Understanding the interaction between the drilling fluid and the natural gas from a gas kick may be of great importance when predicting how a well control incident evolves during drilling operations. This is especially true for oil based mud, which has the ability to dissolve large quantities of gas under high pressure, thus potentially hide any volumetric impact of a gas kick. When the pressure of the dissolved gas decreases below the bubble pressure, free gas will start to emerge. Dangerous situations can occur if the bubble point pressure is low and located close to the surface. This may result in a rapid volumetric expansion of the free gas, as it emerges from solution, thus little to no time to react and initiate proper well control procedures.
Most conventional well control simulators that takes gas solubility into consideration assumes an instantaneous vaporization of gas as the vapour-liquid phase equilibria changes. However, this assumption might not always be realistic. It may take some time before a new equilibrium is reached when the conditions are changed. This will thus affecting the rate of gas liberation from the liquid.
To better understand this complex issue, an analytical expression for the transition rate of dissolved gas to free gas in a supersaturated liquid has been derived for low pressure systems. The analytical model is strongly dependent on the solubility coefficient, Kh, and the transition rate factor, γ, and follows an exponential curve. In this expression, Kh is a measure of how much the liquid is supersaturated at any given time and controls how much gas that will be liberated. γ determines how fast the system will reach a new equilibrium, i.e. how fast the gas will be liberated based on the size of the supersaturation. Both Kh and γ are thought to be values given for a specific gas-liquid combination.
In order to verify the analytical expression, experimental testing has been conducted. The experiment is carried out by pressurizing a tank partly filled with the base oil Exxsol D60 by feeding it with methane gas. Some of the gas will dissolve into the liquid. The rest will flow to the top as free gas and pressurize the tank. By quickly removing some of the free gas, thus depressurize the tank, the liquid will instantaneously become supersaturated, hence triggering liberation of free gas from the solution until a new equilibrium is established. By measuring the tank pressure throughout the degassing phase, values for Kh and γ can be estimated and compared to the analytical model.