This paper presents a comparative evaluation of gas-water interfacial tensions (IFT) measured using the pendant drop technique and computed using either the Selected Plane (SP) or the Computerized Image Processing (CIP) Methods at high- pressure/high-temperature (HP/HT) conditions. Both the SP and CIP Methods are based on solutions to the Young-Laplace equation. The SP Method, which is derived from an approximate solution originally developed by Bashforth and Adams (1883), is dependent on just two pendant droplet dimensions. Further, the solutions were generated for the air-water system at pressures and temperatures much lower than typically encountered in oil and natural gas reservoirs. However, these solutions are often extrapolated mathematically to HP/HT conditions.
Although it is relatively simple to use, the IFT computational accuracy using the SP Method is questionable since it is dependent on the precision to which the two pendant droplet dimensions are measured. However, improvements in computerized data acquisition and imaging systems have made it possible to digitize droplet shapes completely such that IFTs may be computed using the entire shape. So while the CIP Method is also an approximate solution (numerical solution which discretizes the droplet interface) to the Young-Laplace equation, it utilizes many more points on the pendant droplet and incorporates the actual fluid properties directly into the solution. Therefore, the question addressed in this study is: what is the accuracy of the SP Method for computing gas-water IFTs at HP/HT conditions?
To answer this question, we conducted a two-stage study. First, we used the pendant drop method and measured droplet dimensions for the gas-water system at pressures of 1,000 psia to 20,000 psia and temperatures of 122°F, 212°F, 300°F, and 400°F. We then compared gas-water IFTs computed using both the SP and CIP Methods. The comparative evaluation indicates the gas-water IFTs from the SP Method are generally and consistently lower than IFTs from the CIP Method. Differences between the two methods are dependent on pressure/temperature conditions, gas composition, droplet geometry and size, and the illumination when the droplet shape is photographed. However, the maximum absolute mean error for all evaluations was less than 3.5%, suggesting the SP Method is valid for computing gas-water IFTs at HP/HT conditions.