Development of a hydrate-gas-water static equilibrium model and analysis of three-phase stability

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2018-05

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Leung, Ryan Wai-Hung

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Recent evidence suggests that a three-phase stability zone exists at the base of gas hydrate stability (BGHS), where hydrate and gas may coexist due to the pore size distribution. We develop a three-phase stability zone model at static equilibrium based on the idea of minimizing interfacial energy. We use this model to produce three-phase saturations and study the effects of three-phase stability for two applications. The first application is related to the migration of gas from beneath sealing hydrate layers to the seafloor. A proposed mechanism for this upwards gas migration is the generation of fractures through the sealing hydrate sediment due to overpressures caused by the accumulation of gas on geologic timescales. Our study focuses on how the fracturing potential of a three-phase stability zone differs from a discrete BGHS, where hydrate is separated from gas by a sharp boundary. We model gas overpressures at Blake Ridge, Hydrate Ridge, and the Kumano Basin by incorporating mercury intrusion capillary pressure data with our three-phase stability model. Our results show that the overpressures in the three-phase stability model are smaller, reducing the potential for gas-driven fracturing. We also find that hydrate-bearing basins with shallower seafloor depths modeled with three-phase stability need much more methane to generate the overpressures that will initiate fractures. The second application of three-phase stability relates to the bottom-simulating reflection (BSR), which is a common negative polarity reflection in marine sediments that often follows the contour of the seafloor. Recent literature suggests that the BSR indicates the shallowest presence of gas, not the BGHS. This three-phase stability model has an impact on the seismic response of the BSR, and we study this effect by developing 1-D rock physics models of Blake Ridge. By varying the methane quantity and performing fluid substitution with three-phase saturation profiles, we generate synthetic seismograms and analyze the difference in two way travel time (TWTT). For comparison, we use the workflow for a parameter sensitivity model and an original-resolution model. Through this analysis, we find a relationship between the TWTT width of the BSR’s peaks and the methane abundance at the BGHS

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