Experimental study on the feasibility of supercritical CO2-gel fracturing for stimulating shale oil reservoirs

Abstract CO2-based fracturing was widely introduced to stimulate shale oil reservoirs for its multiple advantages. However, fracture height containment may become noticeable as the low-viscosity CO2 used in the shale with massive mechanically weak bedding planes (BPs). To address this issue, a novel CO2-gel fracturing design is introduced in this study. The design consists of initially pumping gel to break through the BPs at the near-wellbore zone and then injecting supercritical CO2 (Sc-CO2) to further extend these gel-induced hydraulic fractures (HFs) and to reactivate far-wellbore BPs. In this paper, the design feasibility was demonstrated through laboratory multi-stage fracturing experiment results, and their comparisons to single-phase Sc-CO2, slickwater, and gel fracturing results. Computerized tomography scanning and acoustic emission monitoring were employed to reveal the fracture geometry and the tension, shear and compression failure mechanisms. Fracturing with low-viscosity Sc-CO2 and slickwater encountered the problem of fracture height containment caused by high infiltration ability of these two fluids, which would lead to opening of wellbore-connected BPs dominated by tensile failure. High-viscosity gel fracturing can create multiple transverse HFs but with a few fractures on BPs. When CO2-gel fracturing fluid was used, the fracture height is enhanced and more far-wellbore BPs are activated mostly in slip. The isolation effect of gel on BPs can lower the leak-off amount of Sc-CO2. Consequently, CO2-gel fracturing is relatively far to the leak-off dominated regime, which means more elastic energy of high-compressibility Sc-CO2 could be attained for fracturing the reservoir rocks. CO2-gel fracturing can provide a useful alternative in improving the vertical fracture height during the fracturing treatment.