Swelling of Oil-Based Drilling Fluids Resulting From Dissolved Gas

Abstract

Summary The method presented in this paper uses an experimentally calibrated equation-of-state (EOS) model to estimate the swelling of oil-based drilling fluids caused by dissolved methane. With this method, the pit gain associated with a given kick size can be determined. The calculation method was verified by experiments conducted in a 6,000-ft [828.8-m] test well. Example calculations are also presented. Introduction A major problem with the use of oil-based drilling fluids is the increased difficulty of detecting gas that enters the borehole and dissolves in the drilling fluid. Thomas et al. used a proprietary computer program to show that the surface responses to a gas kick (i.e., annular flow rate and pit gain) are less in oil-based drilling fluids than in water-based drilling fluids. They attributed the difficulty of detecting a gas kick during use of an oil-based drilling fluid to the increased solubility of the gas in the oil phase of the drilling fluid. In one example well geometry, Thomas et al. compared the observed pit gains and annular flow rates resulting from gas kicks in oil- and water-based drilling fluids and concluded that pit gain was the most reliable indicator of a kick in both oil- and water-based drilling fluids. This paper presents a method that uses an experimentally calibrated EOS model to estimate the swelling of oil-based drilling fluids caused by dissolved methane. With this method, the pit gain associated with a given kick size can be determined. The calculation method was verified by experiments conducted in a 6,000-ft [1828.8-m] test well. Examples that show typical computed values for swelling volumes at various depths, drilling fluid densities, and gas concentrations are presented. Pit-gain comparisons are made with water-based drilling fluids for a wide range of conditions. These examples illustrate situations in which it is difficult to detect a gas kick in an oil-based drilling fluid. The method can also be used to determine the sensitivity requirements of kick-detection equipment for any specified hole geometry. The method applies to surface and subsurface kick-detection equipment. Oil-Based Drilling-Fluid Swelling Liquid swelling caused by dissolved gas is usually represented by an FVF, B, defined as the ratio of the volume of liquid and dissolved gas at a given pressure and temperature to the volume of gas-free liquid at standard pressure and temperature. The FVF for an oil-based drilling fluid, Bf, is a function of the volume fraction of base oil and water used in the preparation of the drilling fluid and the FVF of each phase. Swelling of a base oil caused by dissolved gas, Bo, is calculated with the Peng-Robinson EOS2 (PREOS), which is outlined in the Appendix. Table 1 lists the compositions, critical pressures and temperatures, and acentric factors used m the EOS model for several base oils commonly used in drilling fluids. It also lists the binary interaction coefficients used to calibrate the EOS model.