EXPERIMENTAL DETERMINATION OF THERMODYNAMICAL PROPERTIES OF MIXED GAS HYDRATES AS A BASIS FOR SEAFLOOR STABILITY AND CLIMATE MODELLING

In order to understand the formation and decomposition of natural gas hydrates, detailed data on their thermodynamical properties are of fundamental importance. These data are the general and indispensable basis for the development of models describing seafloor stability or the interaction between climate changes and the stability of natural gas hydrates. Numerous experimental data have been reported for the phase equilibria of gas hydrates in the binary subsystems of H2O- (CH4-CO2-N2-H2S). Little is known about the influence of additional gaseous components on the P-T-x properties. In this study, mixed clathrates were synthesised at defined temperature, pressure, and gas composition. Formation, growth, and decomposition of the gas hydrates were observed in a pressure cell under the microscope. The composition and structure of the hydrates as well as the composition of the gaseous phase were determined using Raman spectroscopy.

In the system CH4-CO2-H2O, measurements at variable P-T-x conditions were conducted to construct phase diagrams. The compositions of the coexisting liquid and solid phases were determined at constant gas composition and at variable pressures between 1 MPa and 9 MPa and temperatures between –20°C and +15°C. Interestingly, the CH4-CO2-ratio of the hydrate phase differs from that of the gas phase. CO2 is preferentially incorporated into the hydrate lattice. With the addition of CO2 the stability of these mixed hydrates increases significantly.

To study the influence of other gases such as H2S on the stability and the composition of the methane clathrates, microthermometric and Raman spectroscopic measurements were conducted on C-H-N-S-bearing gas inclusions in fluorites. The studied samples originate from boreholes in the vicinity of gas reservoirs in the NW German basin. Fluid and gas inclusions in fluorite were trapped under nearly hydrostatic pressure conditions at depth of about 4000m (40MPa) and temperatures between 250°C and 165°C. Gas-rich inclusions in fluorite consist of variable CH4, H2S, CO2, N2, and H2O (NaCl-CaCl2 brines) compositions. Formation of mixed gas hydrates was observed in almost all of the studied inclusions with a water content >1%. These gas hydrates consist of H2S, CO2, and CH4. The data obtained so far show a more preferred incorporation of H2S in the clathrate structure compared to CO2 and CH4.