The Role of Shear Deformation on Shale Fracture Reactivation and Conductivity Evolution

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ABSTRACT In this study, we conduct laboratory experiments reproducing fracture slip on both propped and unpropped fractures in Marcellus shale to explore the role of shear deformation as a primary control on permeability evolution and its correlation with initial stress state, shear stress magnitude and loading rate, and proppant loading concentration. For tests on unpropped fractures, we incorporate the complexity in both form and response of natural fracture topography by using pristine natural fractures directly split along bedding planes. Under low shear stress, unpropped fracture is prohibited from slipping by strongly mated interlocking asperities. As we increase shear stress exceeding the frictional strength of the contact, it exhibits great conductivity enhancement upon fracture reactivation followed by immediate and continuous decay. If shear stress is loaded incrementally instead of instantaneously – broadly representing different fracking fluid injection rate – fracture conductivity response to shear deformation is considerably muted. Unpropped fracture behaviors are also found to be strongly related to fracture roughness and fidelity of the interlocking asperities, while less sensitive to background stress state. For propped fractures, we use manually ground fractures to specifically focus on the proppant impacts. In contrast to unpropped fractures, conductivity enhancement upon shear reactivation only presents where proppant is placed as non-uniformly distributed monolayer, which can be attributed to the generation of interparticle highly permeable flow paths. Otherwise, conductivity decreases as a result of proppant embedment, crushing, and compaction, however the reduction is muted with thicker proppant pack. INTRODUCTION The shale gas revolution dramatically changed the mix of energy supply in the United States over the past decade. The deployment of horizontal drilling and massive hydraulic fracturing to generate highly permeable flow paths has unlocked the massive store of hydrocarbons held in deep, ultralow-permeability shale reservoirs (Warpinski and Teufei, 1987; Maxwell et al., 2002; Taleghani and Olson, 2011; Taleghani et al., 2016).