NUMERICAL SIMULATION OF COUPLED PROCESSES FOR MULTIPHASE FLOW, ROCK DEFORMATION AND HEAT TRANSFER IN ENHANCED GEOTHERMAL SYSTEMS

Most of geothermal reservoirs are situated in low permeable rocks. Well-connected cracks and fractures provide high permeable fluid-flow path to un-fractured tight rock matrix. Under an external stimulus, the contrast of rate of change of thermodynamics properties between high permeable fractures and tight matrix rocks causes large thermodynamics variations between the two medium. In addition, in EGS reservoirs, natural cracks and fractures are typically scarce. Artificial fractures or hydraulic fractures will be vital to provide additional high permeable flow path for an underground fluid flow and heat exchanger. To efficiently simulate mass and energy transport in a fractured rock, special numerical schemes are necessary.

We developed a reservoir simulator for numerical simulation of fully coupled rock mechanics, fluid flow and heat transfer for Enhanced Geothermal Systems (EGS). The continuum modeling approach is used in the model formulation to simulate multiphase fluid and heat flow, coupled with rock deformation in fractured and porous rock. The simulator is built on the TOUGH2-EOS3 module (Pruess et al. 1999). An EGS reservoir may comprise of different scale of fractures under the coupled effects of multiphase fluid and heat flow and rock deformation. The key capability for an EGS simulator is how to handle fluid and heat flow in such different-scale fractures. In the model, we represent different-scaled fractures or fractured zone using different fracture conceptual models, including (1) single continuum; (2) multiple continuum; (3) discrete fracture model, and (4) hybrid approach. In handling effect of rock deformation, we use a simplified assumption, i.e, the in situ total stress in reservoirs is constant or a function of spatial coordinates only. This assumption may provide a reasonable approximation for flow in a deep formation such as in most oil/gas and geothermal reservoirs.

The numerical scheme is verified against the analytical solution classical one-dimensional consolidation problem presented by Terzaghi (1943). The model was used to run a closed loop circulation of EGS reservoir with one injector and one producer. The calculation results showed that thermal induced stresses is more pronounced than pressure induced stresses especially close to the injector well.