Gas kick is an ever-present hazard whose importance is magnified for offshore drilling situations. Modeling gas kick is a complex problem that requires an understanding of the relevant fluid dynamics as well as the solubility of natural gas in oil-based muds (OBM). Drilling fluid swelling due to natural gas solubility in OBM significantly affects the extent of pit gain — one of the primary indicators of a kick in progress. This paper specifically addresses the issue of drilling fluid swelling from gas dissolution in OBM.
Drilling fluid swelling due to gas dissolution is generally expressed the same way as oil swelling due to dissolved gas, by the volume factor, Bo. Many correlations for estimating Bo as a function of temperatures and pressures are available. We have developed a rigorous thermodynamic approach for estimating Bo. Our approach uses the Peng-Robison (1976) equation of state (EOS), van der Waals mixing rules, and binary interaction coefficients appropriate for drilling fluids to account for gas solubility. Solving the cubic form of the Peng-Robinson EOS yields a z-factor for the liquid phase of the mixture. The model uses this z-factor to estimate the liquid-phase volume of dissolved methane and, consequently, Bo. This paper validates the results of estimated Bo from this method with volume factor calculations obtained from Aspen HYSYS. Finally, this paper also presents a section where the methane mole fraction data at different P&T conditions, obtained from HYSYS simulations, is used to validate the solubility model previously developed by Manikonda et al. (2019).