Gas Migration in Unconsolidated Sediments

The research group has developed a grain-scale computational model to investigate the coupling between multiphase fluid flow and sediment mechanics. This has allowed us to elucidate the ways in which gas migration may take place: (1) by capillary invasion in a rigid-like medium; and (2) by initiation and propagation of a fracture. The key finding is that grain size and effective confining stress are the main factors controlling the mode of gas transport in the sediment: coarse-grain sediments under high confinement favor capillary invasion, whereas fracturing dominates in fine-grain media under low confinement. We have confirmed this behavior with carefully controlled laboratory experiments, and this led us to propose a phase diagram that delineates the different invasion regimes (capillary fingering, viscous fingering, and capillary fracturing), controlled by two dimensionless numbers: a modified capillary number and a fracturing number. The first describes the competition between viscous and capillary forces, and the second captures the interplay between capillary forces and the internal frictional resistance of the medium. This mechanistic understanding has allowed us to explain observations of episodic methane venting in nature—both in lakes and in the ocean—through a model of “breathing” conduits which dilate and release gas as falling hydrostatic pressure reduces the compressive effective stress.

We are currently complementing our theoretical and laboratory studies with field work to record high-resolution methane bubbling with a lake-bottom multibeam sonar, with the objective of translating our findings into improved quantitative understanding of the spatiotemporal signature of methane venting from lake and ocean sediments.