Molecular dynamics simulation of real paraffin wax melting points based on OPLS-AA force field parameter optimization

Abstract

The molecular dynamics study on the mechanism of organic solid phase precipitation in crude oil has important guiding significance for ensuring oil and gas production. To address the notable inaccuracies of existing force fields in simulating wax-related properties in crude oil, this study optimizes the force field by integrating quantum chemistry methods with experimental data. Based on the OPLS-AA force field framework, we employed the quantum chemical M06-2X/def2-TZVP method to perform relaxed scanning of dihedral angles in straight-chain alkanes, obtaining all dihedral torsional parameters. The charge parameters were optimized using the measured melting points of C₂₂, C₂₆, and C₃₀ n-alkanes, leading to the development of an improved P-OPLS force field. Validation results demonstrate that this force field provides the most accurate calculations for the evaporation enthalpy and viscosity of n-alkanes. By simulating the melting points of real paraffin wax in actual crude oil from oilfields, the results show that the relative errors between the simulated melting points of paraffin wax in Well A-1 and Well A-2 using the P-OPLS force field and the experimental melting points are 0.56 % and 0.42 %, respectively. This further confirms the accuracy of the P-OPLS force field in simulating mixed-component alkane systems. Additionally, the visualization of orientation order parameters and molecular chains indicates that wax molecules are arranged in a linear order during the crystallization process. Paraffin molecules with similar chain lengths are more likely to match and align with each other when forming crystals, which helps to form stable structures. This study provides a reliable P-OPLS force field for simulating and studying straight chain alkane systems, and provides a potential basis for further investigating the mechanism of wax precipitation in crude oil.