Investigations into the effect of hydraulic fracturing fluids viscosity on the fracture's propagations in unconventional gas reservoir

Hydraulic fracturing was carried out using cylindrical anisotropic shale cores aligned perpendicular to the sediment plane. The viscous oil, water, and liquid-CO2 fracturing fluids were evaluated under both ambient and uniaxial stress conditions. Experiments involving water and oil show that cracks propagate within the load orientation, perpendicular to the sedimentary plane, and along the sediment plane under the uniaxial load conditions. Results imply that in-situ stress conditions greatly affect the development of cracks. Fractures induced by water showed several branches starting from the loading axis. On fluorescent microscopy, however, hydraulic fractures using dense oil produced straight fractures with minimal branching. Viscous oil caused the Mode I fracture according to a statistical analysis of the P wave direction within sound emission frequencies. By contrast, liquid CO2 and water produced the Mode II fracture. Unlike the other two fluids, under loading conditions liquid-CO2 injection had no effect on the propagation of cracks. The low viscosity indicates that low-viscosity fluids, such CO2, cause extensive, branched cracks mostly Mode II fractures. For these reasons, shale gas generation beats slick water injections.