Geochemical mechanisms of water/CO2-rock interactions in EGS and its impacts on reservoir properties: A review

Geothermal energy from Hot Dry Rock (HDR) with poor permeability is recognized as a potential future energy source, and it can be exploited by Enhanced Geothermal System (EGS) technology. In this paper, geochemical mechanisms of typical working fluids interacting with the primary rock types of EGS reservoirs, and their impacts on reservoir properties are summarized. While traditional EGS primarily utilizes granite reservoirs, the technology has been gradually extended to include sandstone and carbonate rock formations. Regarding the working fluid, water is currently the only one that has been put into practice. Mineral dissolution and precipitation and salt precipitation are the two dominant geochemical mechanisms during the heat extraction process using water or CO2. These mechanisms are influenced by fluid pH, reservoir temperature, pressure, and flow rate. The dissolution of feldspar minerals plays an important role in increasing permeability and the availability of cations for the precipitation of secondary minerals. Carbonate minerals are often the quickest to respond to changes in fluid chemistry induced by CO2 injection, and the re-precipitation of carbonates is triggered by increasing temperature and pH. It is difficult to predict variations in permeability due to the involvement of many factors, including particle sizes and salt precipitation. The mechanical properties of rocks are significantly weakened following interactions with working fluids. The weakening effect on rock mechanical properties is more pronounced when CO2 is injected into water-saturated formations. Although water-rock interactions can change the thermal conductivity of the rock, the characteristics of fracture networks appear to be of greater importance because working fluids mainly flow through fractures, and maximizing heat extraction from EGS depends strongly on effective circulation. Further economic evaluations for CO2-EGS are necessary to confirm the economic viability of using CO2 as a working fluid. Additionally, the THMCB (Thermo-Hydro-Mechanical-Chemical-Biological) coupling effects during water/CO2 interactions should be comprehensively studied based on both long-term field tests and truly integrated numerical models.