Permeability-Friction Relationships for Propped Fractures in Shale

We investigate controls on fluid transfer into massive hydraulic fractures 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 soft/weak and hard/strong 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 hard/strong fractures show a continuous permeability decay at near-constant rate throughout a shear deformation. Conversely, permeability of soft/weak fractures declines rapidly during pre-steady-state-friction then declines more slowly after transitioning to steady-state-friction. We posit 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.