Investigation of in situ salinity and methane hydrate dissociation in coarse-grained sediments by slow, stepwise depressurization

Slow (8–15 day), stepwise, degassing of synthesized methane hydrate-bearing sand packs was used to determine in situ salinity to within 0.5 wt%. Rebounds in pressure during shut-in intervals between degassing steps record gradual hydrate dissociation. During successive dissociation steps and pressure rebounds, the pressure decreases more than predicted for a sample with homogenous salinity and temperature at thermodynamic equilibrium. We interpret that hydrate dissociation is limited by local pore water freshening and endothermic cooling around the dissociating hydrate. These results suggest that during production, hydrate dissociation in a reservoir will occur along the freshwater phase boundary even though the bulk salinity of the system may be higher. Pore pressure rebound during shut-in is more rapid in freshwater experiments than in brine experiments, which indicates that salt diffusion plays a role in limiting recovery to homogenous bulk equilibrium during hydrate dissociation. We demonstrate that it is possible to interpret the in-situ salinity from the temperature and pressure at the onset of dissociation. This approach can be applied to actual hydrate-bearing pressure core samples and the results will assist our understanding of how these hydrates form. Depressurization experiments with controlled shut-in periods provide insight into the role of salt and heat diffusion on dissociation in coarse-grained sediments, particularly relevant to pauses in production and in regions of a hydrate reservoir near the hydrate phase boundary.