LAUSR

Abstract Hydraulic fracturing becomes a key technology for unconventional oil and gas development. As a fracturing fluid, CO2 displays considerable advantages, particularly in shale gas exploitation. However, little research has quantitatively analyzed the effectiveness and efficiency of CO2 fracturing in naturally fractured reservoirs. In this study, a CO2 fracturing model is established in naturally fractured reservoirs. A series of fracturing cases in single- and multiple- fracture(s) are conducted based on the simplified three-dimensional displacement-discontinuity-method. Compared to induced fractures by slickwater and gel, the results show that CO2 fracturing can generate relatively longer but narrower fractures due to the low breakdown pressure and pressure drop in fractures. The pressure drop in the single fracture by CO2 fracturing is respectively 60.48% and 78.88% lower than that by slickwater and gel fracturing at the same injection time. Simultaneously, in naturally fractured reservoirs, CO2 fracturing can activate more natural fractures to connect with each other and create a larger stimulated reservoir volume. For a single fracturing, the active natural fractures volume of CO2 fracturing is 3.7 and 9.2 times that of slickwater and gel fracturing, respectively. For multiple fracturing, the active natural fractures volume of CO2 fracturing is 2.5 and 4.4 times that of slickwater and gel fracturing, respectively. Additionally, the sensitivity analyses in natural fracture spacing and length show that both natural fracture spacing and length can significantly determine natural fractures' distribution and connectivity. By CO2 fracturing, a reservoir with relatively smaller natural fracture spacing and longer natural fracture length can be activated more natural fractures. The present work can help the scholars and the readers better understand the effectiveness and efficiency of CO2 fracturing, especially in naturally fractured reservoirs.