COUPLED GEOMECHANICAL AND REACTIVE GEOCHEMICAL SIMULATIONS FOR FLUID AND HEAT FLOW IN ENHANCED GEOTHERMAL RESERVOIRS

摘要

A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are dominated by fracture rock permeability. The evolution of fracture permeability can be made by change in temperature or pressure induced rock deformation and geochemical reaction. Especially in fractured media, the change of fracture apertures due to geomechanical deformation and mineral precipitation/dissolution could have a major impact on reservoir long-term performance. A coupled thermal-hydrological-mechanical-chemical (THMC) model is in general necessary to examine the reservoir behavior in EGS. This paper presents a numerical model, TOUGH2-EGS, for simulating coupled THMC processes in enhanced geothermal reservoirs. This simulator is built by coupling mean stress calculation and reactive geochemistry into the existing framework of TOUGH2 (Pruess et al., 1999), a well-established numerical simulator for geothermal reservoir simulation. The geomechanical model is fully-coupled as mean stress equations, which are solved simultaneously with fluid and heat flow equations. The flow velocity and phase saturations are used for reactive geochemical transport simulation after solution of the flow and heat equations in order to sequentially couple reactive geochemistry at each time step. The fractured medium is represented by multi interacting continua (MINC) model in the simulations. We perform coupled THMC simulations to examine a prototypical EGS reservoir for fracture aperture change at the vicinity of the injection well. The results demonstrate the strong influence of temperature-induced rock deformation effects in the short-term and intermediate- and long-term influence of chemical effects. It is observed that the fracture enhancement by thermal-mechanical effect can be counteracted by the precipitation of minerals, initially dissolved into the low temperature injected water. We conclude that the temperature and chemical composition of injected water can be modified to improve reservoir performance by maintaining or even enhancing fracture network under both geomechanical and reactive geochemical effects.