Today the world is faced with two major energy challenges, i.e., shortage of primary energy sources and climate change. The latter is widely believed to be related to CO2 released from combustion of fossil fuels. Many options are being considered for reducing the emission of CO2 to the atmosphere, including using low carbon fuels (e.g., natural gas) and CO2 capture and storage.
There are vast quantities of methane in the form of gas hydrates in marine sediments and permafrost regions. However, there are many technological challenges in recovering this low carbon fuel. As for CO2 storage, several techniques have been suggested, including their storage in the form of hydrates in sediments. It might be possible to integrate CO2 storage with methane gas production.
In this work, we present the results of a series of experiments on the thermodynamic conditions and kinetics of integrated methane recovery and CO2 sequestration. The preliminary experiments were conducted at different temperature and pressure conditions, in the presence or absence of excess water, in the presence of gaseous or liquid CO2. Silica glass beads and a kaolinite-sand mixture were used to simulate marine sediments. Results of the experimental tests show a higher methane recovery rate in the system inside the methane hydrate stability zone (HSZ) and outside the CO2 HSZ. The presence of excess water noticeably slowed down the CO2 displacement reaction. It was observed that in the kaolinite-sand mixture methane recovery rate was significantly lower than those observed in the tests with silica glass beads. The experimental results infer that mass transfer plays a crucial role in methane recovery through CO2 replacement. The study suggests that the thermodynamic conditions inside methane HSZ and outside CO2 HSZ could be the optimum conditions for integration of methane recovery and CO2 storage in marine sediments.
Introduction
It has been identified by seismic survey there are enormous sedimentary deposits of methane hydrates worldwide (Kvenvolden, 1988 and 1993; Milkov, 2004). The methane trapped in the hydrates has been considered as potential energy source in the near future, while the known geological reserves of conventional natural gas and oil are rapidly declining parallel to expanding demands of fossil fuels. In marine sediments under seafloor naturally-occurring gas hydrates may form from either bacterial or thermogenic methane. Bacterial methane, generated by bacteria via either reduction of CO2 or acetate fermentation, usually forms structure I hydrates, while thermogenic methane is generated from organic matters buried underground (for example, in marine sediments) under high pressure and temperature conditions (Kvenvolden, 1993; Coleman, et al., 1995; Sassen, et al., 1999). Thermogetic methane is always accompanied with non-negligible concentrations of ethane, propane, and butane, and therefore, usually forms structure II hydrates.
