Influence of water-rock interaction on permeability and heat conductivity of granite under high temperature and pressure conditions

Abstract

Water-rock interactions exist between the geothermal host rock (GHR) and the cold injected water for Enhanced (or Engineered) Geothermal Systems (EGS), changing permeability and thermal conductivity of GHR, influencing the heat extraction of EGS. Therefore, a water-rock interaction experiment is performed under high temperature and pressure conditions for heat-treated granite. The variation in permeability and thermal conductivity, mineral composition, and solution composition of the rock matrix is consequently analyzed, and their influence on the heat extraction of EGS is discussed. The thermodynamic analysis of the chemical processes of the experiment is presented using batch chemical simulators, PHREEQC 3.0, reproducing the chemical time series of the water-rock reactions. Water-rock reaction changes the composition of the rock and the circulating fluid. For primary minerals, albite, anorthite, and k-feldspar dissolve, biotite tends to be stable, and quartz gradually increases. Secondary minerals, muscovite, calcite, kaolinite, and chlorite, are formed in our simulation results, and their source and generation mechanisms are discussed. However, the kaolinite and chlorite were not detected in the experiment due to their low quantity. Both the dissolution of primary mineral and the formation of secondary mineral change the permeability and the thermal conductivity of the granite. The permeability exhibits a transition from increasing to decreasing while the thermal conductivity shows an opposite variation. Both approach constant values but only at 60% of the initial value. However, from the long-term perspective, the change in permeability and thermal conductivity of the granitic host rock around the fracture will not exert a strong impact on the heat extraction of EGS. The test results in this context could facilitate our understanding of permeability characteristics and the evolution of thermal conductivity of a granitic host rock during the water-rock interaction, and their influence on the heat extraction of EGS during the operation process.