In Mogul west of Reno, Nevada, USA, in late February 2008 an earthquake sequence occurred that culminated in a magnitude 4.9 mainshock after a foreshock-rich period lasting approximately 2 months on previously unidentified fault structures. In this article, we show that the foreshock sequence may have been driven by a fluid pressure intrusion. We use 1,082 previously calculated earthquake focal mechanisms to infer the local stress field as well as 1,408 relocated foreshock events to determine the required excess fluid pressure field in the source region of the Mogul earthquake sequence to trigger these events. A model of nonlinear pore pressure diffusion is used to model the fluid flow in a highly fractured subsurface. We find a strong correlation between high fluid pressure fronts and foreshock hypocenters, suggesting a natural fluid-driven earthquake sequence.

Massive quantities of fluid are injected into the subsurface during the creation of an engineered geothermal system (EGS) to induce shear fracture for enhanced reservoir permeability. In this numerical thermoelasticity study, we analyze the effect of cold fluid injection on the reservoir and the resulting thermal stress change on potential shear failure in the reservoir. We developed an efficient methodology for the coupled simulation of fluid flow, heat transport, and thermoelastic stress changes in a fractured reservoir. We performed a series of numerical experiments to investigate the effects of fracture and matrix permeability and fracture orientation on thermal stress changes and failure potential. Finally, we analyzed thermal stress propagation in a hypothetical reservoir for the spatial and temporal evolution of possible thermohydraulic induced shear failure. We observe a strong influence of the hydraulic reservoir properties on thermal stress propagation. Further, we find that thermal stress change can lead to induced shear failure on nonoptimally oriented fractures. Our results suggest that thermal stress changes should be taken into account in all models for long-term fluid injections in fractured reservoirs.

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Induced earthquakes from waste disposal operations in otherwise tectonically stable regions significantly increases seismic hazard. It remains unclear why injections induce large earthquakes on non-optimally oriented faults kilometers below the injection horizon, particularly since fluids are not injected under pressure, but rather poured, into the well as observed in the Milan, Kansas area. Here we propose a mechanism for induced earthquakes whereby the karstic lower Arbuckle provides the short-circuit that establishes a tens of MPa stepwise fluid pressure increase within the basement upon arrival of the hydraulic connection to the free surface and ultimately induce slip on the deeper fault. We investigate this scenario through modeling and mechanical analysis and show that earthquakes near Milan are likely induced by large (and sudden) fluid pressure changes when the karst network links two previously isolated hydrological systems. Plan Language Summary We used numerical models to simulate the coupled hydrological and mechanical processes inducing seismicity in southern Kansas. We find that the presence of an extensive karst reservoir, the Arbuckle group, was a necessary condition to produce the M4.9 Milan earthquake, the largest earthquake to occur in over 100 years in Kansas. The results of these models also suggest that a significant percentage of the induced seismicity in Oklahoma would not have occurred without the presence of the Arbuckle. As demonstrated by this work, coupled hydromechanical models are critical to help understanding fluid behaviors in injection reservoirs and can be used to further understand the spatiotemporal distribution of induced earthquakes.

Understanding the dynamics of naturally fractured systems and fractured reservoirs in terms of flow, heat transport and fracture stability (e.g. induced seismicity) is important for a range of applications associated with waste water injection, renewable energy (e.g. geothermal systems), and greenhouse gas mitigation (e.g. geological sequestration of CO2). Here we present the implementation and validation of an open source MATLAB package for efficient numerical simulations of the coupled processes in fractured systems. We take advantage of the embedded discrete fracture model that efficiently accounts for discrete fractures. We perform a series of numerical benchmark experiments to validate the implemented approach against analytical solutions and established numerical methods. Finally, we use a simplified geomechanical model and an integrated fracture stability analysis that allows estimating the potential for shear stimulation, and thus a mechanistic assessment of induced seismic risk during stimulation. The open source distribution of the source code and results can be used as a blue print for the re-implementation of the method in a high performance computing (HPC) framework or as a standalone simulation package for investigating TH(m) problems in geothermal reservoirs.