Pore-scale Mechanistic Study of the Preferential Mode of Hydrate Formation in Sediments: Coupling of Multiphase Fluid Flow and Sediment Mechanics

We present a discrete element model for the simulation, at the grain scale, of gas migration in brine-saturated deformable media. We account rigorously for the presence of two fluids in the pore space by incorporating grain forces due to pore fluid pressures, and surface tension between fluids. The coupled model permits investigating an essential process that takes place at the base of the hydrate stability zone: the upward migration of methane in its own free gas phase. We elucidate the way 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. We find that the main factor controlling the mode of gas transport in the sediment is the grain size, and show that coarse-grain sediments favor capillary invasion, whereas fracturing dominates in fine-grain media. The results have important implications for understanding hydrates in natural systems. Our results predict that, in fine sediments, hydrate will likely form in veins that follow a fracture-network pattern, and the hydrate concentration in this type of accumulations will likely be quite low. In coarse sediments, the buoyant methane gas is likely to invade the pore space more uniformly, in a process akin to invasion percolation, and the overall pore occupancy is likely to be much higher than for a fracture-dominated regime. These implications are consistent with field observations of methane hydrates in natural systems.