Experimental Investigation of Brine-CO2 Flow through a Natural Fracture: Permeability Increases with Concurrent Dissolution/Reprecipitation Reactions

Abstract

CO2 has been proposed as an attractive alternative heat-transmission fluid for engineered geothermal systems, but the geochemical and geomechanical impacts of elevated CO2 content on reservoir sustainability are currently not well understood. The potential effects of using CO2 as a heat-exchange fluid on fracture flow and permeability were investigated by means of a 60-day core-flood experiment in which a pre-existing fracture was exposed to CO2-acidified brine at 200 „aC and 25 MPa. Differential pressures decreased continuously, indicating increasing fracture permeability during both CO2-free and high pCO2 fluid flow, and changes in solution chemistry show enhanced mineral dissolution with the introduction of CO2. Fracture aperture and geometry were analyzed by high-resolution X-ray tomography before and after the core-flooding reaction. Changes in solution chemistry were used to identify the likely reactions responsible for dissolution features and porosity increases noted along some fracture regions, as well as the solid precipitates observed in wide-aperture zones. Particle Image Velocimetry analysis was applied to the tomography data to measure local deformation. Experimental and preliminary simulation results are presented with discussion of implications for CO2-EGS.