EGS Rock Reactions with SCCO2 Saturated with Water and Water Saturated with SCCO2

Abstract

EGS using CO2 as a working fluid will likely involve hydro-shearing low-permeability hot rock reservoirs with a water solution. After that process, the fractures will be flushed with CO2 that is maintained under supercritical conditions (> 70 bars). Much of the injected water in the main fracture will be flushed out with the initial CO2 injection; however side fractures, micro fractures, and the lower portion of the fracture will contain connate water that will interact with the rock and the injected CO2. Dissolution/precipitation reactions in the resulting scco2/brine/rock systems can significantly alter reservoir permeability, so it is important to understand where these precipitates form and how are they related to the evolving ‘free’ connate water in the system. To examine dissolution / precipitation behavior in such systems over time, we have conducted non-stirred batch experiments in the laboratory with pure minerals, sandstone, and basalt coupons with brine solution spiked with MnCl2 and SCCO2. The coupons are exposed to liquid water saturated with SCCO2 and extend above the water surface allowing the upper portion of the coupons to be exposed to SCCO2 saturated with water. The coupons were subsequently analyzed using SEM to determine the extent of reactions in both in and out of the liquid water. Results of these will be summarized with regard to significance for EGS with CO2 as a working fluid.