A Laboratory Study of the Effects of Fluid Compositions on Gas-Water Interfacial Tension at HP/HT Reservoir Conditions
Authors: Shariat A. , Moore R. G. , Mehta S. A. , Van Fraassen K. C. , Newsham K. E. , Rushing J. A. 
Published: October 2011

Abstract
This paper presents the second phase of a comprehensive, three-year laboratory study designed to measure gas-water interfacial tension (IFT) at high-pressure/high-temperature (i.e., HP/HT) reservoir conditions. The first phase of our laboratory study (Rushing, et al., 2008a) evaluated the effects of pressure and temperature as well as common nonhydrocarbon contaminants (i.e., CO2 and N2) in the gas phase on gas-water IFT for several dry gas mixtures with specific gas gravities ranging from 0.5781 to 0.7664. These gas mixtures contained primarily methane but also had small percentages of ethane and propane. Carbon dioxide or nitrogen concentrations ranged from 0, 5, 10 and 20 mole% each. All measurements were made with distilled water.
The second phase of our study, which is presented in this paper, extends the work of Rushing, et al. (2008a) with the primary objective of evaluating the effects of gas phase hydrocarbon composition on gas-water IFTs at HP/HT reservoir conditions. Gas compositions of the tested gas mixtures from Phase I were expanded to include an additional five hydrocarbon components so that the final tested gas mixtures had components ranging from methane to n-hexanes. The resulting gas mixtures had specific gas gravities from 0.8871 to 1.0101. Similar to the first part of our study, we also evaluated the effects of CO2 and N2 in the gas phase. To establish base lines for comparing the effects of hydrocarbon and non-hydrocarbon composition, we also measured IFTs for the CH4-H2O system at HP/HT conditions.
Gas-water IFTs for all gas mixtures were measured at pressures ranging from 1,000 psia to 20,000 psia and at temperatures of 300°F to 400°F. The single-gas-component base line measurements were made at the same pressure range but included two additional temperatures of 122ºF and 212ºF. Note that the gas mixtures for both study phases were designed to be "dry" gases which we define as one that remains in the single hydrocarbon (gas) phase during the entire isothermal pressure depletion path. All gas-water IFTs were measured using a pendant drop method with computer-aided image processing and analysis. Based on the measurements, we observed the following general behavior in gas-water IFTs at HP/HT conditions:
Increasing the concentration of intermediate and heavier molecular weight hydrocarbons has a significant impact on the gas-water IFT. The largest difference occurs at lower temperatures and in the higher pressure range. Temperature still has the most significant impact on gas-water IFTs for all gas mixtures and at all pressures. Generally, increasing temperature decreases gas-water IFT. The presence of CO2 in the gas phase had little to no impact on the gas-water IFTs for both 300ºF and 400ºF. In general, CO2 caused IFTs to increase slightly at lower pressures. At higher pressures, the IFTs either decreased slightly or did not change regardless of the CO2 concentration. The presence of N2 in the gas phase had a much more significant impact than CO2 but only at the lower temperatures. For all gas mixtures, gas-water IFTs increased over most of the pressure range evaluated in this study. However, the decrease in IFT caused by increasing temperature completely countered the increase caused by N2 at 400oF.