Thermo-poroelastic effects on reservoir seismicity and permeability change

Abstract

In this paper we study the role of thermo-poromechanical processes on reservoir seismicity and permeability enhancement using theoretical/numerical analysis. The numerical model is fully coupled, considering non-isothermal compressible single-phase fluid flow in fractured porous rock. It combines the thermo-poroelastic displacement discontinuity method, a nonlinear joint deformation model, and a finite difference method for solving the fluid and heat transport in a fracture network. The model is applied to simulate cool water injection into fracture/matrix systems to examine the role of coupled processes on fracture deformation, matrix pore pressure and stress redistributions to assess their role in induced seismicity and permeability variations. The simulation results are analyzed to draw conclusions regarding injection rate dependence of seismicity, and its transience due to coupled processes. Thermal influence on pore pressure and stress tend to promote delayed seismicity. In presence of coupled processes, rock matrix stress perturbations due to natural fracture deformation can be an influencing mechanism for seismicity. Our results show the induced normal stress in the vicinity of the fracture center where injected water enters, can be significant for higher cooling levels in low permeability matrix, and induces additional pore pressure perturbations in the matrix. These couplings have implications for reservoir stimulation and induced seismicity in geothermal reservoirs. The reservoirrock can experience a series of induced stress/pore pressure regimes with continued cooling (under injection). A potentially destabilizing regime is followed by a stabilizing one, and subsequently the rock approaches a destabilizing state. Each situation can result in potentially different levels of MEQ activity. Finally, the impact of thermo-poroelastic stresses on injection/extraction pressure profiles in a fractured rock is illustrated. Injection pressure tends to initially increase in response to poroelastic stress, but with time the thermal effect dominates resulting in fracture aperture increases and lowering of injection pressure.