Salinity Effects on Pore-Scale Methane Gas Hydrate Dissociation

Methane gas hydrate may become a significant source of methane gas in the global energy mix for the next decades. The widespread distribution of methane gas hydrate, primarily in subsea sediments on continental margins, makes the crystalline compound attractive for countries with shorelines that seek self-sustainable energy. Fundamental understanding of pore-level methane gas hydrate distribution and dissociation pattern in reservoirs is important to anticipate the methane production rate and overall efficiency. Specifically, the local salinity gradients occurring during methane gas hydrate dissociation, and its impact on local dissociation characteristics, must be understood as the aqueous phase in most reservoirs is saline. We experimentally evaluate the salinity effect on methane gas hydrate dissociation using high-pressure silicon-wafer micromodels with realistic sandstone grain characteristics. Methane gas hydrate was formed for a range of brine salinities (0-5wt% NaCl), and we report variations in dissociation patterns during depressurization and thermal stimulation as a function of brine salinity. A strong correlation between initial methane gas hydrate distribution and dissociation characteristic, and subsequent release and mobilization of methane gas, was observed. Local water salinities affected the methane gas hydrate structure leading to distinct dissociation patterns of self-preservation due to water freshening. Plain Language Summary Methane gas hydrates have the potential to become the next big energy resource, and high-end estimates suggest an energy equivalent to conventional fossil fuels combined. The potential in full-scale commercialization drives research and industry to better understand hydrates in natural sedimentary systems, and several short-term field pilots investigating production by depressurization have been undertaken the last decades. This study investigates the link between brine salinity and hydrate melting inside the microsized pores in sedimentary rocks where the hydrates occur.