Promoting CH4/CO2 replacement from hydrate with warm brine injection for synergistic energy harvest and carbon sequestration

Abstract

CH4/CO2 replacement for natural gas hydrates (NGHs) exploitation is a promising method for CO2 geological sequestration and energy recovery simultaneously. However, the puzzles of low replacement efficiency and slow reaction rate caused by the mass transfer obstacle of gas exchange are fatal bottlenecks for field application of gas replacement method. Therefore, we propose a method that uses warm brine injection during CH4/CO2 replacement process to break the barrier of CO2 diffusion and enhance CH4 recovery as well as CO2 storage. Benefiting from the synergistic influence of salt effect and thermal stimulation as well as water flow erosion, warm brine injection provides three dimensionally connected channels in hydrate for subsequent mass transfer, improving CH4/CO2 replacement in the deep layer of hydrate. CO2 sequestration efficiency of the newly proposed method reaches 76.46%, and the maximum amounts of CH4 recovery is nearly treble than that achieved by single CO2 replacement method. Notably, a potential risk of secondary hydrate formation could occur upon the pressure surge and fluid migration attached to warm brine injection, which could be effectively alleviated by increasing salinity to destabilize hydrate lattices by reducing water activity. The gas production is insensitive to the injected heat as the significant heat loss during the transportation of injected brine in pipelines and thermal diffusion through boundaries. The introduction of free water increases the complexity of the reactions in hydrate reservoirs, and the formulation of brine injection regimes should be synergistically optimized based on energy harvest, energy efficiency and CO2 sequestration. This work extends the previous knowledge on CH4/CO2 replacement and shows important practical significance for future field studies of NGHs exploitation and CO2 geological sequestration.