Dissolution and sedimentation patterns of typical minerals in artificial reservoirs under different environments

Abstract

Injection of circulating fluids (usually water) is the predominant means of heat extraction in enhanced geothermal systems (EGS), but the additional injection of circulating fluids can cause a water-rock reaction with the 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 geothermal systems. So, this paper combines the laboratory test with PHREEQC hydrogeochemical simulation. Firstly, the changes and mechanisms of mineral and aqueous chemical fractions in the artificial reservoir fractures were analysed after using groundwater from the sandstone aquifer at the FORGE site, as a heat transfer fluid, in the FORGE project of USA. Secondly, The patterns of dissolution and precipitation of minerals commonly found in artificial fractures in four different environments were then simulated with the aid of Phreeqc after completion of the transconversion. Finally, the effect of different unblocking fluids on the unblocking of these typical minerals is evaluated. The results show that groundwater as a heat transfer fluid causes dissolution of K-feldspar, albite, illite and partial decomposition of kaolinite and proto-silicate, and is also accompanied by rapid production of anorthite, decomposition of dolomite, sodization of Ca-montmorillonite and dolomitization of black mica. Mud acid will be more effective in unblocking fissure closures caused by anorthite, chlorite and kaolinite. NaOH will be more effective in unblocking fissure closures caused by illite and Ca-montmorillonite. Groundwater is not suitable as a blocking remover and can produce secondary mineral blockages(calcite, aragonite)easily. The effect of ultrapure water is relatively stable, it can be used as a heat exchange fluid or blocking remover, but when used as a blocking remover, it is mainly a physical-mechanical action process.