Onset of convection from hydrate formation and salt exclusion in marine sands

Abstract

Short-range diffusion and long-range advection are two endmember mechanisms responsible for transporting methane to locations suitable for hydrate formation in the subsurface. This study introduces a novel transport mechanism in natural porous media: density-driven convection of pore fluids due to hydrate formation and salt exclusion in the pore water. Geologic hydrate systems simulations typically consider the role of salt only as it impacts the equilibrium phase behavior, but have largely ignored its effect on density and mobility impacting fluid flow. In a system sourced with microbially-generated methane, as methane enters a brine-saturated sand from an overlying clay layer, convection is initiated by hydrate formation from methane and water, which excludes salt and thereby increases the salinity of the remaining pore water. As the pore water becomes more saline, the density increases, causing a gravitational instability as the more saline, denser fluid layer overlies a less dense fluid. Fluid parcels are then dragged throughout the layer at a rate as high as 3 cm/yr, which distributes aqueous methane throughout the layer, reaching solubility on a shorter timescale than by diffusion alone. In this study, we examine the factors affecting convection and its ability to transport methane in a layered clay-sand system. We demonstrate the rapid transport of aqueous methane throughout the sand layer with convection compared with diffusion, and how the convective velocity increases with increased hydrate saturation formation rates. The relatively fast rate of convective transport makes convection a viable transport mechanism for supplying deeper sediments with methane generated in shallow regions.