Thermodynamic properties of natural gas mixture in the pre-salt layer: A molecular dynamics approach

Abstract

Accurate determination of thermodynamic properties under extreme conditions, such as those of supercritical fluids, is challenging. Such conditions are typically found in the oil reservoirs in pre-salt layer. In this work, we investigate the behavior of thermodynamic properties of natural gas mixtures, primarily composed of methane (CH₄), light hydrocarbons, and non-hydrocarbon gases, in pre-salt layer conditions employing molecular dynamics simulations with the TraPPE and OPLS force fields. We computed density, speed of sound, isobaric and isochoric heat capacities, Joule-Thomson coefficient, isothermal compressibility, and thermal expansion coefficient over a temperature range of 323.15 to 413.15 K and a pressure range of 10 to 60 MPa. To enhance the accuracy of our results, we employed the Multistate Bennett Acceptance Ratio (MBAR) estimator. The values obtained for density and speed of sound showed strong agreement with available experimental data. Given the scarcity of experimental data for the other properties, we validated our simulation results against the Soave-Benedict-Webb-Rubin (SBWR) equation of state. Notably, the rigid-molecule force fields provided a more accurate description of the thermodynamic properties of the natural gas mixture studied, underscoring its efficacy in modeling complex fluid behaviors under extreme conditions.