Fractures and other discontinuities, such as bedding planes, and faults, usually act as highly permeable flow paths, dominating subsurface fluid and heat transport, which is of importance in ...developing underground energy resources. We investigate coupled transport and fracture deformation/propagation within the framework of the theory of thermo-poroelasticity. A 3D finite element method is developed and utilized to discretize the governing equations. To simulate the thermo-hydro-mechanical behavior of the fracture/matrix system, a special zero-thickness interface element is implemented based on the cohesive zone model (CZM), to simulate both tensile and shear failure. The fluid flux/heat exchange between the fractures and the surrounding permeable rock matrix is determined by fluid/heat transfer coefficients satisfying mass and energy balance across the interior boundaries, and allowing for temperature and pressure drop across the interface. Numerical analyses are performed to verify the model and to illustrate fundamental phenomena observed in the laboratory. Lab-scale fracturing and circulation experiments are studied in detail, revealing the role of hydro-thermo-mechanical properties and coupled processes.
•A thermo-poro-chemo-mechanical model is developed to study injection and production in EGS.•Undersaturated-cold water injection dissolves silica from rock matrix, increasing the fracture ...aperture.•Natural fracture stiffness heterogeneity develops a non-uniform flow path within the fracture.•Higher joint normal stiffness areas show lower aperture increases by poro-thermoelastic processes, but a higher aperture expansion by dissolution.
Coupled thermo-poro-chemo-mechanical processes in geothermal systems impact the reservoir response during injection and production procedures by affecting fracture permeability. A three-dimensional numerical model is presented to analyze these processes during fluid injection into geothermal reservoirs. The solid mechanics aspect of the problem is computed using the displacement discontinuity boundary element method (BEM) while transport processes within the facture are modeled using the finite element method (FEM). The FEM and BEM formulations are integrated to set up a system of equations for unknown temperature, pressure, concentration, and fracture aperture. The fluid diffusion, heat conduction and solute diffusion in the reservoir are treated using BEM so that the need of infinite reservoir domain discretization is eliminated. The numerical model is used to analyze the fracture response to non-isothermal reactive flow in EGS. Numerical examples of SiO2 undersaturated-cold water injection into the geothermal reservoir show that silica dissolves from the rock matrix, increasing the fracture aperture. The zone of silica dissolution spreads into the fracture with continuous fluid injection. At large injection times, thermoelastic stress has a greater impact on fracture aperture compared to poroelastic stress. Simulations that consider natural fracture stiffness heterogeneity show the development of a non-uniform flow path within the crack, with lower rock matrix cooling and thus enhanced silica reactivity in the high stiffness regions. As a result, areas of higher joint normal stiffness show lower aperture increases in response to the thermo-poroelastic processes, but a higher aperture expansion due to silica dissolution. Depending on the injectate saturation state with respect to quartz, silica is added or removed from the rock matrix. This process is likely to impact the rock matrix properties and its mechanical response to stress perturbations associated with fluid circulation.
The complex geological and formation characteristics such as in-situ stress, nonlinearity, and rock discontinuities make three-dimensional hydraulic fracturing problems very difficult to be ...simulated. In this context, we present a numerical framework to predict the propagation pattern of hydraulic and natural fracture interaction by incorporating continuum damage mechanics in a poroelastic finite element method (FEM) model. The three-dimensional element partition method (3D EPM) is introduced to represent the mechanical behavior of fracture surfaces including contact and friction of existing fractures. Taking advantage of the efficiency and simplicity of 3D EPM, the fracture mechanical response and moving boundary conditions in the hydraulic fracturing process are represented without remeshing. The 3D EPM fracture-fluid coupling model is then employed to simulate fracture interaction problems with complex geometry and boundary conditions. A series of synthetic 3D examples are presented to emphasize the response of single and multiple natural fractures under given in-situ stress. The fracture propagation patterns and fluid pressure distributions obtained by the numerical results indicate that the present model is capable to predict and help to understand the complex processes of fracturing and interaction in unconventional reservoirs. For complex three-dimensional problems, our model can be applied to estimate the fracture pattern, then more rigorous coupled models can be used to obtain more accurate results based on the obtained fracture pattern.
Highlights
The coupled models for fluid-driven fracturing.
Modeling the patterns of fluid-driven fracturing without remeshing.
A solution for complex fracture interaction problems.
This study considers three-dimensional (3D) analyses of a fracture network in an enhanced geothermal system (EGS) with special emphasis on the role of coupled thermo-hydro-mechanical processes and ...fractures mechanical interactions. The behavior of the system is modeled by coupling a thermo-poroelastic displacement discontinuity (DD) method (for fracture opening and ride, fluid and heat diffusion in the reservoir matrix) with a finite element method for the fluid and heat convection and conduction inside the fractures. The nonlinear characteristics of the fracture deformation in the normal (change of fracture status from joint fracture to hydraulic fracture) and shear deformation (change of fracture status from stick to slip) are taken into account. The resulting method is then used to simulate relatively short term injection/extraction processes into/from a synthetic fracture network consisting of a major fracture intersected by a set of smaller natural fractures. Injection/extraction into/from the fracture network results in gradual shearing of the fractures that impact the thermo-hydro-mechanical characteristics of the fracture system. It is also shown that the early micro-seismic events are associated with fracture slip on connected fractures due to thermal perturbation. Also, continued injection leads to stress intensity conditions favorable for fracture propagation in shear and tensile modes which could increase reservoir surface area and further contribute to seismicity.
