Macroscale Mechanical and Microscale Structural Changes in Chinese Wufeng Shale With Supercritical Carbon Dioxide Fracturing

Summary Waterless fracturing for shale-gas exploitation using supercritical carbon dioxide (scCO2) is both effective and environmentally friendly, and has become an extensive research topic. Previous researchers have focused on the chemical and physical properties and microstructure of sandstone, carbonate, and shale caprock, rather than on the properties of shale-gas formations. The macroscale mechanical properties and microscale fracture characteristics of Wufeng Shale exposed to scCO2 (at greater than 31.8°C and 7.29 MPa) are still not well-understood. To study the macroscale and microscale changes of shale subjected to scCO2, we obtained Chinese Wufeng Shale crops (Upper Ordovician Formation) from Yibin, Sichuan Basin, China. The shale samples were divided into two groups. The first group was exposed to scCO2, and the second group was exposed to nitrogen (N2). Scanning-electron-microscope (SEM) and X-ray-diffraction (XRD) images were taken to study the original microstructure and mineral content of the shale. To study the macroscale mechanical changes of Wufeng Shale immersed in scCO2 or N2 for 10 hours, triaxial tests with controlled coring angles were conducted. SEM and XRD images were taken after the triaxial tests. In the SEM images, tight bedding planes and undamaged minerals (with sharp edges and smooth surfaces) were found in N2-treated samples both before and after testing, indicating that exposure to N2 did not affect the microstructures. However, the SEM images for the microstructure scCO2-treated samples before and after testing were quite different. The bedding planes were damaged, which left some connected microfractures and corrosion holes, and some mineral types were broken into small particles and left with uneven mineral surfaces. This shows that scCO2 can change rock microstructures and make some minerals (e.g., calcite) fracture more easily. The complex microscale fractures and the decrease in strength for scCO2-treated shale aid the seepage and gathering of gas, enhancing shale-gas recovery. Knowledge of the multiscale physical and chemical changes of shale exposed to scCO2 is not only essential for scCO2 fracturing, but it is also important for scCO2 jets used to break rock and for the geological storage of CO2.