X-ray computed-tomography imaging of gas migration in water-saturated sediments: From capillary invasion to conduit opening

The strong coupling between multiphase flow and sediment mechanics determines the spatial distribution and migration dynamics of gas percolating through liquid‐filled soft granular media. Here, we investigate, by means of controlled experiments and computed tomography (CT) imaging, the preferential mode of gas migration in three‐dimensional samples of water‐saturated silica‐sand and silica‐silt sediments. Our experimental system allowed us to independently control radial and axial confining stresses and pore pressure while performing continuous x‐ray CT scanning. The CT image analysis of the three‐dimensional gas migration provides the first experimental confirmation that capillary invasion preferentially occurs in coarse‐grained sediments whereas grain displacement and conduit openings are dominant in fine‐grained sediments. Our findings allow us to rationalize prior field observations and pore‐scale modeling results, and provide critical experimental evidence to explain the means by which conduits for the transit of methane gas may be established through the gas hydrate stability zone in oceanic sediments, and cause large episodic releases of carbon into the deep ocean. Key Points Controlled experiments elucidate preferential mode of gas migration in sediments Capillary invasion dominates in coarse sediments; conduit opening in fine sediments Conduit opening controls episodic methane release in fine‐grained oceanic sediments