High-Temperature Laboratory Experiments to Support EGS Stimulations: Permeability Response in Fractured Crystalline Samples

Abstract

Enhanced or engineered geothermal systems (EGS) have the potential to provide increased renewable energy resources through created or improved fluid permeability within hot reservoir rocks. The EGS Collab project is focused on intermediate-scale fracture modeling, creation, and monitoring, in support of enhanced permeability in crystalline rocks. Here we propose a series of laboratory-scale experiments to investigate fracture permeability response to chemical reactions at target geothermal temperatures and stresses. The planned experiments utilize samples from crystalline basement at the two proposed FORGE sites (Fallon, Nevada; and Milford, Utah) as well as phyllite from the rock formation hosting the eight boreholes drilled as part of the EGS Collab project. Several experimental samples have been produced by subsampling the original core in plugs of desirable orientations, subjecting these plugs to fracturing techniques, and subcoring. Early experiments will be conducted at variable temperature (~100, 200°C) and fixed confining stress, while later-stage experiments will investigate the effect of varying confining pressure conditions (10-20 MPa). Each experiment utilizes pre- and post-reaction X-ray computed microtomography imaging, as well as changes in fluid pressure and solution chemistry, to evaluate changes to fracture flowpath resulting from high-temperature reaction of mildly saline fluids with common mineral phases (e.g., feldspars, quartz, biotite, muscovite, calcite). The experimental datasets generated through this series of smaller-scale laboratory experiments will be useful for validating reactive transport models for better prediction of fracture conductivity, and will complement the larger datasets being generated in the EGS Collab project in support of the U.S. DOE’s Frontier Observatory for Research in Geothermal Energy (FORGE) mission.