The use of supercritical CO₂ in deep geothermal reservoirs as a working fluid: Insights from coupled THMC modeling

A coupled THMC (thermal-hydrological-mechanical-chemical) model is developed and applied to explore the potential feasibility of using scCO₂ (supercritical carbon dioxide) as a working fluid in geothermal reservoirs. This is achieved by examining the evolution of the kinetics of mineral precipitation-dissolution and its associated impact on the evolution of the rock permeability and porosity. The pH of the reservoir rapidly reduces from 7 to ∼4.5–5 due to the fast dissolution of calcite. Chemical reactions and mineral dissolution and precipitation near the injector are suppressed by the plug-flow penetration of anhydrous scCO₂ displacing the original pore fluid. A conceptual three-zone model is proposed to illustrate the kinetic process of feldspar dissolution and precipitation depending on timing. The initial high concentration of K⁺ prompts feldspar to precipitate in the first stage by consuming K⁺ until 1y, Feldspar were dissolved into precipitations of illite, smectite, and siderite at 1-6y, with albite, muscovite and kaolinite mostly precipitated in the last stage 6–10y. The precipitations of secondary clay minerals and quartz serve to maintain the integrity of caprock sealing. Continuous scCO₂ injection under fully coupled THMC model shows a 1.4-times enhancement of fracture permeability and 1.2-times enhancement of matrix permeability dominated by chemical dissolution and thermal unloading process. The pronounced thermal drawdown is the principal factor in enhancing permeability and porosity near injection well. Furthermore, the expansive capability of CO₂ provides extra benefits in enhancing formation pressure to ensure consistent high flow rates, while achieving a higher thermal energy extraction efficiency and preventing scaling issues in wellbore. The mass concentration of scCO2 in the production well increased to 0.82 after 1.2 × 10⁸s also leads to the enhancement of fluid enthalpy up to 6.5 × 10⁵ J/kg, due to the high heat capacity of scCO₂. The injected CO₂ are sequestered at ∼2 × 10⁷ kg at t = 2 × 10⁸s (6.34y) as the solubility trapping mechanism.