An Experimental Study of Gas Solubility in Oil-Based Drilling Fluids

Abstract

Summary Experimental data are provided for the solubility of C1, C2, CO2, and a natural gas mixture in base oils and emulsifiers used to prepare oil-based drilling fluids over a range of temperatures. In addition, empirical correlation for predicting gas solubility in oil-based drilling fluids at low to moderate pressures is presented and a field application is outlined. Introduction Gas contamination of an oil-based drillig fluid during drilling operations, whether it be by the uncontrolled 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. Before this phenomenon can be well understood and modeled, the solubility of natural gases in a wide variety of oil-based drilling fluids must be known as a function of temperature and pressure. O'Brien1 was the first to report the results of a study on well-control problems eattsed by gas solubility in oil-based drilling fluids. Although O'Brien made no experimental measurements, he concluded that at the same pressure and temperature, the solubility of gas in an oil-based drilling fluid would be 10 to 100 times greater than the solubility in water-based fluids. Thomas et al.2 presented nine experimentally measured data points for C1 solubility in No. 2 diesel oil and three for C1 solubility in an unweighted oil-based drilling fluid, all at 100°F [37.8°C]. A summary of their data is shown in Table 1 and Fig. 1. In their study, Thomas et al. showed that C1 solubility in the oil-based drilling fluid was less than the solubility of C1 in pure No. 2 diesel oil. It was stated that this difference in C1 solubilities in the two liquids was caused by the presence of brine, emulsifier, and solids in the drilling fluid. In addition, air that may be adsorbed by solid components was cited as possibly affecting C1-solubility measurements in the drilling fluid. Ref. 1 also presents curves from computer predictions for methane solubility in diesel oil over a range of temperatures (i.e., 100 to 600°F [37.8 to 3l5.6°C]). These curves are shown in Fig. 2. Note that for low to moderate pressure, the C1 solubility appears to be essentially independent of temperature over the range of temperatures commonly found in boreholes (100 to 300°F [37.8 to 148.9°C]). Matthews3 presented solubility curves for C1, CO2, and H2S in No. 2 diesel oil at 250°F [121.1°C]. For equivalent volumes of the three gases in a mixture at some temperature, Matthews concluded that as pressure is decreased, C1 would come out of solution first, followed by CO2 and then H2S---i.e., H2S is the most soluble gas of the three gases studied, with C1 being the least soluble in No. 2 diesel oil. The curves presented here are shown in Fig. 3. In this paper, the effects of base-oil composition on the solubility of gases in oil-based drilling fluids will be discussed. Experimentally measured solubility data will be presented for C1, C2, a natural gas mixture, and CO2 in base oils, drilling-fluid emulsifiers, and oil-based drilling fluids. Factors affecting the solubility of gases in oil-based drilling fluids will be addressed, and an approximate method will be shown for determining the solubility of gas in an oil-based drilling fluid. The calculation method should be useful for calculating the depth at which drilled gas will break out of solution.