Characterising thermally controlled CH4–CO2 hydrate exchange in unconsolidated sediments
Recovering methane (CH4) via the injection of carbon dioxide (CO2) into a CH4-hydrate-bearing reservoir is a highly attractive mechanism for meeting the world's future energy demand, since it offers the prospect of carbon-neutral energy production. Although the process of CH4–CO2 exchange in hydrate has been demonstrated in numerous laboratory experiments, deployment is hampered by an incomplete understanding of the underlying mechanism of exchange, inconsistencies in reported performance, and insufficient rates of recovery. In this study, the combined effect of thermal stimulation and hydrate-sediment ratio on the rate and degree of CH4 recovery has been quantitatively investigated through a series of large-volume, high-pressure core-flooding experiments on well-characterized synthetic hydrate-sediments using in situ Raman spectroscopy of the effluent vapour. The effect of controlled thermal stimulation has been related to a shell–core model of hydrate exchange, showing that repeated thermal stimulation is effective in temporarily increasing the exchange rate. Furthermore, a numerical analysis of the effective exchange layer thickness for a general particle size distribution is presented: from the results of this work and studies in the literature, we have established a preliminary heuristic for estimating the extent of exchange at constant temperature and pressure. During an initial phase, the effective exchange layer thickness grows at about 9 μm day−1 to a limiting value around 5 μm, where after further exchange occurs at a rate of less than 0.5 μm day−1. The presence of sediment in combination with thermal stimulation was found to significantly increase the rate and overall recovery of CH4, from 28% for a pure hydrate core to 82% for a hydrate-sediment core. This increased recovery is attributed to wetting of the unconsolidated-sediment during exchange. In the context of designing CH4 production from hydrate, these results establish that staged thermal stimulation is an effective means of enhancing CH4 recovery through CO2 exchange and can be improved by applying a series of smaller temperature increments. The enhanced recovery observed in the presence of sediment has important implications for optimized CO2 injection and sequestration, due to the preferential formation of pore-filling hydrate and the associated impact on permeability.