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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. Highlights: The permeability of shear-reactivated and propped fractures evolves synchronously with the evolution of friction. Both factors depend on normal stress and proppant loading concentration but are less sensitive to shear displacement rate. The degree of permeability reduction decreases with increasing effective normal stress and for samples with thicker proppant packs. Coefficients of friction during steady-sliding decrease as normal stress increases. Rock friction/rigidity controls the degree of fracture surfaces damage (as striations or indentations) in shale and consequently impacts the evolution of permeability. Specifically, Strong and rigid fractures show a steady permeability decline during shear deformation, while weak and deformable fractures exhibit a rapid reduction in unsteady-state friction, followed by a slower decline in steady-state friction. Weak and deformable fractures accommodate shear deformation via the combined effects of distributed proppant pack deformation and interface sliding, while strong and rigid rocks primarily reply on distributed proppant pack deformation. [ABSTRACT FROM AUTHOR]

We investigate the evolution of' fracture conductivity as a function of proppant loading concentration under varying effective stresses as an analog to reservoir drawdown. In particular, we define the relative impacts and interactions between proppant crushing, proppant embedment, compaction and particle rearrangement and their impacts on fluid transport. Proppant of realistic concentrations is sandwiched between split core-plugs of Marcellus shale that accommodates embedment as well as rigid steel that excludes it. Impacts of proppant crushing and embedment and roles of particulate transport in fracturing-fluid clean-up are defined. Experiments are performed under triaxial stresses with independent control on confining stress and pore pressure. Normal loading is incremented to represent reservoir drawdown with conductivity evolution recorded continuously via flow-through of brine (20,000 mg/L KCl). Proppant embedment is characterized pre- and post-test by white light optical profilometry with pre-and post-test particle size distributions of the proppant defining the impact of proppant crushing. The conductivity of propped fractures decreases by up to 95% as effective stress is increased by 50 MPa (7000 psi) . This reduction is broadly independent of whether the fracture walls are rigid or deformable. The stress-sensitivity of conductivity is generally muted with increasing proppant loading concentration. We normalize fracture conductivities to equivalent permeabilities of the proppant pack to directly compare pack permeabilities. Low proppant concentrations return higher permeability at low effective stresses but lower permeability at high effective stress, relative to high proppant concentrations. This results since proppant crushing and embedment are both mitigated with increasing proppant loading concentration, as more displacement degree of freedom are added to the system and provide accommodation for interior compaction and rearrangement. Extended effective stress holding times (24 h vs < 1 h) and proppant "aging" exert little impact on transient changes in fracture conductivity. Highlights: Propped shale fracture conductivity decreases as the effective pressure increases. Stress sensitivity reduces with increasing effective stress. Reduced proppant loading concentration results in greater proppant embedment, increased proppant crushing and greater circularity. The mechanisms for permeability reduction between different proppant loading concentrations (thicknesses) are different. Where proppants are injected at low loading concentrations, the most effective approach to maintain permeability is to reduce proppant embedment. Where high proppant loading concentrations are attained, the major impairment mechanism is proppant compaction and rearrangement. [ABSTRACT FROM AUTHOR]