Shear Related Reactivation of Fractures/faults and the Effects on Permeability Evolution and Induced Seismicity PDF Download

Author: Jiayi Yu
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Languages : en
Pages : 0

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Book Description
Large-scale subsurface fluid injection is commonly deployed in geoengineering activities for hydraulic fracturing, enhanced geothermal system (EGS) development, carbon capture and storage (CCS) and underground waste disposal. Preexisting fractures or faults that are favorably orientated to in-situ stresses can be potentially shear-reactivated by the induced fluid overpressure. This is of particular interest in hydraulic fracturing-induced permeability enhancement in shale reservoirs where the presence of intricate natural fracture networks and their interaction with driven hydraulic fractures serve as pathways for the efficient transport of hydrocarbons. Conversely, fault and fracture reactivation in shear may also result in undesired triggered seismicity. The observation that induced seismicity can be of substantial magnitude and hence high hazard necessitates careful management to mitigate any associated risks. The following reports experimental observations that explore the role of shear-related reactivation of fractures and faults, inclusive of both bare surfaces and filled with proppant, and their impacts on the evolution of permeability (Chapters 1 and 2) and induced seismicity (Chapter 3). These activities and outcomes are described in detail in the following. Chapter 1 investigates the factors that influence fluid transfer into massive hydraulic fractures through tightly constrained laboratory experiments, specifically due to the reactivation of oblique fractures and proppant penetration during long-term shale reservoir depletion. We find that the evolution of the propped fracture's friction-permeability relationship is mainly controlled by the rock's friction/rigidity, which is sensitive to normal stress and proppant loading concentration but less sensitive to shear displacement rate. Our experiments examine both shale and steel fractures as analogs for weak and strong fracture surfaces and were calibrated using granular mechanics models (DEM). We observe that propped strong fractures have a continuous permeability decay at a constant rate during shear deformation, while permeability of weak fractures declines rapidly during pre-steady-state-friction and more slowly after transitioning to steady-state-friction. We believe that weak fracture walls accommodate shear deformation through distributed deformation across the proppant pack's interior and sliding at the fracture-proppant interface, whereas strong rocks accommodate shear deformation mainly through distributed deformation within the proppant pack. Chapter 2 reveals the importance of shear deformation in conditioning the fluid transport characteristics of shale reservoirs, specifically due to the prevalent existence of natural fractures. 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 (confining stress). 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. Chapter 3 reports the role of critical stress in quantifying the magnitude of fluid-injection triggered earthquakes. We directly constrain the impact of pre-existing critical stresses on the relation linking seismic moment to injection volume. We report shear reactivation experiments on laboratory faults triggered by fluid pressurization. Experiments are conducted under both zero-displacement and constant shear stress boundary conditions -- differentiating the role of stress relaxation during fault slip. Both are shown capable of linking event magnitude (M) with injected volume ([delta]V). Injection response defines two discrete and linear stages in M-[delta]V space linked by a discrete up-step. The first limb (stage) represents the elastic deformation of the fault, the vertical up-step its reactivation and the second limb the rupture response as incremented sliding. Faults loaded to different pre-stress identify and quantify the controlling role of pre-existing shear stress in conditioning event magnitude. Laboratory results are readily explained by a model that considers the pre-existing stress state in the rupture of a rigid fault with slip limited to the zone of pressurization. This cumulative moment is defined as M=c/(1-c) G[delta]V with c defining the proportion of the static stress drop already applied by tectonic stressing, alternatively viewed as the proximity to failure. The model and confirmatory laboratory observations explain the triggered earthquakes at EGS sites larger than expected based on previous models relative to injection volumes. The three chapters of this dissertation comprise a series of three papers currently in-submittal. By order of chapter appearance, these papers are: Yu, J., Eijsink, A., Marone, C., Rivière, J., Shokouhi, P., Elsworth, D (2023) "Role of Critical Stress in Quantifying the Magnitude of Fluid-Injection Triggered Earthquakes" (In prep.) Yu, J., Wang, J., Li, Y., El-Fayoumi, A., Wu, R., Liu, X., Rijken, P., Rathbun, A., Elsworth, D. (2022) "Role of Shear Deformation on Shale Fracture Reactivation and Conductivity Evolution" (Manuscript submitted: Rock Mechanics and Rock Engineering) Yu, J., Wang, J., Li, Y., El-Fayoumi, A., Wu, R., Liu, X., Rijken, P., Rathbun, A., Elsworth, D. (2022) "Friction-Permeability Relationships for Propped Fractures in Shale" (Manuscript submitted: Rock Mechanics and Rock Engineering).