Impact of Rapid Cooling and Depressurization on Limestone Compressive Strength: Implications for CO₂ Injection in Carbonate Reservoirs

CO₂-injection into depleted hydrocarbon reservoirs can lead to rapid cooling and a CO₂ phase change in the near-wellbore region, which may have adverse effects on rock strength. We report on the effects of thermal shock (ΔT) and rapid CO₂ depressurization (ΔP) on the compressive strength of dry and brine-saturated limestones. Plugs were prepared from Indiana Limestone and a partially dolomitized limestone outcropping in Sanpete County, Utah, USA, and subjected to compressive strength tests untreated and after ΔT- or ΔP- treatment. The ΔT-treatment consisted of emplacement of samples from 60 ºC into dry ice (− 78 ºC), while the ΔP-treatment consisted of three cycles of decompression-recompression between 9 MPa and atmospheric pressure using CO₂ as working fluid. Decompression was achieved in ~ 10 s and carried out at background temperatures of 24 ºC and 60 ºC. Virgin, ΔT-, and ΔP-treated samples were subjected to unconfined compressive strength (UCS) and thick-walled cylinder (TWC) collapse strength tests. A comparison between virgin and treated samples is made using the mechanical data, micro-CT scans, and optical microscopy on sectioned trim-ends. Dry samples are generally stronger than their brine-saturated counterparts, but the effects from ΔT- or ΔP-treatment on rock strength or stiffness are insignificant given sample variability. Based on these findings we suggest that CO₂-injection into depleted limestone reservoirs can lead to strengthening of desiccated regions, but with negligible negative impact from thermal shock or CO₂ depressurization on rock mechanical integrity. More research is needed to understand if these conclusions extend to a wider range of carbonate reservoirs.