Effects Of Asphaltenes And Resins On Foamability Of Heavy Oil

Abstract

Abstract Because of the existence of large reserves, the production of heavy-oils is the object of increasing interest. Some heavy oil reservoirs show anomalous behavior in primary production, with rates of production better than predicted. Interpretations based on the formation of foam (or bubbles dispersion) have been proposed. However there is no evidence for foam production in reservoirs and a mechanism based only on high viscosity has also been proposed. In order to determine the real mechanism, we have studied the "foamability" of heavy oil from South America. The experiments consist of standard measurements in foam studies: qualitative measurements (foam volume, thin film lifetime), and physical properties (volume viscosity, dynamic and static surface tension, elastic modulus). Due to the high viscosity of the crude oil, measurements were performed on solutions of resins and/or asphaltenes in toluene. Asphaltenes were extracted by the HPLC method, and resins by adsorption on silica gel. First, solutions with only asphaltenes were used with concentrations in weight from 1% to 20%. The effect of resins was then investigated using solutions with various resin and asphaltene concentrations. The main results are:The presence of asphaltenes enhances foamability and film lifetime, with a large variation in all measurements at around 10% asphaltene concentration.A very long characteristic time of dynamic tension and a large increase of elastic modulus were observed.Added resins have no effect on foam volume. However, the threshold in properties around 10% asphaltene concentration is no longer observed with resins. The results can be interpreted with existing models for polymer clustering and interfacial behavior of proteins. As proteins, asphaltene molecules can reorganize themselves at the interface, leading to a long time of tension decrease and a high increase of elastic modulus. The effect of resins is analyzed by comparison with crude-water emulsion. In conclusion, with the systems we have studied, asphaltene fraction enhances significantly foam properties (foamability, single film lifetime, surface rheology) above 10% in weight. Because problematic is flow of heavy oil in porous media. Then we have also performed experiments in glass micromodel with the same fluids, but no proofs of interfacial effects were observed. Introduction Extensive use of fossil energy has been made in the recent years. Oil production is expected to begin to decline before 20101. Because of the existence of still important reserves, the production of heavy oils2, which are highly viscous oils is the object of much interest. The high viscosity of these oils is attributed to the large quantity of asphaltenes, which are polyaromatic compounds of high molecular weight, ranging from 1,000 to over 50,000 g/mol. They are a solubility class defined as the oil soluble fraction in toluene and insoluble fraction in alkanes, in practice pentane or heptane. They contain a large variety of chemical species, with functional groups including acids and bases. Their composition depends on the alkane nature and amount, and on the origin of the crude oil3. Some of these groups are hydrophobic, whereas the polyaromatic skeleton is more polar and as a result, asphaltenes are surface active4. In the bulk oil, they pack into stacks of layers and form extended aggregates, fractal-like when dispersed in good solvents and more globular although large (several microns) in marginal solvents such as mixtures of toluene and heptane5.