Permeability–Friction Relationships for Propped Fractures in Shale

Controls on fluid transfer into massive hydraulic fractures are investigated due to reactivation of, and proppant penetration into, oblique fractures transecting the main fracture face during long-term reservoir depletion through tightly constrained laboratory experiments. Permeability evolution of fracture-contained proppant permeability/conductivity is highly sensitive to both normal stress and proppant loading concentration and less sensitive to shear displacement rate. By experimentally examining the shale and steel fractures—as an analog to end-member manifestations of weak/deformable and strong/rigid fracture surfaces—and calibrating using granular mechanics models (DEM), we conclude that the evolution of friction–permeability relationship of a propped shale fracture is largely controlled by the rock friction/rigidity. To be specific, propped strong/rigid fractures show a continuous permeability decay at near-constant rate throughout a shear deformation. Conversely, permeability of weak/deformable fractures declines rapidly during pre-steady-state friction and then declines more slowly after transitioning to steady-state friction. It is posited that weak fracture walls accommodate shear deformation via the combined effects of distributed deformation across the interior of the proppant pack and from sliding at the fracture–proppant interface. However, strong rocks accommodate shear deformation primarily through distributed deformation within the proppant pack.