A thermo-hydro-mechanical simulation on the impact of fracture network connectivity on the production performance of a multi-fracture enhanced geothermal system

Connectivity is an inherent feature of heterogeneous hydraulic conductivity fields and determines the paths of least resistance along which fluid fluxes converge in multi-fracture enhanced geothermal systems (EGS). In this work, numerical algorithms are used to construct fractured porous media models to analyze fractures and the rock matrix in EGS systems. Connectivity coefficients are defined to quantify the connectivity of the fractured system. Based on this, the impacts of fracture connectivity, non-connected fractures, and injection-production pressure differences on EGS production performance are investigated. The research results confirm that the connected area ratio (R_s) is an effective indicator of the connectivity level of the fracture network. Fluid flow and heat exchange predominantly take place within the interconnected area of fractures. When R_s = 0, geothermal energy cannot be efficiently extracted, resulting in significant waste of geothermal resources; when 0 < R_s < 0.1, thermal short-circuit easily occurs in the reservoir; then, as R_s continues to increase, the thermal extraction performance improves. The thermal output power increases with the square of R_s. Dead-end and discontinuous fractures improve the reservoir's fluid mobility, but their contribution to fluid flow is limited. In addition, very high fluid mobility in the matrix of the connected zone weakens the ability of fractures to act as flow channels and triggers a premature thermal breakthrough, shortening the EGS's lifespan. Therefore, striking a balance between the reservoir's lifespan and heat extraction capacity requires precise control of the injection-production pressure difference. Developing a 500 × 500 m² (on a horizontal plane) multi-fracture EGS in deep granite formations, R_s of the fractured zones should be greater than 0.5 to reduce fluid resistance and ensure effective heat extraction. In such a case, it is advisable to maintain a pressure differential of no more than 10 MPa between the injection and production wells to achieve at least 30 years of continuous heat extraction.