A NOVEL FULLY COUPLED GEOMECHANICAL MODEL FOR CO2 SEQUESTRATION IN FRACTURED AND POROUS BRINE AQUIFERS

Numerical simulation of thermal-hydrologic-mechanical (THM) processes in fractured and porous media can be applied to solving practical problems in many areas including CO2 sequestration in saline aquifers. THM simulators are based on Darcy's law for multiphase flow with conservation of mass, Biot's theory of poroelasticity (extended to fractured media) with conservation of momentum, and Fourier's law of heat conduction with conservation of energy. One approach to simulating THM processes in reservoirs is fully coupled, where flow (pore pressure and saturations) and geomechanical variables (stresses and displacements), and temperature are solved simultaneously. This approach was taken by Chin et al. and Osario et al., but both formulations were isothermal and single phase. This paper presents a fully coupled, fully implicit THM simulator. The geomechanical equations relating stresses and displacements are combined to yield an equation for mean stress as a function of pore pressure and temperature. The multiphase and heat flow formulation is that for TOUGH2, the starting point for our simulator, and we add the mean stress equation (with mean stress as an additional primary variable) to that formulation. In addition, theories of poroelasticity and experimental studies have correlated porosity and permeability to effective stress (the difference between mean stress and pore pressure); we incorporate those dependencies into our simulator as well. The simulator formulation and numerical implementation are verified using analytical solutions and example problems from the literature. We simulated a double porosity onedimensional consolidation problem that has as analytical solution. Problems from the literature include simulation of CO2 sequestration in a hypothetical aquifer-caprock system and CO2 storage in a gas field.

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