Investigation of Fracture Permeability Evolution in Phyllite Reservoir Rock Specimen from Blue Mountain Geothermal Field

Abstract

Permeability reduction due to mineral dissolution/precipitation in fractured reservoirs is a major concern in geothermal reservoir operations. In order to investigate the evolution of fracture aperture/permeability caused by fluid-fracture surface interactions, a flow-through test was performed on a phyllite specimen retrieved from DB2 well (depth of 1.26 km) at the Blue Mountain geothermal field, Nevada, USA. Permeability evolution of the fractured phyllite specimen was investigated under different states-of-stress (confining pressure and differential pore pressure) at rock temperature of 130 °C and injected geothermal fluid temperature of 65 °C. Fracture aperture/permeability evolution was analyzed using hydraulic data recorded during the course of the experiment. Pre- and post-test X-ray Micro-CT imaging were performed to investigate the flow-induced changes in fracture aperture. Influent and effluent chemistry were analyzed using inductively-coupled plasma optical emission spectrometry (ICP-OES) to determine mineral dissolution during the test. Results indicated a decline in permeability of the specimen due to fracture closure caused by stress corrosion and geo-chemical interactions between the injected geothermal fluid and fracture surface. ICP-OES analyses of the effluent and image analyses indicated that the decrease in fracture aperture was driven by both mechanical and chemical processes.