Effects of Rapid Cooling and CO2 Depressurization on the Compressive Strength of Limestone

Abstract

ABSTRACT CO2-injection into depleted reservoirs can lead to rapid cooling and a CO2 phase change in the near-wellbore region, potentially affecting rock mechanical integrity. We report unconfined compressive strength (UCS) and thick-walled cylinder (TWC) collapse strength tests on dry and brine-saturated limestones subjected to cooling (ΔT) or CO2 depressurization (ΔP). Plug samples were prepared from Indiana limestone and a limestone from the Green River Formation, outcropping in Sanpete Valley, Utah, USA. The ΔT-treatment consisted of a thermal shock from 60°C to −78°C, while the ΔP-treatment consisted of three cycles of rapid depressurization from ∼9 MPa CO2 pressure to atmosphere. The ΔP-treatment was performed at room temperature and at 60°C. The results show that dry samples are generally stronger than their brine-saturated counterparts, while effects from ΔT- or ΔP-treatment on rock strength or stiffness are indiscernible. Based on our findings on the Indiana and Green River Formation limestones, we suggest that CO2-injection-induced cooling or depressurization in limestone reservoirs can lead to strengthening of desiccated regions with negligible negative impact on rock mechanical integrity. INTRODUCTION Carbon capture and storage (CCS) is a key strategy on the way to a net-zero CO2-emissions industry, including in carbonate reservoirs, which constitute vast potential storage volumes (e.g., Bonto et al., 2021). However, large-scale implementation of CCS will only be viable if operational risks are understood and mitigated. One of the important geomechanical concerns for geological CO2 sequestration are thermal stress effects due to injection of relatively cold CO2 (Rutqvist, 2012; Roy et al., 2018). Especially in cases where the reservoir (residual) pore pressure is substantially lower than the intended CO2 supply pressure (typically >7 MPa), adiabatic, isenthalpic expansion of CO2 in the near-wellbore region can lead to sharp temperature drops, possibly even reaching below freezing conditions (Oldenburg, 2007; Mathias et al., 2010). Rapid temperature changes can result in thermal strains, fracturing, and, potentially, loss of containment (Pašić et al., 2007; Wang et al., 2022).