Laboratory-scale observations of geochemical and hydrologic fracture response at sustained geothermal flow conditions

In coordination with the Enhanced Geothermal Systems (EGS) Collab project, fractured crystalline rock samples were flooded with synthetic brines at representative geothermal temperature and stress conditions, to monitor permeability responses in geochemically reactive systems. Oriented fractures were induced in rock cores obtained at target reservoir depths from both a metamorphic phyllite (4850' level, Sanford Underground Research Facility, South Dakota) and a plutonic granodiorite unit (6800' depth, Frontier Observatory for Research in Geothermal Energy (FORGE) site, Milford, Utah). The induced fractures typically consisted of several similarly oriented fracture splays, aligned with foliation in the case of the phyllite and cutting across large grains in the granodiorite. Confining pressures and temperatures were varied from 10-20 MPa and 100-200°C in multiple experiments conducted under constant flowrate conditions. Differential pressure variations indicated changes to bulk permeability, although individual splays experienced opposing responses in aperture. Correlation of X-ray computed tomography gray-scale with mineralogical composition allowed us to spatially correlate regions of enhanced reaction with corresponding mass loads from solution chemistry monitoring. The datasets and observations from this series of smaller-scale laboratory experiments will be used to validate reactive transport models for better prediction of fracture conductivity at the intermediate scales interrogated by the EGS Collab project, or at the potential field scales relevant to the FORGE mission. The EGS Collab Project is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Geothermal Technologies Office. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.