Experimental Study of the PVTX Properties of the System H2O-CH4
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The system H2O-CH4 is found in a variety of geological environments in the earth's crust, from sedimentary basins to low grade metamorphic terrains. Knowledge of the Pressure- Volume-Temperature-Composition (PVTX) properties of the H2O-CH4 system is necessary to understand the role that these fluids play in different geological environments. In this study the properties of the H2O-CH4 fluid system at elevated temperatures and pressures has been investigated experimentally to determine the PVTX properties of H2O-CH4 fluids in the P-T range equivalent to late diagenetic to low grade metamorphic environments, and CH4 mole fraction less than and equal to 4mol%. A study has also been conducted to determine methane hydrate stability over the temperature range of -40~20 degree Celsius. Synthetic fluid inclusions were employed in both studies as miniature autoclaves. Experimental data for the PVTX properties of H2O-CH4 fluids under late diagenetic to low grade metamorphic conditions was used to calculate the slopes of iso- Th lines (the line connecting the P-T conditions of the inclusions at formation and at homogenization) at different PTX conditions. An empirical equation to describe the slope of iso-Th line as a function of homogenization temperature and fluid composition was developed. The equation is applicable to natural H2O-CH4 fluid inclusions up to 500 degree Celsius and 3 kilobars, for fluid compositions less than and equal to 4 mol% CH4. The Raman peak position of CH4 gas is a function of the pressure and temperature. This relationship was used to determine the pressure along the methane hydrate stability curve in the H2O-CH4 system. The combined synthetic fluid inclusion, microthermometry and Raman spectroscopy method is a novel experimental approach to determine the P-T stability conditions of methane hydrates. The method is fast compared to conventional methods, and has the potential to be applied to study other gas hydrate systems.
- Doctoral Dissertations