A fully coupled thermal-hydrological-mechanical-chemical model for CO2 geological sequestration

The importance of thermal-hydrological-mechanical-chemical (THMC) interactions is well recognized in the procedure of CO2 geo-sequestration. Geo-mechanics and geo-chemistry may have significant effects on the aqueous phase composition, porosity and permeability of the formation, which in turn affect flow and transport processes. Using a mean stress formulation, geomechanical effects are considered such as stresses, displacements, and rock deformation in CO2 sequestration. Chemical equilibrium and kinetics are taken into account in the mass balance equation, which is able to quantitatively simulate fluid flow, solute transport and geo-chemical reaction in the operation of CO2 geo-sequestration. Since rock strength decreases with increasing amounts of reactive minerals, substantial dissolution/precipitation of rock composition may lead to significant changes in the mechanical behavior, a generic computational scheme has been developed to take the mechanical and chemical coupling effects into account. Based on these theories, a novel mathematical model of the THMC processes is developed in this paper. A fully 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 in a three-phase mixture, (2) stress and displacement related to mean stress, (3) non-isothermal effects on fluid properties and reaction processes, and (4) the equilibrium and kinetics of water-rock and gas-rock chemical interactions. A practical reactive transport examples with cold supercritical CO2 injection into saline aquifer has been proposed to analyze the THMC processes quantitatively. It is indicated that the geochemical reactions do not have significant impact on pore pressure, mean stress and temperature. The thermal energy transport with low temperature significantly affects the mean stress and geochemical reactions in the saline aquifer. Low temperature accelerates equilibrium dissolution of gas and mineral, but slows down kinetic dissolution/precipitation of minerals, such as anorthite and kaolinite.