Effect of Gas Flow Rate on Hydrate Formation Within the Hydrate Stability Zone

Abstract

We form methane hydrate in brine-saturated, coarse-grained samples, under hydrate-stable conditions, by injecting methane vapor at various flow rates. Decreasing the flow rate results in higher hydrate saturation, lower brine saturation, a smaller affected volume, and larger average pressure differentials across the sample. We interpret that the longer execution times at lower flow rates allow for additional methane transport and hydrate formation at the hydrate-brine interface. As a result, the hydrate skin is thicker at lower flow rates and thus is capable of sustaining larger pressure differentials. In several experiments, we stop brine flow and supply methane gas to the sample for an additional 800 hrs. During this period, hydrate continues to form, pressure differentials develop, and the bulk density changes within the affected volume. We interpret that there is gas present in the sample that is disconnected from the gas source. Hydrate forms around the disconnected gas due to methane transport through the skin that surrounds it, causing the internal gas pressure to decline and leading to inward collapse and net volume decrease. This lowers the brine pressure and creates a differential pressure across the sample that induces gas flow. This study indicates that lower gas flow rates through the hydrate stability zone can produce very high saturations of hydrate but require a larger differential pressure to sustain flow. Ultimately, this process is an alternative mechanism for sustained upward gas flow and hydrate formation far above the base of the hydrate stability zone.