Numerical modeling of shear stimulation in naturally fractured geothermal reservoirs

Shear-dilation-based hydraulic stimulations are conducted to create enhanced geothermal systems (EGS) from low permeable geothermal reservoirs, which are initially not amenable to energy production. Reservoir stimulations are done by injecting low-pressurized fluid into the naturally fractured formations. The injection aims to activate critically stressed fractures by decreasing frictional strength and ultimately cause a shear failure. The shear failure leads to a permanent permeability enhancement of the fractures, which contributes to the overall reservoir permeability, owing to the damage in fracture surface characteristics during the shear failure. Shear stimulation is considered a key for geothermal energy development; however, seismicity is a critical by-product, which has to be controlled. Numerical modeling can provide a deeper understanding on governing mechanisms, which is essential for reservoir assessments and the control of seismicity. The primary goal of this thesis is to aid further development of EGS by contributing to the current state-of-the-art for numerical modeling of shear-dilation-based hydraulic stimulations. Numerical modeling of shear-dilation-based hydraulic stimulations requires mathematical modeling of flow and mechanical deformation in fractured formations. The initial focus of the thesis is the modeling of the mechanical deformation of naturally fractured rock. The deformation and stress state of the rock are controlled by the deformation of pre-existing fractures, which is governed by different equations than the deformation of the surrounding formation. A cell-centered finite-volume approach is developed where the fractures are represented as co-dimension one inclusions in the domain. The method is capable of modeling deformation considering open and closed fractures with complex and nonlinear relationships governing the displacements and tractions at the fracture surfaces. The method aims to provide benefits for studies including flow and deformation couplings in a discontinuous rock. Hydraulic stimulations are essentially coupled hydro-mechanical processes, where the deformation of fractures has an impact on the permeability as well as on the stress state of the rock. We develop a computational model, which has the capability to capture these interrelations in two- or three-dimensional domains. Considering the significance of the pre-existing fractures, we model the reservoir as a network of explicitly represented large-scale fractures immersed in a permeable rock matrix. The model can forecast the permeability evolution of geothermal reservoirs with complex fracture networks. To be able to mitigate the seismic hazards, the contributing processes and the interaction between them should be examined. The computational model developed here also has the capability to investigate the induced seismicity. By using the developed model, a novel hypothesis regarding the induced seismicity generated after the termination of injections has been tested. During the fluid injections, the pressure builds up inside the fractures, which causes normal deformation and increases the void place between the fracture surfaces. The termination of the injections reverses the void increase; i.e., the fracture starts to close owing to the pressure decrease. We identify that the fracture closure is one of the mechanisms that are responsible for the induced seismicity generated after the termination of injections

Thermo-hydro-mechanical Analysis of Fractures and Wellbores in Petroleum/Geothermal Reservoirs

The thesis considers three-dimensional analyses of fractures and wellbores in low-permeability petroleum/geothermal reservoirs, with a special emphasis on the role of coupled thermo-hydro-mechanical processes. Thermoporoelastic displacement discontinuity and stress discontinuity methods are elaborated for infinite media. Furthermore, injection/production-induced mass and heat transport inside fractures are studied by coupling the displacement discontinuity method with the finite element method. The resulting method is then used to simulate problems of interest in wellbores and fractures for related to drilling and stimulation. In the examination of fracture deformation, the nonlinear behavior of discontinuities and the change in status from joint (hydraulically open, mechanically closed) to hydraulic fracture (hydraulically open, mechanically open) are taken into account. Examples are presented to highlight the versatility of the method and the role of thermal and hydraulic effects, three-dimensionality, hydraulic/natural fracture deformation, and induced micro earthquakes. Specifically, injection/extraction operations in enhanced geothermal reservoirs and hydraulic/thermal stimulation of fractured reservoirs are studied and analyzed with reference to induced seismicity. In addition, the fictitious stress method is used to study three-dimensional wellbore stresses in the presence of a weakness plane. It is shown that the coupling of hydro-thermo-mechanical processes plays a very important role in low-permeability reservoirs and should be considered when predicting the behavior of fractures and wellbores