Coupled THM Analysis of Long-term Cold CO2 Injection: Caprock Stress Evolution and Mechanical Integrity

ABSTRACT: Injection of carbon dioxide (CO2) at temperature lower than the target geological formation alters the reservoir-caprock stresses due to both pressure and temperature changes. The stresses evolve spatially and temporally during long-term injection. Predicting such stress changes is crucial to ensure mechanical integrity of sealing caprock. We implement a robust coupled thermal-hydro-mechanical (THM) formulation based on Cheng's thermo-poroelasticity into a commercial finite difference solver and adopt a 2D axisymmetric model for injection via a single vertical well. This study explores long-term cold CO2 injection (ΔT = -30°C) with special emphasis on exploring the role of caprock permeability on varying drainage responses. Numerical results show that stress redistribution caused by THM response leads to variations in the minimum total horizontal stress (i.e., fracture gradient in normal faulting stress regime) that depend on the extent of the cooling front. In general, total horizontal stresses decrease within the cooling front and increase above the cooling front. Pore pressure in the caprock may reduce significantly as caprock response tends to undrained conditions for ~nanodarcy caprock permeability. The simulations help predict stress changes and dynamic fracture gradients including the effects of temperature for safe and massive geological carbon storage.