A CAPTIVE DROP INSTRUMENT FOR SURFACE OR INTER-FACIAL TENSION MEASUREMENTS AT ELEVATED-TEMPERATURES AND PRESSURES

Measurements of very low surface and interfacial tension are seldom undertaken in crude oil-containing systems at elevated temperatures and pressures due to the experimental problems involved. We describe a novel method that has been developed to allow such measurements to be made. In the captive drop instrument a droplet or bubble is held captive against a specially prepared surface which is strongly hydrophilic and forces a zero degree contact angle (measured through the aqueous phase). This arrangement ensures that the captive droplet or bubble has the necessary geometry to permit surface or interfacial tension measurements over a wide dynamic range. These interfacial tension values are determined by digitizing a droplet or bubble image, statistically fitting the profile to a mathematical function, and then using that function to provide geometrical co-ordinates required to compute the tension by means of one of several axisymmetric-type calculation procedures chosen according to the magnitude of the tension to be determined. The fluid phases and capture surface are all contained within a cell that permits imaging under conditions of equilibration at elevated temperatures and pressures. Comparisons between the captive drop method and either standard systems or independent measurements show that good agreements were attained for both surface and interfacial tensions ranging from 83 mN m(-1) for aqueous brine/air systems down to 0.003 mN m(-1) for a crude oil/natural surfactant cosurfactant solution system (i.e, over five orders of magnitude). The technique is capable of measuring surface or interfacial tensions over a wide range of temperatures, up to 200 degrees C, and pressures, up to 69 MPa (10000 psi).