Experimental Study of Fracture Propagation of CO2 Fracturing in Conglomerate Reservoirs

Employing CO2 as a fracturing fluid could effectively limit the hydration swelling of clay minerals and reduce the damage to conglomerate reservoirs. The gravel particles increase the heterogeneity of conglomerate, making fracture propagation more complex than in sandstone and shale. True triaxial fracturing experiments were performed using artificial conglomerates. The initiation pressure, fracture morphology and fracture surface characteristics were analyzed. Studies showed that the heterogeneity of the conglomerate greatly affected fracture initiation and propagation. Compared with hydraulic fracturing, CO2 fracturing has significant advantages, such as degrading initiation pressure and creating more complex fractures. The fracture induced by CO2 mostly appeared as deflection, making the fracture surface rougher. The stress difference greatly influenced the fracture propagation. As the stress difference grew, the fracture morphology shifted from complex to simple, and the expansion mode shifted from deflection to penetration. Increasing the injection rate can lead to a more balanced fracture propagation. However, large injection rates lead to higher initiation pressure. Research results are expected to provide a theoretical basis for CO2 fracturing and help promote the integration of conglomerate reservoir exploitation and CO2 storage. 1. INTRODUCTIONS Mahu conglomerate oil reservoir is by far the world's first whole conglomerate oil field with huge resource potential (Hou et al., 2018). Since 2015, the development method of volumetric fracturing has been adopted to realize effective mobilization (Rafieepour et al., 2020, Lin et al., 2022, Zhang et al., 2024a, and Zhang et al., 2024b). But the single well EUR of some sensitive reservoirs is low and the annual decline rate is large. The analysis results show that one of the reasons for the faster decline in production and pressure is the high content of water-sensitive minerals in the reservoir rock, which soften in contact with water and cause high damage to permeability (Xv et al., 2019). Therefore, CO2 is considered as a fracturing medium to explore the low-injury fracturing mode without or with less water (Shen et al., 2010, Wang et al., 2012).