Geothermal Anywhere

NREL researchers have developed tools and models to simulate various types of AGSs and assess their technical and economic performance for different subsurface, surface, and operating conditions. One such tool is the Slender-Body Theory Reservoir Heat Transfer Simulator (SBT-RHT Simulator). This model is based on the computationally fast slender-body theory, can handle multiple thermally interacting subsurface closed-loop heat exchangers with any geometry, and has been validated against finite element simulations.

The key benefit of this simulator is its ability to perform transient simulations over the reservoir lifetime (e.g., 20–30 years) in a matter of seconds, which allows scientists to quickly evaluate designs and conditions using a Monte Carlo-type approach. Although computationally fast, this simulator does have limitations. It does not account for geomechanical effects, requires uniform and homogenous rock properties, and has limited ability to simulate coupled reservoir fluid convection. But high-performing geometries can be modeled using more complex reservoir modeling tools.

The EGS Collab project, funded by the U.S. Department of Energy and managed by Lawrence Berkeley National Laboratory, seeks to improve understanding of EGSs by studying stimulations in crystalline rock at a 10-m-scale testbed. The Experiment 1 testbed consists of a hydraulically stimulated fracture that intersects several natural fractures and is located at the 4850 level (approximately 1.5 km depth) of the Sanford Underground Research Facility in Lead, South Dakota. One producer and one injector intersect the hydraulic fracture, and six observation wells equipped with sensors allow for real-time monitoring of temperature, seismicity, electrical resistivity, and other parameters. Research focused on interpreting the tracer and thermal response measured within the production well.

NREL analyses have highlighted the potential for coproduction of geothermal with oil and gas, and demonstration sites such as the Rocky Mountain Oilfield Testing Center have also shown this potential. An Estimate of the Near-Term Electricity Generation Potential of Coproduced Water from Active Oil and Gas Wells concludes that up to 362 MW of electricity could be produced with 2012 technology and produced water flow rates. That number has likely increased as power plant technology has advanced and U.S. oil and gas production has increased significantly. However, there are substantial data gaps in the reported amounts and temperatures of water produced from oil and gas wells. Considering the potential, an updated and more in-depth study is warranted.

Additional NREL research into manufacturing and supply chain analyses has shown that widespread manufacturing of small power units could make this type of application more economic. Global Value Chain and Manufacturing Analysis on Geothermal Power Plant Turbines posits that instead of manufacturing fit-to-purpose power units, producing a smaller number of standard designs at larger volumes could decrease costs by up to 60%.

The most attractive geothermal resources have high temperature gradients, low drilling costs, and reservoir permeabilities greater than 10 millidarcies. In An Estimate of the Geothermal Energy Resource in the Major Sedimentary Basins in the United States, NREL researchers estimated the recoverable geothermal from 15 major known sedimentary basins. Prospects, in order of favorability, in Nevada, Utah, Texas, and Colorado exhibit attractive reservoir characteristics and were chosen for further analysis. Resources in Wyoming and Louisiana exhibit lower geothermal gradients and were not evaluated. Reservoir modeling and techno-economic analysis were performed at Mary’s River Basin–North in Nevada.