Predicting Gas Kick Behavior in Oil-Based Drilling Fluids Using a PC-Based Dynamic Wellbore Model

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ABSTRACT The high solubility of natural gas in oil-based drilling fluids can mask the true nature of a gas kick. The sudden evolution of gas near the surface can lead to severe well control problems. The problem is accentuated to varying degrees by the presence of CO2 or H2S. Previous authors measured gas solubility and swelling behavior of oil-based drilling fluids and natural gases to tune the equation of state (EOS). Multiphase flow and EOS models run on mainframe computers were used to predict the effects of gas kicks on surface indi-cators such as pit gain and annular flow rate while gas kicks were allowed to travel to the surface uncontrolled. This paper presents a new approach to predicting gas kicks in oil-based drilling fluids utilizing an interactive PC well control model originally developed for water-based drilling fluids. Experi-mental swelling and solubility data were obtained for mixtures of C02-rich gas and "non-toxic" base oil at simulated downhole temperature and pressure conditions. Instead of the EOS, a modular subroutine was written to describe the expansion of these mixtures as a function of pressure, temperature and mixture composition. The modified model was used to predict pit gain, annular flow rate, casing pressure and gas distribu-tion in the annulus during simulated gas kicks in a hypothetical well. A comparison to water-based drilling fluids under similar conditions was made. It is concluded that small but potentially dangerous gas kicks in oil-based drilling fluids may not be detected by an early increase in annular flow or pit gain.The dampening of surface response is due to the high gas solubility in the oil phase of the drilling fluid.Stopping circulation to "check for flow" can be used to detect gas kicks in both oil-based and water-based fluids. The well will be flowing in an oil-based drilling fluid because not enough drilling fluid is present deep in the annulus to mask a flow increase. Lower increases in casing pressure, annular flow and pit gain result when circulating a gas kick to the surface under controlled conditions in an oil-based fluid than in a water-based fluid, because all or a portion the gas is forced into solution by the back-pressure that is maintained on the annulus.