Analysis of CH4 Solubility Characteristics in Drilling Fluids: Molecular Simulation and Solubility Experiment

Based on molecular simulation methods, this paper constructs a molecular model of the CH4-drilling fluid system to conduct an in-depth analysis of the microscopic dissolution behavior of CH4 in drilling fluids. By utilizing key parameters such as molecular motion trajectories, interaction energies and solubility free energies, the mechanisms of CH4 dissolution and diffusion are revealed. The factors influencing CH4 solubility and their variation mechanisms are elucidated at the molecular level. Through gas solubility experiments, the variation patterns of CH4 solubility in drilling fluids under different temperature and pressure conditions are investigated, and optimized solubility models for CH4-drilling fluid systems are selected. The results indicate that the dissolution and diffusion behavior of CH4 in drilling fluids can be described using free volume, interaction energy and solubility free energy, with the degree of influence ranked as follows: interaction energy > free volume > solubility free energy. The interaction and free volume of CH4 in oil-based drilling fluids are both greater than those in water-based drilling fluids, suggesting a higher solubility of CH4 in oil-based drilling fluids. Solubility models of CH4 in drilling fluids under conditions of 30~120 °C and 10~60 MPa are obtained by regression. The research findings not only deepen the understanding of the dissolution and diffusion behavior of CH4 in drilling fluids for shale gas horizontal wells, but also provide crucial parameters for establishing wellbore pressure models in managed pressure drilling.