothermal energy. The Administration’s efforts through GTO specifically have tremendous applications for conventional energy, including knowledge gains in reservoir characterization and extraction techniques, and induced seismicity.

Based on increasingly interdisciplinary geosciences research, such knowledge could hold major societal implications – and creates extraordinary anticipation for the next groundbreaking discovery.

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Meet Scott Miller

Scott Miller is the spring 2014 American Geosciences Institute/AAPG Geoscience Public Policy intern. He has a bachelor’s degree in geology from Appalachian State University, and before coming to AGI he worked at the Florida State University Antarctic Marine Geology Research Facility in Tallahassee, Fla. He plans to finish graduate school and use his higher degree to promote geosciences education and understanding to the public.

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