Induced seismicity
Reactivation of Pre-existing Fractures
Injecting fluid into a geological formation could be more complex than extracting fluid for several reasons: First, injected fluid, whether similar to an original formation fluid (single-phase fluid flow such as wastewater injection and EGS) or a different one (two-phase flow such as CO2 geologic sequestration and EGS with CO2), causes pore-pressure buildup and poroelastic effects. Second, injected fluid (nonisothermal fluid flow such as CO2 geologic sequestration, wastewater injection, and EGS) could retain a different temperature. Poroelastic and thermal stress effects can increase or oppose each other, depending on the condition of the injection/extraction and the bottom-hole temperature of fluids. In particular, injecting fluid colder than an original fluid will cause thermal contraction and corresponding decreases in stresses, which yield an effect opposite of what volume expansion and increases in stresses driven by isothermal fluid injection do.
Reactivation of pre-existing fractures via shear slip (or induced-seismicity) would occur once the state of stresses reaches a failure criterion. Therefore, we can determine the maximum sustainable pressure limit by accounting for factors such as: (1) initial stress state, (2) rock properties, (3) poroelastic effect, and (4) thermal stress effect for the safe operation of CO2 geologic storage.
Hydro-Thermo-Mechanically Couplined Numerical Simulation
Accounting for both poroelastic and thermal effects during fluid flow in a porous medium greatly increases the complexity of the problem. If this type of problem were to be solved analytically, initial and boundary conditions and geometry should be limited to a simple setting. In this regard, numerical simulation is a powerful tool to enable us investigate spatio-temporal evolutions of poroelastic and thermally-induced stresses and corresponding changes in stability when imposing isothermal or nonisothermal cold-fluid injection into a porous medium.
We use a commercial software COMSOL for the simulations of the isothermal or nonisothermal injection of pressurized fluid into a porous medium. The single-phase fluid flow condition can simplify the problem and thus facilitate focusing on thermo- and poro-mechanical effects; yet we are currently extending the problem to multi-phase flow conditions. In COMSOL, we couple physical modules for the fluid flow in porous media, heat transfer in porous media, and poroelasticity. (Names of built-in modules in COMSOL are Poroelasticity, Solid Mechanics, Heat Transfer in Solids, and Multiphysics). For more information, please refer to "Kim, S. and Hosseini, S.A. 2015. Hydro-thermo-mechanical analysis during injection of cold fluid into a geologic formation. International Journal of Rock Mechanics and Mining Sciences, 77, 220-236".
Example of hydro-thermo-mechanically coupled numerical simulation model (Darcy's law in porous media + Solid mechanics + Thermal diffusion in porous media + Thermal stress)
Maximum Sustainable Pressure Limit
Reactivation of preexisting fractures via shear slip (i.e., induced seismicity) is likely to occur, in most cases, prior to shear failures of intact rock or tensile fractures, so it will determine the maximum sustainable pressure limit. Figure below suggests a flow chart for determining maximum sustainable pressure limit - in case of CO2 geologic storage. Please refer to "Kim, S. and Hosseini, S. A. 2014. Geological CO2 storage: Incorporation of pore pressure/stress coupling and thermal effect to determine maximum sustainable pressure limit. Energy Procedia, 63, 3339-3346." for more information in detail.
Flow chart to determine maximum sustainable pressure limit - in case of CO2 geologic storage.