We develop a coupled thermo-hydro-mechanical (THM) model to study the fluid flow and heat extraction processes in hot fractured vuggy reservoirs consisting of natural fracture network and vug. Fluid flow along fractures and vugs are determined according to the cubic law and Navier-Stokes equation, and an extended Beavers-Joseph-Saffman boundary condition is adopted to couple the porous media-vug interface. Heat exchange between the interface of fracture, vug and matrix are calculated based on local thermal nonequiuilibrium. We implement a fracture constitutive model to capture the variation of fracture apertures due to normal compression-induced closure and shear dislocation-induced dilation. We conduct a series of numerical experiments to systematically analyze how hydraulic properties and heat extraction parameters are affected by the combined effects of geometrical distribution and geomechanical deformation of fracture vug networks. The results show geometrical connectivity of fracture vug networks plays a critical role in dominating the thermo-hydro-mechanical processes of fractured vuggy rocks and the geomechanical deformation of fractured vuggy reservoir exerts a secondary-order influence on the response of hydraulic and thermal performance.
Hot fractured vuggy reservoir consisting of rock matrix, fractures and vugs are naturally existed in the fracture/karst dominated carbonate rock, classified as one of the resource types of geothermal play . Well-developed Fractures (distributed with disconnected or connected form) and vugs (isolated or connected with fractures, varied in size from centimeter to meters in diameter) are existed due to the tectonic movement and paleokarst dissolution effects . Long-term circulation of cold fluid into a fractured vuggy reservoir tends to disturb the hydraulic, thermal and mechanical equilibrium of the reservoir, leading to spatial and temporal variations of fracture transmissivity, due to compression-induced closure and shear-induced dilatancy of rough fractures [3–6]. Meantime, the fractures play a vital role in enhancing the conductivity of low permeable rock mass and increase the efficiency of heat extraction. Therefore, the understanding of fluid flow and heat transport in the context of combined effects of geometrical distribution and geomechanical deformation of fracture vug networks in fractured vuggy geothermal reservoirs is of great importance for optimizing the long-term heat extraction [7, 8]. However, due to the complex geometry of fracture and vug and distinct flow patterns of different reservoir space, it is challenge to conduct numerical simulation of fluid flow and heat transfer in fractured vuggy reservoir with the consideration of geomechanical process.