Dissolution and Flow Channeling in Hydrate‐Bearing Sediments: Implications for Permeability

Abstract Estimates of in situ effective permeability of hydrate reservoirs are tens to hundreds of times less than most laboratory measurements on natural hydrate‐bearing sediments. The measured permeabilities are high because methane‐free water creates and expands flow channels in the sample during pressure core storage and laboratory testing. We study this process in natural hydrate‐bearing sediments retrieved from the deep‐water Gulf of Mexico. During a 90‐day methane‐free water flow experiment, the effluent methane concentration decreases from the equilibrium concentration to zero, with the concentration inversely proportional to flow rate. The permeability increases 20‐fold as hydrate saturation decreases from 0.88 to 0. In a second experiment, permeability remains constant when using methane‐saturated water as the permeant, but immediately increases when switching to methane‐free water. We develop a one‐dimensional reaction‐transport model to simulate our observations. Model results indicate that flow channeling is required to explain the data. Dissolution starts at the specimen outer rim during long‐term storage and evolves into a complex geometry during the flow test, leading to high effective permeability. Our interpretation of artificially high measured permeability on natural hydrate sediments is compatible with permeability estimates based on field data, synthetic hydrate samples, and theoretical models. In turn, a more realistic low in situ permeability would limit the ability to produce methane by pressure drawdown and hinder CO 2 injection to form hydrate.