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 un dertaken in crude oil-containing systems at elevated temperatures and pressures due to the experimental problems involved. We describe a nov el method that has been developed to allow such measurements to be mad e. 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 phas e). This arrangement ensures that the captive droplet or bubble has th e necessary geometry to permit surface or interfacial tension measurem ents over a wide dynamic range. These interfacial tension values are d etermined by digitizing a droplet or bubble image, statistically fitti ng the profile to a mathematical function, and then using that functio n to provide geometrical co-ordinates required to compute the tension by means of one of several axisymmetric-type calculation procedures ch osen according to the magnitude of the tension to be determined. The f luid phases and capture surface are all contained within a cell that p ermits imaging under conditions of equilibration at elevated temperatu res and pressures. Comparisons between the captive drop method and eit her standard systems or independent measurements show that good agreem ents were attained for both surface and interfacial tensions ranging f rom 83 mN m(-1) for aqueous brine/air systems down to 0.003 mN m(-1) f or a crude oil/natural surfactant cosurfactant solution system (i.e, o ver five orders of magnitude). The technique is capable of measuring s urface or interfacial tensions over a wide range of temperatures, up t o 200 degrees C, and pressures, up to 69 MPa (10000 psi).