In-situ laboratory study on influencing factors of pre-SC-CO2 hybrid fracturing effect in shale oil reservoirs

Supercritical CO2 (SC-CO2) fracturing, being a waterless fracturing technology, has garnered increasing attention in the shale oil reservoir exploitation industry. Recently, a novel pre-SC-CO2 hybrid fracturing method has been proposed, which combines the advantages of SC-CO2 fracturing and hydraulic fracturing. However, the specific impacts of different pre-SC-CO2 injection conditions on the physical parameters, mechanical properties, and crack propagation behavior of shale reservoirs remain unclear. In this study, we utilize a newly developed "pre-SC-CO2 injection → water-based fracturing" integrated experimental device. Through experimentation under in-situ conditions, the impact of pre-SC-CO2 injection displacement and volume on the shale mineral composition, mechanical parameters, and fracture propagation behavior are investigated. The findings of the study demonstrate that the pre-injection SC-CO2 leads to a reduction in clay and carbonate mineral content, while increasing the quartz content. The correlation between quartz content and SC-CO2 injection volume is positive, while a negative correlation is observed with injection displacement. The elastic modulus and compressive strength exhibit a declining trend, while Poisson's ratio shows an increasing trend. The weakening of shale mechanics caused by pre-injection of SC-CO2 is positively correlated with the injection displacement and volume. Additionally, pre-injection of SC-CO2 enhances the plastic deformation behavior of shale, and its breakdown pressure is 16.6% lower than that of hydraulic fracturing. The breakdown pressure demonstrates a non-linear downward trend with the gradual increase of pre-SC-CO2 injection parameters. Unlike hydraulic fracturing, which typically generates primary fractures along the direction of the maximum principal stress, pre-SC-CO2 hybrid fracturing leads to a more complex fracture network. With increasing pre-SC-CO2 injection displacement, intersecting double Y-shaped complex fractures are formed along the vertical axis. On the other hand, increasing the injection rate generates secondary fractures along the direction of non-principal stress. The insights gained from this study are valuable for guiding the design of pre-SC-CO2 hybrid fracturing in shale oil reservoirs.