Injection of supercritical CO2 for geothermal exploitation from single- and dual-continuum reservoirs: Heat mining performance and salt precipitation effect

The geothermal exploitations from the single-continuum (i.e. sandstone here) and dual-continuum (i.e. hot dry rock here) reservoirs using CO2 have unique advantages and huge application potentials. However, the salt precipitation caused by the evaporation can damage the reservoir and reduce the heat mining rate. In this paper, two comprehensive simulation models are established to analyze the fluid flow, salt precipitation, and its effects on the reservoir properties and heat mining rate during the geothermal exploitation using CO2 from the sandstone and hot dry rock reservoirs. For the sandstone reservoir, the formation water will flow back to the injection well under the actions of the gravity and the gas-liquid capillary pressure driven by evaporation, which can result in a lot of NaCl precipitating in the injection well. The salt precipitation in the sandstone reservoir can make the steady heat mining rates decrease by about 1/3. For the hot dry rock reservoir, the formation water in the matrix system can be drawn to the fracture through the fracture-matrix interface due to the capillary pressure, which complicates the distribution of the salt precipitation in the fracture system. Under the actions of the back flow in the fracture and flow exchange between fracture and matrix, the salt precipitation mainly occurs in the areas near the injection and production wells, which makes the steady heat mining rate decrease by about 1/2. Both the low-salinity water injections prior the CO2 injection and after the salt precipitation occurred can effectively reduce the salt precipitation for the sandstone, but only the low-salinity water injection after the salt precipitation occurred can dissolve the precipitated salt and effectively recover the heat mining rate.