Resistance of caprock to hydraulic fracturing due to CO2 injection into sand lens reservoirs

The geomechanical stability of caprock is critical to the long-term safety of geologic storage of CO2 in depleted oil reservoirs. Based on the theory of rock fracture mechanics in deep formations, we propose a quantitative assessment framework for the resistance of caprock to hydraulic fracturing due to CO2 injection and storage in a sand lens reservoir. By simplifying the sand lens reservoir as a lenticular crack model, we developed a formula for the Mode-I stress intensity factor at the crack tip (pinch-out tip of the sand lens reservoir) and the in situ fracture toughness of the surrounding caprock. We then established a preliminary threshold for assessing the resistance of caprock to hydraulic fracturing. Based on this threshold, the critical hydraulic fracturing pressure at the pinch-out tip of a sand lens reservoir and the allowable injection pressure at the bottom hole of an injection well can be estimated. This quantitative assessment framework has been verified for use in sand lens reservoirs with a case study of the Daqingzijing CO2 storage site in the Jilin oil field, China. These results provide important implications for assessing the resistance to hydraulic fracturing in similar reservoirs with stratigraphic pinch-outs in impermeable formations, preventing caprock fracturing and water or gas channeling during CO2 injection.