Multiscale Change Characteristics of Chinese Wufeng Shale Under Supercritical Carbon Dioxide

Objectives/Scope. Waterless fracturing for shale gas exploitation with supercritical carbon dioxide (scCO₂) is effective and environment-friendly, which has become an international fascinating research topic. The formers mainly focus on the chemical and physical properties, and microstructure of sandstone, carbonate and shale caprock, rather than the properties of shale gas formation. And the macroscale mechanical properties and nanoscale fracture characteristics of Wufeng shale under scCO₂ conditions (above 31.8 °C and 7.29 MPa) is still not clear.

Methods, Procedures, Process. To study multiscale change characteristics of shale under scCO₂ condition, the Chinese Wufeng shale crops (upper Ordovician formation) is obtained from Yibin, Sichuan Basin, China. Firstly, SEM and XRD are taken to study the original microstructure and mineral content of shale. Then, triaxial tests with controlled coring angles and gas conditions (immersed scCO₂ and N₂ for 10 hours) have been carried out on shale φ25 × 50 mm cylindrical specimens for the objective of studying macro mechanical change of Wufeng shale. In addition, SEM and XRD are also conducted after the triaxial tests.

Results, Observations, Conclusions. Some results are concluded as follow. (1) In the SEM figures, the Group 2 (after testing) are similar to the original ones: the tight bedding planes and undamaged minerals (with sharp edges and smooth surfaces) are found in Group 1 (before testing) and Group 2 (before and after testing), which indicating that the triaxial test with N₂ can't damage the microstructures of shale. (2) However, the microstructures in Group 1 SEM figures (after testing) are quite different than the others: the bedding planes are damaged, which lefts some connectivity micro-fractures and corrosion holes; the minerals are broken into lots of small debris and left some uneven mineral surfaces. That demonstrates the scCO₂ can change the microstructures and make some minerals (e.g. calcite) fracture more easily. (3) A relative content reduction of calcite, dolomite and pyrite have been found in Group 1, significantly. (4) The scCO₂ reduce the triaxial compressive strength of shale with a decline of 3.82% to 19.38%. (5) huger fractures in macroscale have been found in the specimens of Group 1, scCO₂ and bedding plane lead to the diversities of failure modes.

Novel/Additive Information. The multiscale physical and chemical changes of shale under scCO₂ are essential for scCO₂ fracturing, scCO₂ jet breaking rock and the geological storage of CO₂. The complex microscale fractures, the macro decrease of strength in shale are helpful to the seepage and gathering of shale gas, and finally enhance the shale gas recovery.