X-Ray Computed-Tomography Imaging of Preferential Mode of Gas Migration through Water-Saturated Sediments

Field observations suggest that gas transport through water-saturated soft sediment is an essential component of seafloor dynamics, and that it exerts a fundamental control on natural gas seeps, the creation of pockmarks in the ocean floor, and the growth and form of gas hydrates. In this study, we use controlled experiments and computed tomography (CT) imaging to investigate the preferential mode of gas migration in three-dimensional samples of water-saturated silica-sand, silica-silt, kaolin-clay, as well as multi-layered sediments. Our experimental system allows independent control of radial and axial confining stresses and pore pressure while performing continuous x-ray CT scanning. The CT image analysis of the gas migration provides the first experimental confirmation, in three-dimensional systems, that capillary invasion preferentially occurs in coarse-grained sediments whereas grain displacement and conduit openings are dominant in fine-grained sediments [1]. Our findings rationalize prior field observations and pore-scale modeling results which suggest the strong control of grain size and sediment mechanics on the nature of gas invasion into water-filled porous media. These data 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.