Well Control Problems Associated With Gas Solubility in Oil-Based Drilling Fluids.

Gas contamination of an oil-based drilling fluid during drilling operations, whether it be by the flow of formation gas into the wellbore (gas kick) or by the drilling of gas-bearing formations (drilled-gas), poses a potential hazard to the drilling equipment, environment, and personnel. This danger is the greatest when bottomhole conditions are such that the gas will completely dissolve into the drilling fluid and rapidly evolve as the gas-cut drilling fluid is circulated up the well. This work summarizes a study of well control problems associated with gas solubility in oil-based drilling fluids. The solubilities of various gases (i.e., methane, ethane, carbon dioxide, etc.) in base oils used in oil-based drilling fluid preparation as well as an oil-based drilling fluid over a range of pressures and temperatures were measured and a method for predicting the solubility of a gas mixture containing methane, ethane, and carbon dioxide in an oil-based drilling fluid is presented. In addition, methods for predicting the pit gain to be expected for a given gas kick taken while drilling with an oil-based drilling fluid and for predicting the annular behavior to be expected when drilled-gas contaminates an oil-based drilling fluid during drilling were developed. Both methods were verified using data measured during experiments conducted in a 6000 ft test well