Measuring Interfacial Tensions in a Gas-Condensate System With a Laser-Light-Scattering Technique

Abstract

Summary A laser-light-scattering technique was used to measure a range of interfacial tensions (IFT's) from 1 to 10(-3) mN/m in a methane/propane gas-condensate system. The data were compared with the Weinaug-Katz (WK) and the Hough-Stegemeier (HS) correlations. Results show that the HS correlation performed better when used on the critical isotherm. On other isotherms, the WK correlation gave the better fit. Introduction Low IFT between fluid phases plays a particularly important role in the success of most tertiary recovery processes in hydrocarbon reservoirs. In miscible systems, for example, enhanced hydrocarbon recovery relies on the interaction between the displacing and inplace fluids producing near-zero IFT. Here, mixing and mass transfer take place across the phase boundary, producing changes in fluid properties that lead to thermodynamic miscibility and reduced capillary pressures. This result produces low residual liquid saturation and is the ultimate objective in such processes as miscible and surfactant flooding. In gas-condensate reservoirs below the dewpoint, hydrocarbon fluid separates in the rock formation into its constituent liquid and vapor phases. Here, the EFT between phases varies with pressure. The volume of liquid condensing in the formation is often small, close to the critical saturation. This leads to permanently trapped liquid hydrocarbon. Laboratory experiments with condensate fluids, however, show that fluid flow rates are considerably improved and residual liquid saturations much reduced when IFT is low (less than 0.04 mN/m). These works demonstrate the importance of low IFT in correlating improved flow rates and low residual saturations with fluid composition and hence reservoir pressures. At present, a significant gap exists in experimental work on condensate systems reporting IFT data below 0.04 mN/m, partly because of the difficulty of controlling fluids at high pressures and temperatures. Moreover, when very low tensions are measured, extra care must be taken to avoid extraneous vibrations. Thus, equipment must be mounted on vibration-free tables, adding to the overall cost. Stegemeier appears to be the only one reporting data below 0.04 mN/m. His work is based on the pendant-drop technique, but the method perturbs the fluid system and is subject to errors when fluids are measured close to their critical points. For these reasons, existing correlations are used to extrapolate points. For these reasons, existing correlations are used to extrapolate to values below 0.04 mN/m. Two types of correlations, which are often used at high tension values with pure-component systems, have been shown to be reasonably accurate when predicted values are compared with experimental data:methods that correlate the tension as a unique function of reduced temperature, often called corresponding states correlations, andthe method proposed by Macleod and modified by Sugden that uses the parachor. In mixture systems, these methods are modified and empirical mixing equations are introduced. The most widely used correlations in the oil and gas industry for predicting EFT in hydrocarbon fluid systems are the WK correlation and predicting EFT in hydrocarbon fluid systems are the WK correlation and its modified version derived by Hough and Stegemeier. More recently, Lee and Chien derived a correlation based on critical scaling laws that can be used to predict IFT near the fluid's critical point. While these correlations predict high tension values with reasonable accuracy in the absence of good experimental data, their precision at lower values is somewhat uncertain. In this paper, we summarize the results of an experimental study on IFT measurements on a two-component gas-condensate system and compare our results with data derived from the WK and HS correlations. We used a methane/propane fluid system with critical properties similar to those described by Sage et al. and a laser-light-scattering technique as the preferred method of IFT measurements. This measurement technique has preferred method of IFT measurements. This measurement technique has the advantage of being nonperturbative and needs only a small volume of liquid to make measurements. Thus, it is capable of measuring surface properties of a fluid close to its critical point. properties of a fluid close to its critical point.