Swelling of Oil-Base Drilling Fluids Due to Dissolved Gas

Abstract

ABSTRACT A major problem associated with the use of oil-base drilling fluids is the increased difficulty of detecting gas which enters the borehole and dissolves in the drilling fluid. Previous authors have made computer-simulated comparisons between a water-base and an oil-base drilling fluid for one specific field example. However, a method is needed that will permit field personnel to quickly estimate the amount of dissolved gas that can be associated with an observed pit gain for the field conditions present. In this paper, a method is presented for estimating the swelling of oil-based drilling fluids due to dissolved gas. The method can be applied both (1) when the gas is fully miscible with the drilling fluid, and downhole mixing is limited and (2) when gas initally contacts the drilling fluid in volumes above the solution gas-mud ratio, and mixing is enhanced by the initial developement of gas bubbles. Experimental PVT data were used to verify the calculation method presented for a range of compositions, temperatures, and pressures. The method was also verified by experiments in a 6000-foot test well. Examples are presented showing typical computed values for swelling volumes at various depths, mud densities, and gas concentrations. Pit gain comparisons are made with water-base drilling fluids for a wide range of conditions. These examples illustrate situations in which it is difficulty to detect a gas kick in an oil-base drilling fluid. In addition to determining the amount of the dissolved gas present in a given field situation, the method can also be used to determine the sensitivity requirements of kick-detection equipment for any specified hole geometry. The method applies to both surface and subsurface kick-detection equipment.