An Analysis of CO2, CH4 and Mixed CO2-CH4 Gas Hydrates: Experimental Phase Equilibria Measurements and Simulations with State-of-the-Art Software

Abstract

The enormous amount of CH4 trapped in gas hydrates has led to a growing focus on gas hydrates as a potential future energy source. One of the proposed methods for the production of CH4 from these deposits, is to exchange the CH4 molecules with CO2 molecules. In this context, the determination of the phase equilibria of mixed CO2-CH4 hydrates is essential. In this thesis, the phase equilibria of mixed CO2-CH4 hydrates has been investigated through simulations in PVTsim. The capability of simulating these systems to a significant degree of accuracy is a vital factor in potentially securing the practical application of the CO2-CH4 exchange process. The predictability and accuracy of PVTsim is hence evaluated by means of agreement with published -and experimental data. It has been found that PVTsim is accurate in its prediction of the dissociation conditions of mixed CO2-CH4 hydrates, while compositional predictions has been found to be less accurate. From analysis of the phase equilibria data predicted by PVTsim for mixed CO2-CH4 hydrates, it was observed that a sI/sII structural transition occurred for a range of CO2/CH4 ratios when utilizing the SRK equation of state. Based upon subsequent analysis and literary evidence, it has been concluded that this structural transition is incorrect. This error is not observed for simulations performed utilizing the PR equation of state, where sI hydrate is predicted for all CO2/CH4 ratios. Data regarding the phase equilibria of CO2-, CH4-, and mixed CO2-CH4 hydrates has additionally been acquired from experiments performed with the hydrate cell at the Department of Chemistry at the University of Bergen. The experimental data indicates that the updated hydrate cell serves as an accurate apparatus for the intended experimental purposes - where PVTsim serves as the tool for validation. Data regarding the phase equilibria of mixed CO2-CH4 hydrates at high isobaric conditions (> 7 MPa) has been acquired from experiments performed at the Statoil's research laboratory in Bergen. Predicted dissociation conditions from PVTsim with regards to the experimental data acquired indicates that PVTsim predicts these conditions less accurately. This has been attributed to the possibility of the hydrate former being in a liquid phase