(688b) Investigating the Role of Coupled Thermal-Hydrological-Mechanical (THM) Processes During Geological Sequestration of CO2

Geologic sequestration of CO₂ is one of the technologies being considered for mitigating impact of anthropogenic emissions of CO₂. During injection of large quantities of CO₂ into geological formations, the coupled fluid flow, thermal and geomechanical effects are expected to be important. Mechanical failure (fracturing) of the caprock, or to reactivation of existing geological faults due to high injection pressures are some of the potential risks in sequestration reservoirs. Developing effective reservoir management strategies to minimize these risks will require accurate models for the fracture-stress interaction due to variations in temperature and fluid pressure. The equations describing each of the THM processes are complex, involving material properties with highly nonlinear behavior – for example the permeability of fractured rocks can vary by many orders of magnitude over the pressure and temperature ranges of interest. The time spans of interest range from minutes to tens of years, and the spatial scales range from centimeters to kilometers. The need to accommodate the wide range scales also requires the simulator to handle a wide range of pressure and temperature gradients.  To address these issues, a fully coupled nonlinear Thermo-Hydro-Mechanical (THM) model is being developed as part of the Finite Element Heat and Mass Transfer (FEHM) code at the Los Alamos National Laboratory. FEHM is a porous flow simulator that uses continuum mechanics approach with control volume (CV) and finite element (FE) discretization. It can be used to simulate non-isothermal, multiphase, multi-component fluid flow, heat transfer and mechanical processes using either implicit or explicit solution approaches. We have implemented a novel technique for mapping between CV formulation for fluid-flow equations and FE formulation for stress equations to handle stress-dependant variations in permeability. This approach allows for implicit solution of the coupled THM balance equations as well as effective representation of stress-dependent permeability changes. In this presentation, we will describe development of the model and its numerical implementation. We will demonstrate application of the approach using an example of a faulted reservoir. Our numerical studies are focused on understanding how faults/fault gouges behave during large-scale CO₂ injection including coupled fault displacements, potential permeability enhancement due to shear as well as tensile failures and CO₂ migration.