Browsing by Subject "Alaska North Slope"
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Item Physical controls on hydrate saturation distribution in the subsurface(2012-12) Behseresht, Javad; Bryant, Steven L; Mohanty, Kishore K; Hesse, Marc A; Prodanović, Maša; Sharma, Mukul MMany Arctic gas hydrate reservoirs such as those of the Prudhoe Bay and Kuparuk River area on the Alaska North Slope (ANS) are believed originally to be natural gas accumulations converted to hydrate after being placed in the gas hydrate stability zone (GHSZ) in response to ancient climate cooling. A mechanistic model is proposed to predict/explain hydrate saturation distribution in “converted free gas” hydrate reservoirs in sub-permafrost formations in the Arctic. This 1-D model assumes that a gas column accumulates and subsequently is converted to hydrate. The processes considered are the volume change during hydrate formation and consequent fluid phase transport within the column, the descent of the base of gas hydrate stability zone through the column, and sedimentological variations with depth. Crucially, the latter enable disconnection of the gas column during hydrate formation, which leads to substantial variation in hydrate saturation distribution. One form of variation observed in Arctic hydrate reservoirs is that zones of very low hydrate saturations are interspersed abruptly between zones of large hydrate saturations. The model was applied on data from Mount Elbert well, a gas hydrate stratigraphic test well drilled in the Milne Point area of the ANS. The model is consistent with observations from the well log and interpretations of seismic anomalies in the area. The model also predicts that a considerable amount of fluid (of order one pore volume of gaseous and/or aqueous phases) must migrate within or into the gas column during hydrate formation. This work offers the first explanatory model of its kind that addresses "converted free gas reservoirs" from a new angle: the effect of volume change during hydrate formation combined with capillary entry pressure variation versus depth. Mechanisms by which the fluid movement, associated with the hydrate formation, could have occurred are also analyzed. As the base of the GHSZ descends through the sediment, hydrate forms within the GHSZ. The net volume reduction associated with hydrate formation creates a “sink” which drives flow of gaseous and aqueous phases to the hydrate formation zone. Flow driven by saturation gradients plays a key role in creating reservoirs of large hydrate saturations, as observed in Mount Elbert. Viscous-dominated pressure-driven flow of gaseous and aqueous phases cannot explain large hydrate saturations originated from large-saturation gas accumulations. The mode of hydrate formation for a wide range of rate of hydrate formation, rate of descent of the BGHSZ and host sediments characteristics are analyzed and characterized based on dimensionless groups. The proposed transport model is also consistent with field data from hydrate-bearing sand units in Mount Elbert well. Results show that not only the petrophysical properties of the host sediment but also the rate of hydrate formation and the rate of temperature cooling at the surface contribute greatly to the final hydrate saturation profiles.Item Simulation of ice wedge polygon geomorphic transition, Prudhoe Bay, Alaska(2015-08) Abolt, Charles Joseph; Young, Michael H.; Johnson, Joel P; Sharp, John MA numerical model is presented to simulate the changes in topography associated with ice wedge polygon transition from low-centered to high-centered form. The model applies a hillslope diffusion equation to an eroding polygon using a finite-difference approach. It is calibrated using a LiDAR dataset from a site where low-centered polygons exist within meters of high-centered polygons, whose formation appears to have been triggered by construction of the Dalton Highway. The loss of hydrologic storage and the transport of soil from the polygon center into polygon troughs during transition are estimated from model simulations. Optimized values of the hillslope diffusion coefficient suggest that multiple physical processes, including frost heave and continuous soil creep, may drive lateral soil transport at the site. The optimized parameters, furthermore, capture the decreasing influence of anthropogenic disturbances (in this case, the Dalton Highway) on polygon form at distances greater than 35 meters. Overall, a match between the topography of simulated and observed high-centered polygons confirms that the hillslope diffusion paradigm approximates much of the complexity of polygon transition. Future refinements to the model should include more process-based treatment of the mechanisms that drive soil transport and control rates of polygon erosion.