Permeability-Friction Relationships for Propped Fractures in Shale

Abstract

ABSTRACT: 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. 1. INTRODUCTION Recent advances in methods of recovery – horizontal drilling and massive hydraulic fracturing – enable oil and gas recovery from deep, ultralow-permeability shale reservoirs. This new resource has dramatically changed energy supply in the United States and worldwide over the past two decades. However, the deployment of massive hydraulic fracturing is accompanied by controversy. Large scale fluid injection into the subsurface potentially generates overpressure and may result in the reactivation of faults and fractures (Warpinski and Teufei, 1987; Maxwell et al., 2002; Zhou et al., 2008; Taleghani and Olson, 2011; Zhou and Xue, 2011; Taleghani et al., 2016). In addition, the intersection of natural fractures by the driven hydraulic fracture results in complex fracture networks with the architecture controlled by constraints on the crossing of these fractures (Olson and Taleghani, 2009; Cheng et al., 2015; Zhang et al., 2017). Oblique fractures intersecting the main hydraulic fracture may reactivate in shear as the effective stress state is modified by the passing hydraulic fracture (Wang et al., 2018). Other plausible causes for fracture slip include stress reorientation and poroelastic effects due to nonuniform pressure depletion in heterogeneous permeability fields (Segall and Fitzgerald, 1998; Rousssel and Sharma, 2012; Zhang et al., 2017), undesired fluid leakage into pre-existing hydraulic fractures (Guindon 2015), fluid reinjection (Dohmen et al., 2017) and the enhanced interactions between the natural fractures and hydraulic fractures during fracture propagation (Weng, 2015; Fang et al., 2017a). Some in situ observations suggest that the induced shear deformations can also influence the fluid transport characteristics of the reservoir formation (Guglielmi et al., 2015). Thus, concurrent observations of shear deformation and fluid transport are important in understanding the evolution of fracture permeability in response to fracture reactivation, especially during long-term reservoir depletion.