| In the past several years, induced microearthquakes (MEQs) related to energy development projects have garnered public attention. Large-scale fluid injection in geoengineering activities, such as enhanced geothermal reservoir stimulation, geological storage of CO2, shale reservoir stimulation, and deep disposal of wastewater can generate significant fluid overpressures and induce microearthquakes by reactivating preexisting faults and fractures that are widely distributed throughout the upper crust. The monitoring of fluid-injection-induced microearthquakes can provide significant information (e.g., timing, special distribution, and moment magnitude) in evaluating reservoir development (e.g., fracture distribution and reservoir hydraulic conductivity evolution), in informing production strategies (e.g., accommodation of production wells), and in assessing the risks of fluid injection activities (e.g., caprock integrity of CO2 reservoir). The following questions are addressed in this study: (1) what are the mechanisms and implications of in-situ feedbacks ( e.g., monitored fluid-injection-induced MEQs distribution, seismic moment magnitudes, and fracture behaviors) in geothermal stimulation? (2) What information can be derived from MEQs to inform geothermal reservoir stimulation strategies? (3) Are there any potential relationships between induced seismic or aseismic slip and permeability evolution of fractures in such unconventional reservoirs and caprocks? (4) What factors play an important role in controlling these relationships? These questions are addressed in the five individual chapters of this thesis.;Chapter 1 describes a case study of anomalous MEQs distribution: A bimodal depth distribution of fluid-injection-induced MEQs was observed in the 2012 stimulation phase of the Newberry Volcano EGS Demonstration project in Oregon, US. During 7 weeks of hydraulic stimulation of well NWG 55-29, 90% of MEQs occurred in the shallow reservoir (∼500 m to ∼1800 m), only a few occurred adjacent to the bottom of the open borehole (∼2500 m to ∼3000 m) while almost no seismicity was observed in the intervening interval (∼1800 m to ∼2500 m). Our analysis of frictional stability using spatial models for fluid pressure diffusion of injected fluids shows that the distribution of MEQs is consistent with observed casing damage, and a possible leak at ∼700 m, and is inconsistent with migration of fluids from the casing shoe. The role of fluid injection through the ruptured casing is further supported by the analyses of shear failure and pore-pressure diffusion. Finally, the absence of seismicity at intermediate depths is consistent with our laboratory determinations of frictional stability, showing velocity strengthening frictional behavior for samples from intermediate depths, bracketed by velocity neutral and weakening behavior for samples from shallower and greater depths.;Chapter 2 introduces a method to constrain the evolution of fracture permeability at sufficiently fine resolution with observed in-situ MEQs data to define reservoir response. In this method, we propose a model that couples the moment magnitude to fracture aperture and then estimates the reservoir permeability at relatively high resolution. The critical parameters controlling fracture aperture and permeability evolution are stress-drop, the bulk modulus of the fracture embedded matrix, and the dilation angle of fractures. We employ Oda's crack tensor theory and a cubic-law based analog to estimate the permeability of a synthetic fractured reservoir at various scales, demonstrating that the resolution of permeability is largely determined by the cellular grid size. Finally, we map the in-situ permeability of the Newberry EGS reservoir using observed MEQs during two rounds of reservoir stimulations in 2014. The equivalent mean permeability evaluated by each method is consistent and unlimited by representative elementary volume (REV) size. With identical parameters, Oda's crack tensor theory produces a more accurate estimation of permeability than that of the cubic law method, but estimate differences are within one order of magnitude. The permeability maps show that the most permeable zone is located within the zone of most dense seismicity providing a reference for the siting of the production well. This model has the potential for mapping permeability evolution from MEQs data in conventional and unconventional reservoirs and at various scales.;However, the impact of induced seismicity on fracture permeability evolution remains unclear due to the spectrum of modes of fault reactivation ( e.g., stable vs. unstable). To better understand the hydro-mechanical behavior of reservoir due to stimulation, it becomes essential to understand the fundamental relationship between induced seismic and aseismic slip and permeability evolution of a fracture. As seismicity is controlled by the frictional response of fractures, (Abstract shortened by ProQuest.). |
