Thermodynamics and Kinetic Mechanisms for CH<sub>4</sub>/CO<sub>2</sub> Swapping in Natural Sediments
Bjørn Kvamme
Abstract
The combination of safe long-term storage of CO2 in CH4 hydrate-filled sediments is a win-win situation through associated energy production. Formation of a new hydrate from the injection gas releases heat that will be used for dissociation of in situ CH4 hydrate. Adding another gas, like for instance N2, will increase the injection gas permeability. The question is then how the added N2 will affect thermodynamic properties of the system and potentially change the possibility for creating a new hydrate from the injection gas. In this work I utilize a residual thermodynamic model system that includes all phases, including hydrate and liquid water. With an ideal gas as a reference system for all components in all phases it is feasible to compare thermodynamic properties of different phases directly. It is found that the Gibbs free energy for hydrate formed from CO2 and average seawater is 2 kJ/mol lower than Gibbs free energy for CH4 hydrate. This difference is slightly smaller for hydrate formed from a CO2/N2 mixture with 25 mol % N2. Hydrate stability is reduced with increasing salinity up to mole fraction of NaCl 0.08, which is the maximum NaCl content evaluated. The difference in hydrate stability between the injection gas and CH4 hydrate, however, is almost the same. The difference in enthalpies of hydrate formation between the CO2/N2 mixture and CH4 hydrate is reduced with increasing salinity, but the CH4/CO2 swap is still considered as feasible for a NaCl mole fraction of 0.08. A possible increase in salinity up to NaCl mole fraction 0.08 will therefore not stop the CH4/CO2 swapping. Heat transport through condensed water phases is very fast. It remains unclear if there will be an ion increase in the pores. If released liquid water from CH4 at least compensates for consumed water for creation of injection gas hydrate then the ion is not an issue. Calculated properties, and models, presented in this work will be incorporated into a kinetic model intended to shed more light on this for various realistic cases.