Enhanced Geothermal Systems (EGS): Hydraulic fracturing in a thermo-poroelastic framework

The natural permeability of geothermal reservoirs is typically low and therefore needs to be enhanced to enable efficient use and economic viability. Hydraulic Fracturing (HF) methods, often referred to as hydraulic stimulation, are one of the primary methods used for permeability enhancement. In this research paper, physical processes associated with HF are simulated within a thermo-poroelastic (THM) framework. A fracturing model is introduced as a novel component to the THM framework. This fracturing model is expressed in terms of Terzaghi's effective stress and it governs fracture length and aperture evolution in all directions of space. The distribution and properties of new fractures are used in a parametric analysis to calculate anisotropic fracture-induced permeability tensor at each point in the computational grid. Mechanical, hydraulic, and thermal effects interact in different ways at the vicinity of the inlet and of the outlet. Our results show that thermal cooling primarily contributes to the fracturing process within a zone close to the inlet. In addition, injection pressure and/or flow rate must be increased during the lifetime of the reservoir to compensate for pressure drop caused by increased aperture due to fracture growth. We tested our fracturing model at the laboratory scale using observations from a homogeneous triaxial test. The model is then integrated into an in-house FORTRAN 90 finite element code that solves the thermo-poroelastic boundary value problem. Numerical simulations for fluid circulation and thermal recovery at the geothermal reservoir of Soultz-sous-Forets are performed using the fully integrated code. As expected, at early times heat convection controls the spatial progression of the effective stresses and hydraulic permeability from inlet to outlet. However, the enhancement of permeability due to HF causes a rapid propagation of the heat front in the reservoir. Therefore, the applied injection flow rate effectively controls the evolution of the hydraulic permeability at moderate-to-long times of the HF process. The borehole stability against shear failure is also investigated. (C) 2016 Elsevier B.V. All rights reserved.