An experimental study of mixed CO2-CH4 hydrate phase equilibria and the CO2-CH4 exchange reaction

Abstract

Large amounts of CH4 are trapped in naturally occurring gas hydrate deposits. Extensive research on these species has generated a considerable interest in developing technologically and economically viable recovery methods for this potentially enormous energy source. Extraction of CH4 from the hydrate phase by replacing it with CO2 is the most recently proposed approach. As this simultaneously offers geological sequestration of CO2 it is regarded as a very promising option. Extensive knowledge and experimental data on the CO2-CH4 hydrate phase equilibrium is a requirement for further development of this approach. This thesis features phase equilibria studies on simple CO2, simple CH4 and mixed CO2-CH4 gas hydrates. In addition to this the CO2-CH4 exchange reaction has been investigated by injecting CO2 into a system containing stable CH4 hydrates in the presence of available water and CH4. The experiments were performed in a high pressure cell fitted with a state of the art data acquisition system. CO2 injection led to additional mixed hydrates due to available CH4 and water, which suggest that CO2 injection into a reservoir rock containing excess water may lead to a decrease in permeability. Over a series of six CO2-CH4 hydrate experiments it was found that gradually changing the gas composition from CO2 dominant to CH4 dominant, gradually shifts the L_w-V-H phase equilibrium line towards higher requirements for thermodynamic driving forces. CO2 was found to be the preferred guest molecule. The simple hydrate experiments showed that quantitative changes in a simple hydrate system does not affect the L_w-V-H phase equilibrium line. Predicted equilibrium curves calculated in PVT have been compared to experimental PT curves during dissociation of the hydrate phase. Phase equilibrium calculations show good agreement with experimental data. However, some deviations are seen for the mixed CO2-CH4 hydrate systems.