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 (Delta T) and rapid CO2 depressurization (Delta P) 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 Delta T- or Delta P- treatment. The Delta T-treatment consisted of emplacement of samples from 60 degrees C into dry ice (- 78 degrees C), while the Delta P-treatment consisted of three cycles of decompression-recompression between 9 MPa and atmospheric pressure using CO2 as working fluid. Decompression was achieved in similar to 10 s and carried out at background temperatures of 24 degrees C and 60 degrees C. Virgin, Delta T-, and Delta P-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 Delta T- or Delta P-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.