Water Storage of Water-Based Enhanced Geothermal System Based on a 3D Thermal-Hydrologic-Mechanical Model

The thermal-hydrologic-mechanical (THM) coupled processes in water-based enhanced geothermal system (EGS) greatly influence the heat extraction performance of EGS. Many THM models have been proposed, however, there is a lack of detailed analysis of water storage, which is caused by the increments of reservoir porosity and water density, and the influence of water storage on the heat extraction performance needs to be uncovered. In this paper, a 3D THM model is established to simulate the water storage amount and heat extraction rate for a water-based EGS. The 3D THM model is verified against an analytical solution. Then, the influences of water storage are investigated, and comparisons between the THM and thermal-hydrologic (TH) processes are made for different initial reservoir porosities. The results show that the increment of reservoir porosity has a larger influence on water storage than that of water density. If ignoring water storage, the injection flow rate would be underestimated, while the production flow rate and heat extraction rate would be overestimated, and the reservoir would be cooled a little slower. Compared with the TH processes, the THM processes show larger cumulative water storage amount, higher steady-state heat extraction rate and higher cooling rate of reservoir, indicating that mechanical process has important influences on EGS performances. For higher initial reservoir porosity, the cumulative water storage amount is larger. It can be inferred that the water storage amount is related to the cooling rate of reservoir. The results of this paper show that water storage has a certain influence on the heat extraction rate, and that the mechanical process and initial reservoir porosity have important effects on the water storage amount, which should be simulated based on a THM model.