Experimental Investigation of Gas Flow and Hydrate Formation Within the Hydrate Stability Zone

We form methane hydrate by injecting methane gas into a brine-saturated, coarse-grained sample under hydrate-stable thermodynamic conditions. Hydrate forms to a saturation of 11%, which is much lower than that predicted assuming three-phase (gas-hydrate-brine) thermodynamic equilibrium (67%). During hydrate formation, there are temporary flow blockages. We interpret that a hydrate skin forms a physical barrier at the gas-brine interface. The skin fails periodically when the pressure differential exceeds the skin strength. Once the skin is present, further hydrate formation is limited by the rate that methane can diffuse through the solid skin. This process produces distinct thermodynamic states on either side of the skin that allows gas to flow through the sample. This study illuminates how gas can be transported through the hydrate stability zone and thus provides a mechanism for the formation of concentrated hydrate deposits in sand reservoirs. It also illustrates that models that assume local equilibrium at the core-scale and larger may not capture the fundamental behaviors of these gas flow and hydrate formation processes. Plain Language Summary Methane hydrate is a crystalline solid composed of water and methane that forms at low temperature and high pressure. It is a major component of the carbon cycle and an enormous potential energy resource. It can be found in deep ocean sediments within thick, sandy layers above accumulations of methane gas. These hydrate deposits may form by the upward flow of buoyant methane gas from warm regions, where gas is stable, to cold regions, where hydrate is stable. We simulate this process experimentally by injecting gas into a cold, pressurized sand core. A significant amount of hydrate forms, and methane gas advances through the sand despite being in a region where it should solidify into hydrate. Our analysis of the X-ray images and the fluid volumes and pressures suggests that, during injection, a thin hydrate skin forms at the gas-brine interface. The hydrate skin separates the gas from the liquid and limits hydrate formation, allowing gas to flow through the sand. This mechanism may form high concentrations of hydrates in sands in geologic systems.