Assessment of CO2 Injectivity in Highly Deformable Chalk Reservoirs: A Thermo-Hydro-Mechanical Analysis

ABSTRACT Depleted hydrocarbon chalk reservoirs are attractive candidates for CO2 storage in Denmark. However, their highly deformable nature presents challenges for CO2 injectivity and storage. This study investigates the potential for thermal effects on reservoir and caprock integrity caused by a 16 year-long supercritical cold CO2 injection in a depleted gas reservoir using a thermo-hydro-mechanical simulation. We focus on quantifying the additional stresses and strains induced by the thermal expansion and contraction of the rocks and examine whether these changes could trigger fracturing/fault reactivation in the chalk and/or the overlaying shale deposits. To address convergence issues in non-isothermal flow modeling, we assume that the density and viscosity of CO2 remain constant with temperature change in the coupled simulation. Our results demonstrate that, with constant rate injection of CO2, temperature propagation is limited to a short distance around the well, and CO2 only cools down the temperature around the injection site. This study provides insights into the feasibility of cold CO2 injection in hot depleted hydrocarbon reservoirs and highlights the importance of considering thermal effects on faults and fractures behavior. INTRODUCTION Carbon Capture and Storage (CCS) can help reduce greenhouse gas emissions while transitioning to renewable energy (IPCC, 2005). Depleted oil and gas reservoirs offer significant potential for CO2 storage (Bonto et al., 2021), using natural subsurface storage space that previously contained hydrocarbons for millions of years and reusing existing infrastructure, such as wells, platforms and pipelines (Amour, Hajiabadi, Hosseinzadehsadati, et al., 2022). Storing CO2 securely underground in depleted oil and gas fields is critical, and the success of this approach depends on ensuring geomechanical stability during CO2 injection, which can have significant implications for cap rock integrity and fault reactivation (Akhurst et al., 2015). Injecting cold CO2 may cause thermal contraction of the reservoir matrix and unload normal compression on existing fractures and faults (Salimzadeh, Paluszny, & Zimmerman, 2018; Vilarrasa & Rutqvist, 2017). This process can lead to an increase in the aperture of impermeable faults and fractures, potentially creating new pathways for CO2 leakage and inducing seismic activity (Min et al., 2005; Rutqvist, 2012). Therefore, the injection of large amounts of low-temperature CO2 into hot, depleted hydrocarbon chalk reservoirs may induce a range of complex and strongly coupled thermal, hydraulic, mechanical, and chemical (THMC) processes (Yin et al., 2011) involving interactions among reservoir formation fluids, CO2, and chalk minerals. As a result, accurately identifying the stresses, pressures, and temperature changes around the injection well is essential to obtaining practical information for injection design, thermal fracturing, and leakage prevention.