Researchers at the University of Oklahoma (OU) recently conducted an unprecedented rock fracturing experiment that showcased the potential for enhanced geothermal systems (EGS) as a viable power source for the United States. Supported by funding from the Energy Department’s Geothermal Technologies Office (GTO), Dr. Ahmad Ghassemi and his research team of graduate students successfully fractured and stimulated a series of 13-cubic-inch blocks of granite and other igneous rocks found in EGS reservoirs. The OU team was able to extract more than 50 watts of power production from the small rocks, which is equivalent to a household lightbulb, showing promise that field-scale fractures and a properly engineered EGS can produce considerable amounts of power.
Much of the OU team’s work was made possible by the university’s unique laboratory capabilities, which includes newly designed geomechanical testing equipment sponsored by GTO. The equipment allows researchers to hold blocks of rock in place in order to subject them to extreme environments similar to actual EGS reservoirs without the need to drill expensive test wells that are thousands of feet deep. The testing equipment has the capability to simulate pressures and temperatures approaching 70 megapascal (MPa) and 100°C, respectively – a significant achievement at laboratory-scale.
An elusive but important measure for evaluating EGS reservoir sustainability is the ability to accurately map fracture propagation. The testing equipment allowed the OU team to measure seismic emissions during each month-long experiment, and proved effective in identifying the exact location of rock breakdown over time. When combined with self-potential – a method used to measure conductive fluid content in rocks – the research team was able to map the direction and extent of permeable fracture propagation.
The monitoring of seismic emissions provided the opportunity to manage the circulation of fluids to optimize heat extraction. The OU team tailored injection rates and production pressures to more evenly distribute flow through the rock to avoid fluid channeling, which is caused by circulating fluid too rapidly. This heat extraction method provides an important consideration for future field-scale efforts such as at the Field Observatory for Research in Geothermal Energy (FORGE) – a first-of-its-kind dedicated site to develop, test, and accelerate breakthroughs in EGS technologies and techniques and where experimentation similar to the OU team’s will take place on a much larger scale, at 8,000-10,000 foot depth ranges.
Investing in EGS technologies could eventually lead to more than 100 gigawatts of economically viable electric generating capacity, enough to power 100 million U.S. homes. Efforts to accelerate EGS development such as those from the OU team will help diversify the domestic energy portfolio, enhance energy access, and increase energy security.