Numerical simulation of heat mining performance of hot dry rocks with fracture network based on a local thermal non-equilibrium model

Key words:  hot dry rocks  discrete fracture network  thermo-hydro-mechanical coupling  local thermal non-equilibrium  injection-production well pattern

Abstract:

Heat carrying fluids can be injected and circulated in fractured networks formed in hot dry rocks for effective heat mining. In order to simulate the heat transfer process between the rock mass and the working fluid realistically, a local thermal non-equilibrium model was adopted, considering the reservoir as a dual-medium model composed of matrix rock and discrete fractures. The interactions of the fluid flow, heat transfer and rock mechanics during the heat mining process was simulated via a full thermo-hydro-mechanical(THM)coupling model, and the heat mining performance with different inject-product well patterns and operation parameters was investigated. The simulation results show that the adoption of the THM coupling model is necessary and adequate for the simulation of the heat resource mining process, and the heterogeneous distribution of the fractures has an significant influence on the performance of heat mining. Reasonable layout of the injection-production well patterns is conducive to the large-scale utilization and efficient exploitation of the geothermal energy. The thermal expansion coefficient of the rock matrix can affect the thermal stress induced by temperature difference, thus affecting the heat mining performance. Rational selection of injection-production pressure difference is beneficial heat extraction, but an excessive pressure difference can shorten the duration of the process.