Improving Fracture Complexity and Conductivity in Shale Formation with Calcite-Filled Natural Fractures by a Novel CO2-Assisted Fracturing: An Experimental Study

Summary Given the advantages of using CO2 as a fracturing fluid to enhance unconventional oil/gas production and urge of carbon neutrality, CO2-assisted fracturing draws increasing attention in China recently. However, several critical issues related to this fracturing technology, such as the mechanism of hydraulic fracture (HF) growth, still need to be clarified. A novel CO2-assisted fracturing design, which can increase the HF complexity and conductivity, as well as improve the porosity/permeability of surrounding rock matrix, bedding planes (BPs), and natural fractures (NFs), was proposed. In the design, the carbonated water, formed by dissolving surpercritical CO2 in the slickwater, is used as the slug fluid to soften the calcite-sealed NFs that intersect with the precreated HFs. Subsequently, the slickwater is injected as the carrying fluid to dilate the NFs. To verify this design, a series of true triaxial fracturing simulations and static soaking experiments were conducted on the Longmaxi shale in Sichuan basin, China. Scanning electron microscopy results show that carbonated water, a weakly acidic fluid, can react vigorously with the carbonate-rich shale with time going on, thereby resulting in numerous dissolved pores with the diameter of dozens of microns. Eventually, the reaction between rock and carbonated water increases porosity/permeability and reduces mechanical strength. Notable dissolution of calcite, which could soften the calcite-sealed NFs, can occur in a short time (0.5 hours). Pretreating the specimen with carbonated water can lower the breakdown pressure of the rock by 2.7 MPa for half an hour and 11.7 MPa for 2 hours and promote HFs to propagate along the BPs and NFs in the shear-dominant mode. The shear dislocation and uneven erosion of fracture surface are of great significance in improving the permeability or conductivity of HFs. Notably, well shut-in for an optimized period may allow the sufficient interaction between carbonated water and shale, thereby improving the effectiveness of composite fracturing. This innovative design, which takes advantage of the special physical-chemical properties of supercritcal CO2, is feasible and conducive to enhancing production from unconventional reservoirs.