Laboratory Investigation of Fluid Flow and Permeability Evolution Through Shale Fractures

Shear slip caused by increased pore pressure due to injection has been explored as a mechanism for permeability enhancement of unconventional hydrocarbon reservoirs. The process reactivates pre-existing fractures around a hydraulic fracture causing them to slip and dilate and can also cause fracture propagation in the shear and tensile modes creating secondary cracks resulting in increased permeability. Control and optimization of shear stimulation can be achieved by studying how fluid flows through fractures as the stresses (shear and normal) change and how fracture permeability evolves with slip. However, laboratory data on fluid flow and fracture slip in reservoir rocks particularly shale rocks are rare, and the mechanisms of permeability evaluation with shear slip and dilation are still not well understood. In this paper, we present the results of a laboratory scale testing program to address these questions. Salt water (7% kcl) was injected into the two Eagle Ford shale samples under triaxial conditions having a single natural or an induced tensile fracture to induce shear slip, and flow rates during shear slip processes were measured to characterize fracture permeability evolution. In addition, fluid flow through shale fractures under different values of confining stress and injection pressure were examined to investigate the stress-dependent permeability of shale fractures. The hydrostatic flow tests show that flow rate linearly rises with the increase of injection, while an exponential relationship can be observed between flow rate and effective confining pressure. In the injection-driven shear tests, we achieved 6 to 15 times increase in flow rate even with only small shear sliding (<0.1 mm) induced. In addition, permeability tends to linearly evolve with the increase of shear slip and normal dilation. The results quantify the role of shear slip in enhancing permeability of shale fractures, and would help engineer solutions for maintaining these fractures open, reducing costs (proppant/water and additive cost savings).