A comparative study of fracture surface roughness and flow characteristics between CO2 and water fracturing

Fracture surfaces were measured using a 3-D optical topography scanner, and the fracture surfaces caused by CO2 fracturing are much rougher than those caused by water fracturing due to the stronger penetrability of CO2 through tiny pores and channels. The fracture roughness of siltstone with intensive macro-pores is greater than that of shale, because fractures preferably propagate along the orientation of macro-pores. Fracture flow conductivity is greatly enhanced after small shear displacement between fracture surfaces, because of the generation of flow pathways on rough fracture surfaces. Shear deformation perpendicular to flow direction creates relatively unobstructed and straight flow pathways along the flow direction, and flow conductivity is around one order of magnitude higher than that with shear deformation along the flow direction and 300-400 times that without any shear deformation. Our results suggest that CO2 fracturing has an advantage over water fracturing in improving flow capacity of created fractures.