TOUGH2Biot: A Coupled Thermal-Hydrodynamic-Mechanical Model for Geothermal Development

摘要

Geothermal development and exploitation involve coupled thermal-hydrodynamic-mechanical (THM) processes. Amechani-cal module based on the extended Biot consolidation model is developed and incorporated into the well-established thermal-hydrodynamic simulator T0UGH2, resulting in an integral numerical THM simulation program TOUGH2Biot. A finite element method is employed to discretize space for rock mechanical calculation and the Mohr-Coulomb failure criterion is used to determine if rock undergoes shear-slip failure. Mechanics is partly coupled with the thermal-hydrodynamic processes and gives feedback to flow through stress-dependent porosity and permeability. TOUGH2Biot is verified against an analytical solution for the 1D Terzaghi consolidation and cooling-induced subsidence. TOUGH2Biot has been applied to evaluate thermal, hydrodynamic and mechanical responses of geothermal exploitation at the Geysers geothermal field, California. The results demonstrate that change in effective stress due to temperature decrease is greater than that due to pressure decrease near the production. The most likely shear-slip occurs near the injection because of an increase in shear stress. The formations are compressed and the maximum vertical displacement is up to 0.6m after 40 years of production, half of which is contributed from temperature decline. At the same time, TOUGH2Biot is proved to have the capability of analyzing change in pressure and temperature, displacement, stress and potential shear-slip failure for geothermal exploitation.