Unconventional Resources (Jan 2022)
Dissolution and sedimentation patterns of typical minerals in artificial reservoirs under different environments
Abstract
The injection of circulating fluids (usually water) is the predominant means of heat extraction in an enhanced geothermal system (EGS); however, the additional injection of circulating fluids can cause a water-rock reaction with a dry thermal rock mass, resulting in the generation of a large number of secondary minerals. These secondary minerals cause fracture closure in artificial reservoirs and severely impede the sustainable use of these geothermal systems. Therefore, this study combined laboratory tests with a PHREEQC hydrogeochemical simulation. First, the changes and mechanisms of mineral and aqueous chemical fractions in the artificial reservoir fractures were analyzed after using groundwater from the sandstone aquifer at the Frontier Observatory for Geothermal Energy Research (FORGE) site as a heat transfer fluid in the FORGE project. Second, the dissolution patterns and precipitation of minerals commonly found in artificial fractures were simulated in four different environments with the aid of PHREEQC after completing transconversion. Finally, the effects of different fluids were evaluated for the unblocking of typical minerals. The results show that as a heat transfer fluid, groundwater causes the dissolution of K-feldspar, albite, and illite, partial decomposition of kaolinite and proto-silicate, rapid production of anorthite, decomposition of dolomite, sodization of Ca-montmorillonite, and dolomitization of black mica. Mud acid was more effective in unblocking fissure closures caused by anorthite, chlorite, and kaolinite. NaOH was more effective in unblocking fissure closures caused by illite and Ca-montmorillonite. Groundwater is not suitable as a blockage remover and can easily produce secondary mineral blockages (calcite and aragonite). The effect of ultrapure water is relatively stable; it can be used as a heat exchange fluid or blockage remover, but when used as a remover, it behaves primarily as a physical-mechanical action process.
