Mechanisms Leading to Co-Existence of Gas Hydrate in Ocean Sediments [Part 2 of 2]

Abstract

The experimental boundary conditions do not replicate the situation in gas reservoirs converted to hydrate in the Arctic. The key differences are that in the experiment water can move 117 to the hydrate stability zone only from below, while in nature water can move from below and from above. In experiment gas can move to the hydrate stability zone from below and from above, while in nature it can move only from below. Finally, the fluid phase pressures decrease with time in the experiment (though the conditions remain within the hydrate stability field throughout), while the water phase pressure does not change with time in nature. These differences are minor and do not alter the essential similarities to the natural situation. In the experiment and in nature, the base of the gas hydrate stability zone descends gradually through an existing gas accumulation, and fluid phases are free to move in response to any gradients that arise as hydrate forms. The latter point is important: no fluid flow (neither pressure-driven nor capillarity-driven) is imposed in the experiment. Instead the system evolves a combination of buoyancy-, pressure-and capillarity-driven fluxes of the gas and water phases, and these fluxes balance the various resistances to transport.The model predicts that the contribution of capillarity-driven flux is significant in these experiments. In fact R v~0 .69 when capillary-dominated flow is taken into account while R v~1 when only the viscous dominated portion of the flow was considered. This is consistent with our findings in previous sections of this report, namely, pressure-driven flow cannot supply water from below the BGHSZ fast enough to sustain hydrate formation. Moreover the model predicts that the amount of gas flows to the GHSZ from above quickly diminishes as both the effective permeability, due to hydrate formation within the GHSZ, and gaseous phase relative permeability, due to decrease in gas saturation within the GHSZ, decrease. Therefore, the main contribution for gas flow would be flow from the middle gas inlet. While the gas flux from above and from below could not be independently measured in this experiment, the consistency of the model with other observations suggests that all the fluids needed during conversion to hydrate can be supplied from below. But this is possible only if capillarity-driven flux is accounted for.The consistency of the capillarity-driven flux model with the experimental observations suggests that the finding in the previous section, viz. that the observed hydrate saturation profile in Mt. Elbert arises naturally from the capillarity-driven fluxes, is not a coincidence. That is, these findings suggest that the gas reservoir conversion process is slow, that it introduces modest, even negligible pressure gradients but impose significant gradients in saturation which, along with gas buoyancy, feed the appropriate volumes of fluids to the GHSZ.A complete validation of the model would compare the predicted hydrate saturation profile to the observed profile in F...