Conductivity Evolution in Propped Fractures During Reservoir Drawdown

Abstract

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 $\mathrm{psi})$. 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.