•Thermo-poromechanical coupling is shown to have implications for reservoir stimulation and induced seismicity.•These couplings cause the reservoir to experience a series of stress perturbations with ...injection.•An initial destabilizing stage is followed by a stabilizing stage, after which another destabilizing stress regime is reached; each potentially impacting the MEQ activity.•Consideration of only injected volume is not adequate for understanding the spatio-temporal distribution of injection related MEQ.•The impact of thermo-poroelastic stresses on injection/extraction profiles in a fractured rock has been studied.
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.
Rock mechanics and geomechanical studies can provide crucial information for economic geothermal reservoir development. Although significant progress has been made in reservoir geomechanics, ...technical challenges specific to the geothermal area (high temps, data collection, experimentation issues) have prevented widespread use of geomechanics in geothermal reservoir development. However, as the geothermal industry moves to develop more challenging resources using the concept of enhanced geothermal systems (EGS), and to maximize productivity from conventional resources, the need for improved understanding of geomechanical issues and developing specific technologies for geothermal reservoirs has become critical. Rock mechanics research and improved technologies can impact areas related to in-situ stress characterization, initiation and propagation of artificial and natural fractures, and the effects of coupled hydro-thermo-chemo-mechanical processes on fracture permeability and induced seismicity. Rock mechanics/geomechanics research, including experimental and theoretical investigations as well as numerical and analytical solutions, has an important role in optimizing reservoir design and heat extraction strategies for sustainable geothermal energy development. A number of major areas where rock mechanics research can facilitate geothermal systems development are reviewed in this paper with particular emphasis on EGS design and management.
In most simulations of gas production from shale reservoirs, only the elastic deformation of reservoir rock and fractures is modeled. However, many experimental studies and field investigations ...indicate shale experiences viscoelastic deformation. In this work, a numerical model is constructed by implementing a poro-viscoelastic model into a dual permeability model (DPM) by using finite element method (FEM), to investigate the coupled time-dependent viscoelastic deformation of shale, and fracture permeability evolution in response to compressible flow of gas and gas desorption. The viscoelastic effect is considered through both deviatoric and mean effective stresses to allow for the effect of shear strain localization. The geomechanical model is first verified against available analytical and numerical solutions. Then, the model is applied to a few synthetic production cases to investigate the geomechanical evolution of a fractured reservoir. Comparing the case of poroelasticity and poro-viscoelasticity shows that the pore pressure differences throughout the domain are small, however, the stress evolution is quite divergent, with higher fracture closure in the poro-viscoelastic case. Comparison of the cumulative gas productions for poroelastic and poro-viscoelastic cases shows that cumulative gas production predicted by the poro-viscoelastic case is always lower than that of the poroelastic one. The difference between the two cases increases for long production times, as the viscous deformation (creep) of the reservoir rock closes the fracture. Results of the numerical simulations suggest that viscous deformation-enhanced closure of natural fractures that feed the main propped fractures can have a critical role in production decline.
•A viscoelastic gas shale reservoir model based on the dual permeability concept is developed.•Normal and shear deformation of fractures are considered.•Simulation of several gas production cases show the viscoelastic effect on pore pressure is small.•Viscoelastic effect on the stress evolution and hence fracture deformation is noticeable.•The permeability of the fracture is very sensitive to the creep behavior of the reservoir rock.
Lab-scale cold water circulation test results and analysis are presented as a proxy for field-scale heat mining in enhanced/engineered geothermal systems. The cold water circulation tests were ...conducted in hydraulically stimulated igneous rock blocks heated to a uniform temperature. The test blocks had a five-spot system with one injection well at the center and four producers drilled about 9 cm away from it. Two kinds of igneous rocks (granite and gabbro) with different permeability were tested. Due to the heterogeneity of the rock and the notch at the injection well, different fracture geometries with different number of producers connected by the hydraulic fracture were obtained. The circulation tests illustrate for the first time a number of interesting phenomena in a laboratory setting. The great potential of geothermal energy has been demonstrated by nearly 50W of power produced by the cold water flowing through a hydraulic fracture in a mini-EGS reservoir, yielding a good heat extraction percentage of 6.24%~7.85% after 2000 ml of cold water injection. Also, the cooling effect on the injection pressure and thus the fracture aperture has been clearly observed in the experiments i.e., with continued cold water injection the pressure drops, indicating fracture aperture increase due to thermal stress. In addition, reservoir growth during circulation has been observed based on the AE record and production data. Production well competition has also been observed when multiple producers were connected to the injection well by the induced hydraulic fracture, emphasizing the necessity of proper field management to maximize heat production. The test result indicates that a more “complex” flow path enhances heat production, and early thermal breakthrough and undesirable fracture propagation can be avoided by close monitoring of AE, pressure, and production responses.
One of the most significant characteristics of unconventional petroleum bearing formations is their heterogeneity, which affects the stress distribution, hydraulic fracture propagation and also fluid ...flow. This study focuses on the stress and pore pressure redistributions during hydraulic stimulation in a heterogeneous poroelastic rock. Lognormal random distributions of Young’s modulus and permeability are generated to simulate the heterogeneous distributions of material properties. A 3D fully coupled poroelastic model based on the finite element method is presented utilizing a displacement–pressure formulation. In order to verify the model, numerical results are compared with analytical solutions showing excellent agreements. The effects of heterogeneities on stress and pore pressure distributions around a penny-shaped fracture in poroelastic rock are then analyzed. Results indicate that the stress and pore pressure distributions are more complex in a heterogeneous reservoir than in a homogeneous one. The spatial extent of stress reorientation during hydraulic stimulations is a function of time and is continuously changing due to the diffusion of pore pressure in the heterogeneous system. In contrast to the stress distributions in homogeneous media, irregular distributions of stresses and pore pressure are observed. Due to the change of material properties, shear stresses and nonuniform deformations are generated. The induced shear stresses in heterogeneous rock cause the initial horizontal principal stresses to rotate out of horizontal planes.
