Effect of thermal shock on the permeability and seismic wave velocity of the caprock and reservoir during CO2 injection

The injection of relatively cold CO2 into warmer reservoirs causes rapid cooling of the host reservoir and the overlying caprock in the near wellbore region. This rapid cooling may cause thermal fracturing, which may potentially lead to escape of the CO2. We have here investigated the effect of thermal fracturing, due to rapid quenching, on the permeability and P-and S-wave velocities of caprock and reservoir samples from the In Salah CO2 storage site, Krechba field, Algeria. Injection of CO2 may have cooled the caprock and reservoir by up to 65 degrees C. Laboratory experiments were conducted where caprock and reservoir samples were heated to various temperatures (50, 150, 250, 350 and 500 degrees C) and rapidly quenched to room temperature (20 degrees C). Permeability and P- and S-wave velocities were measured pre- and post-thermal treatment at effective pressures up to 60 MPa. Backscattered scanning electron microscopy analysis revealed that most of the fracturing is mainly within siderite (Fe carbonate). X-ray diffraction analysis showed that the minerals chlorite (Fe-clay) and siderite were destroyed in specimens heated to 500 degrees C. Specimens that were rapidly quenched by 230 degrees C experienced less than half an order of magnitude increase in permeability and very small to negligible changes in the P- and S-wave velocities. We conclude that the effect of thermal shock fracturing in the Krechba field due to CO2 injection is negligible because the change in the permeability and velocities are insignificant for the predicted maximum degree of cooling (approximately 65 degrees C) of the formation due to CO2 injection. The conclusion of the lack of thermal quenching effects due to CO2 injection at In Salah may be applicable to all global Carbon Capture and Storage prospects.