Viscometric Determination of the Onset of Asphaltene Flocculation

Abstract A new technique for the determination of the onset of asphaltene flocculation (OAF) has been developed through viscosity measurement of the oil mixed with a gaseous solvent under miscible conditions. A capillary tube was used for the viscosity measurements. The method is based on the detection of a sharp increase in the viscosity of the oil-solvent mixture, at increasing concentrations of precipitating solvent. High pressure experiments with three oils, and propane as an asphaltene flocculent, were performed. For these oil-propane systems the OAF values determined by the new method were compared to those obtained using a photometric method. In the new method, detection of OAF caused by hydrocarbon solvent gases at high pressure, is done in a very straightforward way, and does not require any reference curves on oil mixed with non-precipitating solvents (i.e. toluene, benzene, etc). The main advantage of the new method as compared to the photometric methods, is that it is applicable for both light and heavy oils, and no dilution with toluene is necessary. Introduction The oil industry has faced the problem of asphaltene deposition for many years. Asphaltene deposition in the reservoir, well bores, well head equipment, pipelines and other downstream equipment can lead to serious problems and can increase the cost of producing oil. Presently, most methods for dealing with the asphaltene problem are remedial in nature. It would be highly desirable to use preventive methods to combat asphaltene deposition or at least to reduce the late of asphaltene deposition, thereby decreasing the cost of remedial techniques. The test methods used to determine the propensity of oil to deposit asphaltene are more complicated in nature. For this reason, given the diversity of laboratory procedures, very often, contradictory results are presented for the same combination of oil and a flocculating agent. Asphaltene deposition increases whenever anything disrupts or destabilizes the asphaltene-resin micelles. For example, the addition of low-molecular weight hydrocarbons - such as propane, n-pentane, petroleum naphtha or gasoline - will flocculate asphaltenes by solubilizing resins in the bulk oil phase. There are two different types of asphaltene deposition: - Asphaltene deposition during primary recovery of oil, which is due mainly to the changes in pressure and temperature. - Asphaltene deposition during miscible flooding involving rich hydrocarbon gases and carbon dioxide. For the latter, the most important problem is to determine the minimum amount of solvent gases that would trigger the flocculation/deposition process, at the prevailing pressure. The quantity of asphaltene so deposited is also of interest. Determination of Onset of Asphaltene Flocculation Using Light Scattering Method As mentioned, accurate determination of onset of asphaltene flocculation (OAF) and onset of asphaltene deposition (OAD) are very important for the choice of methods for preventing/combating asphaltene deposition. To the best of our knowledge, there are no methods for the determination of OAD, while there are several methods for determining of OAF having different accuracy and difficulty. The most important methods for OAF determination are: a) light scattering method, b) gravimetric method, c) interfacial tension measurement between water and oil upon dilution with a precipitant agent, and d) visual method.