Numerical analysis on the effects of hydrothermal development on the permeability of porous reservoir based on coupled THMC modeling

Abstract

As a clean and renewable energy, geothermal is gaining widespread attention worldwide. The development of hydrothermal resources is accompanied by the coupling processes of thermo-hydro-mechanical-chemical (THMC) caused by production and injection, which have a complex effect on the porosity and permeability of reservoir. Exploring the effects of these processes on the changes in various physical fields within the reservoir is of great significance for the efficient and sustainable development of geothermal energy. Based on the fully coupled framework of fluid flow and transfer in TOUGH2, a method that considered both mechanical and chemical processes was proposed and taken in this study, to analyze the effects of long-term injection and production on reservoir porosity and permeability. The results show that the effective stress changes under fluid pressure and thermal stress, as well as mineral dissolution and precipitation under chemical action, jointly led to the variation of reservoir porosity and permeability. The overall variation trend was mainly controlled by the mechanical action, as the influence of fluid pressure was more obvious and rapid, which dominated the mechanical changes of the whole reservoir in the early stage of injection (within 3.5 years). With the continuous injection, thermal stress played a more obvious role around the cooling area. By 30 years, the significant change in effective stress has extended to the entire reservoir. In the later stage of operation (after 20 years), as the temperature field of the reservoir gradually stopped changing, porosity began to increase under the dominance of fluid pressure. The effect of chemical action was long-term and lag in time. Mineral precipitation only occurred near the injection well, and the precipitation range and amount increased gradually with the continuous injection. By 30 years, the precipitation of dolomite injected into the grid reached 0.06%.