Dynamic surface properties of asphaltenes and resins at the oil-air interface

Because of the existence of large reserves, the production of heavy oils is presently the object of much interest. Some heavy oil reservoirs show anom alous behavior in primary production, with rates of production better than predicted. In Canada and Venezuela some heavy oils are produced in the form of "bubbly" oil, which is stable for several hours in open vessels. These crude oils are therefore commonly called "foamy oils." Since the presence o f bubbles could be responsible for an enhanced rate of production, a better knowledge of the properties of the gasoil interface is desirable. We have experimentally studied the effect of concentration of asphaltenes and resin s on static and dynamic properties of oil-air interfaces and also on bulk v iscosity. The experiments include surface tension measurements using the pe ndant-drop method, surface viscosity by the oscillating-drop method, foamab ility by continuous gas injection, and film lifetime, All the experiments w ere performed using resins and asphaltenes in toluene solutions at 20 degre esC. At first asphaltenes enhance foamability and film lifetime. All the ex periments performed showed a change in regime for asphaltene concentrations around 10% by weight, possibly due to clustering. At the studied concentra tions, the adsorption process at the air-oil interface is not diffusion con trolled but rather involves a reorganization of asphaltene molecules in a n etwork structure. The formation of a solid skin is well identified by the i ncrease of the elastic modulus. This elastic modulus is also an important p roperty fbr foam stability, since a rigid interface limits bubble rupture. The interface rigidity at long times decreases with increases in resin frac tion, which could decrease foam stability as well as emulsion stability. (C ) 2001 Academic Press.