Modelling of Thermally Induced Fracture Slip Caused by Fluid Injection During Geothermal Production

Abstract

In geothermal reservoirs where fluid is injected during stimulation and production, both hydraulic and thermal effects can cause slip of fractures, resulting in permeability changes and induced seismicity. Upon injection, slip may occur as the effective normal traction is reduced through elevated fluid pressure and/or cooling of the host rock. In general, the relative importance of the pressure and thermal effects is assumed to depend on the rate and temperature of the injection. Direct observation in the subsurface being largely unattainable, numerical simulations may provide valuable insights on the processes mentioned above. However, simulation is also highly demanding, due to the coupling of the thermo-hydro-mechanical-chemical (THMC) processes and their interaction with the fractures. For extensively fractured reservoirs, the 3d geometry of the fracture network causes further challenges. In this work, we focus on the thermal component of the normal traction reduction caused by injection of colder fluid during operation of a reservoir with highly permeable fractures. We model a coupled thermo-mechanical problem and its interaction with fluid flow, explicitly accounting for both the matrix and the fractures using a discrete fracture-matrix model. For the mechanical problem, the fractures act as internal boundaries on which a friction law is imposed. Explicitly representing the most prominent fractures facilitates capturing their critical influence on flow. The model allows for dilation and deformation of these fractures due to slip and THM effects. Hence, it also considers the back-coupling effect of changing apertures on fluid flow and deformation. Thermo-hydraulic stimulation of fractured geothermal systems as a consequence of fluid injection during production are studied through transient 3d simulations, utilizing our recently implemented functionality in the open-source simulation toolbox PorePy.