Sequentially coupled THMC model for CO2 geological sequestration into a 2D heterogeneous saline aquifer

Abstract

The significance of thermal-hydrological-mechanical-chemical (THMC) interactions is well recognized in the operation of CO2 geo-sequestration. Geo-mechanical and geochemical effects may significantly affect aqueous phase composition, porosity and permeability of the formation, which in turn influence flow and transport. The TOUGHREACT simulator has the capability to quantitatively simulate fluid flow, solute transport and geochemical reaction in CO2 geo-sequestration using sequential coupling. Using a mean stress formulation, geomechanical effects such as stresses, displacements, and rock deformation in CO2 sequestration have been simulated by the recently developed TOUGH2_CSM. Based on these simulators, in this paper a novel mathematical model of the THMC processes is developed. A sequentially coupled computational framework is proposed and used to simulate reactive transport of water, CO2 gas and species in subsurface formation with geomechanics. The novel frameworks are designed to keep a generalized computational structure for different THMC processes. The coupled THMC simulators focus on: (1) fluid and heat flow, solute transport within a three-phase mixture, (2) stresses and displacements related to the mean stress, (3) non-isothermal effects on fluid properties and reaction processes, and (4) the equilibrium and kinetics of fluid-rock and gas-rock chemical interactions. A set of partial differential equations is presented to model the THMC processes of the fluid and heat flow, solute transport in aqueous and gaseous phase, mean stress, and geochemical reactions under both equilibrium and kinetic conditions. A 2D reactive transport model with complex chemical compositions is presented to analyze the THMC processes quantitatively on the coupled effects of geochemical reaction and geomechanics during CO2 geo-sequestration process. The model is able to analyze the long term fate of CO2 and the efficacy of different trapping mechanism (structural, residual, solubility and mineral trapping) with respect to key minerals. The modeling algorithm can also be used to simulate CO2 EOR and other secondary and EOR processes in petroleum reservoirs.