Induction of convective flow due to salt exclusion during hydrate formation in coarse-grained sediments
Abstract
The mechanisms of supplying methane to reach the high hydrate saturations (>80%) in the coarse-grained sand layers of hydrate systems such as those in Walker Ridge 313 and Daini Atsumi Knoll remain an active area of research. In systems where locally-generated microbial methane can be supplied to reservoirs by diffusion from fine-grained layers to adjacent coarse-grained layers, modeling indicates that hydrate is expected to accumulate mainly at the interface between the two layers because of the sharp contrast in methane solubility due to differences in pore size. This suggests that a separate mechanism is needed to deliver methane more uniformly throughout the coarse-grained layer to produce the more homogeneous distributions of hydrate that are observed in natural systems. We report for the first time a Rayleigh-Taylor instability driven by porewater salinity differences within a single coarse-grained layer as a mechanism of homogenizing the distribution of hydrate. In this system, methane is produced microbially in the clay layers surrounding the coarse-grained layer. As methane diffuses from the clay into the sand layer, hydrate forms at the edges. Since hydrate formation excludes salt, the salt concentration within the pore water increases, and the pore water becomes denser. At a certain critical hydrate saturation, the pore water becomes dense enough to initiate convective flow, transporting dissolved salt and methane throughout the layer. We use the Rayleigh number to parameterize the relationship between density-driven convection and diffusion, and the Damköhler number to parameterize the relationship between the rate of density change due to hydrate formation and convection. This convective transport of dissolved methane within the sand layer limits spikes of large hydrate saturation at the edges of the sands and homogenizes hydrate saturations within thick sand layers. Initial observations indicate the onset of convection can occur at hydrate saturations below 25%. As this convective flow can be initiated at fairly low hydrate saturations, it may be responsible for accumulations of methane in laterally extensive coarse-grained layers. The presence of a slight dip can further enhance the impact of this transport process, leading to high accumulations of dissolved methane and higher hydrate saturations down-dip.
SAND2020-7570 A- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMOS034..07F
- Keywords:
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- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3025 Marine seismics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3036 Ocean drilling;
- MARINE GEOLOGY AND GEOPHYSICS