Numerical simulation of thermal hydrological mechanical chemical processes during CO2 geological sequestration

Abstract

The significance of thermal-hydrological-mechanical-chemical (THMC) interactions is well recognized in the operation of CO2 geo-sequestration. Geo-mechanical and geo-chemical effects may significantly affect aqueous phase composition, porosity and permeability of the formation, which in turn influence flow and transport. The TOUGHREACT simulator (Xu et al. 2004) 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 (Winterfeld & Wu 2012). Based on these simulators, in this research a novel mathematical model of the THMC processes is developed. Two computational frameworks, sequentially coupled and fully coupled, are proposed and used to simulate reactive transport of CO2 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 formed to model the THMC processes. The capabilities of the models are verified by four analytical solutions. Finally, nine reactive transport models with general chemical compositions are presented to analyze the THMC processes quantitatively, especially on the coupled effects of geo-chemical reaction and geo-mechanics on CO2 geo-sequestration process, and the long term fate of CO2 and its sensitivity on mineralogical compositions with respect to key minerals. The THMC models have the previously unavailable capability to simulate the fluid and heat flow, solute transport in aqueous and gaseous phase, mean stress, and geochemical reactions under equilibrium and kinetic conditions. The preliminary findings from the modeling studies are summarized below: geochemical reactions leading to favorable mineral trapping include dissolution of plagioclase feldspar and chlorite minerals, and precipitation of carbonate and silicate minerals. In terms of efficacy for the trapping mechanisms, structural trapping is the dominating mechanism during supercritical CO2 injection period, and mineral trapping dominates during long term storage period. In a typical sandstone formation, the efficacy of mineral trapping increases from zero percent at the early injection period to 65 percent after 10,000 years. The efficacy of mineral trapping increases with the increasing volume fraction of oligoclase, and decreases by 25 percent when there is no oligoclase in the formation. The geochemical reactions do not have significant impact on the mean stress, pressure or temperature, but the thermal transport affects mean stress and geochemical reactions. Cold CO2 injection cools down the formation leading to accelerated CO2 and calcite dissolution. The kinetic reaction rate for anorthite dissolution and kaolinite precipitation are small, and dissolution of anorthite leads to kaolinite precipitation. As a result of large volume injection of CO2, the pore pressure and mean geo-stress increase significantly under the caprock, and this may lead to stability problems. The upward migration of supercritical CO2 speeds up through permeable faults created by permeability and porosity increase. The mineral trapping of supercritical CO2 mainly occurs in the upper caprock saline aquifers.