Environmentally Friendly Production of Methane from Natural Gas Hydrate Using Carbon Dioxide

Huge amounts of natural gas hydrate are trapped in an ice-like structure (hydrate). Most of these hydrates have been formed from biogenic degradation of organic waste in the upper crust and are almost pure methane hydrates. With up to 14 mol% methane, concentrated inside a water phase, this is an attractive energy source. Unlike conventional hydrocarbons, these hydrates are widely distributed around the world, and might in total amount to more than twice the energy in all known sources of conventional fossil fuels. A variety of methods for producing methane from hydrate-filled sediments have been proposed and developed through laboratory scale experiments, pilot scale experiments, and theoretical considerations. Thermal stimulation (steam, hot water) and pressure reduction has by far been the dominating technology platforms during the latest three decades. Thermal stimulation as the primary method is too expensive. There are many challenges related to pressure reduction as a method. Conditions of pressure can be changed to outside the hydrate stability zone, but dissociation energy still needs to be supplied. Pressure release will set up a temperature gradient and heat can be transferred from the surrounding formation, but it has never been proven that the capacity and transport ability will ever be enough to sustain a commercial production rate. On the contrary, some recent pilot tests have been terminated due to freezing down. Other problems include sand production and water production. A more novel approach of injecting CO 2 into natural gas hydrate-filled sediments have also been investigated in various laboratories around the world with varying success. In this work, we focus on some frequent misunderstandings related to this concept. The only feasible mechanism for the use of CO 2 goes though the formation of a new CO 2 hydrate from free water in the pores and the incoming CO 2 . As demonstrated in this work, the nucleation of a CO 2 hydrate film rapidly forms a mass transport barrier that slows down any further growth of the CO 2 hydrate. Addition of small amounts of surfactants can break these hydrate films. We also demonstrate that the free energy of the CO 2 hydrate is roughly 2 kJ/mol lower than the free energy of the CH 4 hydrate. In addition to heat release from the formation of the new CO 2 hydrate, the increase in ion content of the remaining water will dissociate CH 4 hydrate before the CO 2 hydrate due to the difference in free energy.