Surface Tension Of Water-Hydrocarbon Systems At Reservoir Conditions
Authors: Firoozabadi Abbas , Ramey Henry J. 
Published: March 2010

Abstract
A correlation for the estimation of surface tension of hydrocarbon gas and hydrocarbon liquids against water has been developed. Two correlating parameters, density difference between phases, and reduced temperature jar the hydrocarbon phase, were the basis in the correlation of data. New measurements the surface tension for three reservoir oils-brine are also presented.

Introduction
Many reservoir engineering studies require knowledge of the surface tension of water-gas and water-oil systems. Examples are imbibition studies and calculation of fluid saturations in water-gas and water-oil regions.
Capillary pressure is directly proportional to the surface tension between the two phases. Because at reservoir conditions the surface tension of water-gas and water-oil systems is generally about one order of magnitude higher than surface tension of equilibrium oil-gas systems, one would expect the capillary pressure to strongly influence the flow process in water-gas and water-oil systems. In the case of imbibition in fractured reservoirs, capillary pressure is perhaps the most important parameter affecting the flow performance of the reservoir.
Macleod's empirical equation(l) and later theoretical deductions(2) give the relationship between the tension at the interface and the density difference between phases for a pure substance:
Equation (Available In Full Paper)
"In this equation, ∆p is the density difference between phases, M is the molecular weight, σ is the surface tension and P is a temperature independent parameter known as parachor. Eq. 1 yields constant values of P over wide temperature ranges for pure hydrocarbons. Weinaug and Katz(3) extended Eq. 1 to mixtures in the following form:
Equation (Available In Full Paper)
Eq. 2 or various forms of this equation have been applied to binary hydrocarbon systems successfully. Apparently no attempt has been made to employ a Macleod-type relationship for the correlation and/or estimation of the surface tension of pure liquid hydrocarbons-water, gas-water and oil-water systems. As it will be demonstrated later, Eq. 2 is not applicable even for the case of water-methane surface tension estimation.
For the tension between water and pure hydrocarbons, measurements at reservoir temperatures and pressures have been reponed in the literature. Examples are: methane, propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-dodecane and benzene surface tension against water. However, tension measurements between reservoir crude and water which also provide density and property of phases are scarce in the literature.
The present study was undertaken to:develop a correlation for the tension of methane, and other hydrocarbons against water at reservoir conditions;provide data on the tension between water and reservoir oils together with compositional analyses and properties of the phases; andobserve the difference between distilled water and brine surface tension with pure hydrocarbons and reservoir oils, and to examine the appropriateness of a single correlation for both water-pure hydrocarbons and water-reservoir oils.

Literature Review
In this section, the literature data on the surface tension betweenwater-methane and water-natural gases,water-pure liquid hydrocarbons, andwater-reservoir oils will be reviewed.
The main goal is the correlation of the surface tension data at reservoir pressures and temperatures.