Geothermal energy production requires sufficient temperature at depth, a working fluid to transport heat from the reservoir to the surface, and a network of fracture pathways that connect injection and production wells. These requirements constrain the locations where geothermal development is viable. Enhanced Geothermal System (EGS) development seeks to expand geothermal energy production to areas that do not possess an existing fracture network. Hydraulic and thermal fracturing techniques can artificially generate and maintain a network of interconnected fracture pathways within a geothermal reservoir. Hydraulic stimulation improves the thermal deliverability of the geothermal reservoir by opening up new and/or existing fracture pathways in the reservoir. Subsequently, injection of sub-reservoir temperature fluid, typically plant injectate, causes thermal contraction of the rocks adjacent to these fracture pathways maintaining, and in some cases, improving the conductive pathways generated through hydraulic stimulation.
EGI’s unparalleled expertise and experience in geothermal science and project management has been instrumental to the successful EGS demonstration project at the Raft River geothermal field. The Raft River geothermal field and Department of Energy enhanced geothermal system test site is located in Cassia county Idaho roughly 100 miles northwest of Salt Lake City on the Utah-Idaho border. Since 2010, the $10.2 million test project has been led by Dr. Joseph Moore and the EGI Geothermal Group.
About 5,000 gpm of geothermal water is produced from four production wells, generating 11 MWe of power before being reinjected back into the reservoir through four injection wells. The objective of the project was to take a sub-commercial injection well, RRG-9 ST1, and hydraulically and thermally stimulate it to improve its injectivity to commercial levels.
EGI has conducted extensive analyses of geologic data, water chemistry, microseismic activity, borehole imaging, and numerical modeling to characterize the reservoir and infer fluid pathways within the geothermal system. The wells at Raft River encountered 5,000 ft. of Quaternary and Tertiary volcanic and volcanoclastic rocks before their completion in the Precambrian basement. The Elba quartzite, located in Precambrian rocks, is the primary geothermal reservoir with an average resource temperature of 300 °F. Analysis of water chemistry indicates that the field is bisected into two regions separated by a shear fault termed the Narrows Zone, as described by Ayling & Moore (2013). The Narrows zone strikes to the northeast through the middle of the field. Microseismic activity attributed to plant activity suggest that while acting as a barrier between the two regions the Narrows zone allows for fluid movement along its length. The RRG-9 ST1 wellbore was imaged shortly after completion leading to the identification of 82 intersecting fractures. Among the identified fractures, a fracture zone located between 5,640 and 5,660 ft. MD and striking to the northeast is permeable. Temperature data indicates that this zone has nominally accepted all of the fluid injected into RRG-9 ST1. The rocks in Precambrian basement have very low porosities, and fluid flow in the system is highly dependent on fracture conductivity. Temperature and microseismic data suggest that fluid injected into RRG-9 ST1 passes through the intersecting fracture zone, connecting into the Narrows Zone, and then moves along its length to the northeast.
Since 2012, test well RRG-9 ST1 has been successfully stimulated hydraulically and thermally. Shortly after the completion of drilling, RRG-9 ST1 was hydraulically stimulated in February 2012. Injection flow rates were increased in a step-wise fashion from 11 gpm to 760 gpm at a wellhead pressure of 1,150 psi. The well was shut in for a year and a half while a 10-inch pipeline was constructed between the plant and the RRG-9 ST1 wellhead. Injection of sub-reservoir temperature fluid from the plant began in June 2013. The well accepted less than 20 gpm. Then, between late August and early September 2013 the well was hydraulically stimulated to improve the poor injectivity. Agricultural pumps were used to increase injection rates up to 330 gpm during the course of the stimulation. Injection resumed through the 10-inch line following the hydraulic stimulation. This second stimulation improved flow rates from 50 gpm to 120 gpm. Injection was maintained through March 2014 with little improvement in the injectivity. A third hydraulic stimulation was conducted in April 2014 using pump trucks to pump fluid up to injection rates of 1,260 gpm at a wellhead pressure of 980 psi. Following the third hydraulic stimulation, injection through the 10-inch resumed with injection maintained on a nearly continuous basis. Unlike the previous hydraulic stimulation, the injectivity of the well began to improve continuously. Flow rates improved from 120 gpm prior to the third hydraulic stimulation to roughly 550 gpm in April 2015. In April and August 2015 the well was shut in to conduct pressure fall off testing. When the well was brought back online the rate of injectivity improvement increased. Currently the well nominally accepts 960 gpm at an injection pressure of 190 psi.
RRG-9 ST1 is now in commercial use at the Raft River geothermal plant.
EGI seeks to replicate the success seen at Raft River at the U.S. Department of Energy FORGE Utah site near Milford, Utah. FORGE— the Frontier Observatory for Research in Geothermal Energy— is a U.S. Department of Energy initiative to help develop EGS techniques and promote viable, clean, domestic sources of energy. The Milford, Utah site is uniquely suited to the task with well-characterized reservoir rocks (granite and granitic gneiss) at > 175 °C, permeability, and depth ranges at 2 to 4 km. The site is located in southwestern Utah roughly 220 miles south of Salt Lake City. Positioned within Utah’s renewable energy corridor, the site is adjacent to the Blundell Geothermal Power Plant, a 300 MW wind farm, and a 200 MW solar power plant. A major transmission line is also nearby.
The geothermal resource at Milford has been well characterized by analysis of existing data from numerous preexisting wells drilled in the vicinity as well as several surveys, including seismic monitoring by the University of Utah. The target geothermal reservoir is located in granite and gneiss metamorphic rocks. Resource temperatures of 350 °F were encountered at a depth of 6,600 ft. and reached roughly 440 °F at 1,300 ft., well within the site requirements issued by the Department of Energy. The site will make an ideal subsurface laboratory to explore different EGS technologies such as hydraulic and thermal stimulation techniques.
The same monitoring techniques that were successfully applied at Raft River will be invaluable at the Milford FORGE site. EGI has over 44 years of experience working on geothermal projects. This experience and the lessons learned at Raft River will be applied to successfully develop the Milford, Utah FORGE site.
EGI Graduate Student Researcher Jacob Bradford earned a Bachelor of Science degree in Chemical Engineering in 2010. He is currently pursuing a Ph.D. in Chemical Engineering from the University of Utah with a research focus in Enhanced Geothermal System (EGS) development. Jacob has worked on the Raft River Project since 2012 and authored this article.
Ayling, B., & Moore, J. N. (2013). Fluid geochemistry at the Raft River geothermal field, Idaho, USA: New data and hydrogeological implications. Geothermics, 116-126.
Williams, P. P., Covington, H. R., & Pierce, K. L. (1982). Cenozoic stratigraphy and tectonic evolution of the Raft River basin, Idaho, in Cenozoic Geology of Idaho. Idaho Bureau of Mines and Geology Bulletin, 26, 491-504.
For more information about geothermal resources throughout Utah, watch this short video highlighting EGI’s Utah FORGE project
Video courtesy of the Utah Governor’s Office of Energy Development.