Experimental study and molecular simulation of gas dissolution and diffusion behavior in drilling fluid

An appropriate knowledge of annulus multiphase flow behavior is critical to ensure a safe well control operation in the event of a gas kick scenario during well drilling activities. However, owing to formation gas solubility in drilling fluids, the dynamic distribution of dissolved gas and free gas in the annulus will induce a complex annulus flow pattern, which provokes potential safety risks in the operation. To determine the magnitude of this effect, some fundamental works have been carried out in this paper. Experiments to measure methane solubility in brine, white oil and poly-alpha-olefin, which represent water-based, oil-based and synthetic-based drilling fluids, respectively, were performed under different temperature and pressure conditions. The attained solubility data are helpful in understanding the distribution of annulus dissolved gas and free gas, they can provide some references for evaluating the risks associated with safety well control in a gas kick scenario. Molecular dynamics simulations were also performed to study the mechanism of methane dissolution and diffusion in drilling fluids at microscopic level. It was observed that the "capacity", "power" and "path" of methane molecules in the solution will determine the extent of its solubility in the drilling fluid. According to the results, the "capacity", "power" and "path" for methane molecules to dissolve in white oil and poly-alpha-olefin is greater than in brine, suggesting the same dissolution features as the experimental results. Therefore, the molecular simulation can be an effective approach to predict the dissolution performance of different gas/drilling fluid systems with these evaluation criteria. In addition, with a better recognition of the gas dissolution process, the simulation results also provide support for further research on annulus transient multiphase flow behavior. (C) 2016 Elsevier B.V. All rights reserved.