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.

Introduction to Special Issue on Gas Hydrate in Porous Media: Linking Laboratory and Field‐Scale Phenomena

“…Meyer, Flemings, DiCarlo, You, et al () start with methane migrating through the sediment sample already held within the P‐T conditions for hydrate stability, whereas Sahoo et al () inject methane gas and then allow the system to equilibrate before lowering the temperature, following a method first described by (Waite et al, ). These studies and Meyer, Flemings, and DiCarlo () postulate that gas hydrate initially forms as a film or skin around gas bubbles in pore spaces.…”

“…For their experimental configuration, lower gas flux leads to much higher final saturations of gas hydrate, a result that may seem counterintuitive. Meyer, Flemings, and DiCarlo () interpret this finding by noting that lower methane flux allows more time for hydrate growth and the formation of thicker hydrate skins on gas bubbles, but also larger pressure differentials. The laboratory studies (Meyer, Flemings, & DiCarlo, , Meyer, Flemings, DiCarlo, You, et al, ; Sahoo et al, ) imply that gas bubbles may sometimes be stranded within pore space, armored by hydrate skins, and therefore unconnected to the flow of pore fluid and gas through the medium.…”

“…The role of a vapor phase in the nucleation and accumulation of gas hydrate in porous media is the focus of several laboratory studies in the Special Section as well (Meyer, Flemings, & DiCarlo, , Meyer, Flemings, DiCarlo, You, et al, ; Sahoo et al, ). Both Meyer, Flemings, DiCarlo, You, et al () and Sahoo et al () describe experiments in which gas hydrate does not form to saturations as high as are predicted based on thermodynamic equilibrium for the three phase system (gas‐pore fluid‐gas hydrate).…”

“…Meyer, Flemings, and DiCarlo () add a sensitivity study to this framework by varying the rate at which gas flows through a brine‐saturated porous specimen held within the P‐T conditions for gas hydrate stability field. They then maintain upstream methane pressure for 800 hr after stopping brine withdrawal.…”

“…The laboratory studies (Meyer, Flemings, & DiCarlo, , Meyer, Flemings, DiCarlo, You, et al, ; Sahoo et al, ) imply that gas bubbles may sometimes be stranded within pore space, armored by hydrate skins, and therefore unconnected to the flow of pore fluid and gas through the medium. Like the dissolved phase models of VanderBeek and Rempel (), the laboratory experiments conducted by Meyer, Flemings, & DiCarlo, (), Meyer, Flemings, DiCarlo, You, et al, () and Sahoo et al () with a free gas phase can provide an explanation, albeit with a different physical grounding, for high saturations of gas hydrate forming far above the base of the GHSZ.…”