Mechanisms for CH4/CO2 Swapping in Natural Sediments

Abstract

Production of natural gas from hydrates involves multiple complex competing phase transitions, which are rarely analyzed thermodynamically. Hydrates in sediments are typically examined in terms of the local conditions of indirect thermodynamic variables, such as temperature and pressure. This can be very misleading in the evaluation of hydrate production methods. Any hydrate production method is governed by the thermodynamic laws. The combined first and second laws determine phase distributions in terms of Gibbs free energy minimum. This minimum is constrained by the first law of thermodynamics through enthalpy. The entropy changes during a specific action for hydrate production need to be sufficient to overcome the bottlenecks of breaking hydrogen bonds. In this work, I point out some important drawbacks of the pressure reduction method. The main focus is, however, on combined safe long-term storage of CO2 and release of CH4. It is demonstrated that CO2 hydrate is more stable than CH4 hydrate, in contrast to interpretations of pressure temperature diagrams, which are frequently used in discussions. Pressure and temperature are independent thermodynamic variables and merely determine at which conditions of these independent variables specific hydrates can exist. Gibbs free energy is the dependent thermodynamic variable that determines the level of phase stability. The first law determines the need for supply of thermodynamic driving forces for hydrate dissociation. Unlike in conventional analysis, it is pointed out that chemical work is also a driving force in the pressure reduction method. The release of heat from the formation of a new CO2 hydrate from injection gas is the primary source for CH4 hydrate dissociation in the CO2 method. Increased salinity due to consumption of pure water for new hydrate could potentially also assist in dissociation of in situ CH4 hydrate. Based on thermodynamic calculations, it is argued that this effect may not be significant.