The deposition of asphaltenes in oil producing formations and production systems has caused problems for years. Selection of chemical control agents in the past has been limited to bulk dissolution studies on samples retrieved from production systems. Until recently, the accepted way to treat these problems has been through the use of xylene, toluene or other aromatic solvents. This method requires the use of large amounts of these solvents, as well as a high frequency of treatment. This paper desribes the results of field testing and application of asphaltene control chemicals, and the use of laboratory tests to select asphaltene deposition removal and prevention chemicals.
Preliminary dispersant and solvency tests are conducted by an asphaltene dispersant test in hexane. Chemicals which provide promising results in dissolving and dispersing asphaltenes in the non-solvent medium of hexane are selected as candidates for field application, or for additional testing in a core flow deposition removal test. The core flow test apparatus provides a method to introduce asphaltene fouling into a core and study its removal by the use of chemical agents. Using core samples and asphaltenes from the production resource under consideration allows the selection of the best removal chemical.
Asphiltenes are complex hetroatomic polar macro-cycles containing carbon, hydrogen, sulfur, and oxygen. They are large and highly aromatic in nature, and occur in crude oils as stabilized micelles. Resins and maltenes, which are molecular precursors of the asphaltenes, act to stabilize a dispersion of asphaltene particles. The asphaltenes are surrounded by the resins and maltenes polar heads, while their increasingly aliphatic tails extend into the hydrocarbon (oil) phase. When mechanical or chemical forces become sufficiently great, these stabilizing species are lost and the asphaltene particle becomes susceptible to interaction with a similarly destabilized asphltene leading to flocculation and precipitation. These destabilizing factors include: streaming potential caused by the fluid flow in the porous media of the formation which destabilizing asphltenes by electrical potential, mechanical factors, and by the addition of foreign agents. These agents can be acid or other stimulation or workover fluids, and recovery assist additives such as CO2 or other miscible gases. These materials can destabilize the asphaltenes by changing the pH or the solubility characteristics of the crude oil. 1,2,3,4
Because the asphaltene particle is polar, it may take on a charge by induction resulting in secondary aggregation. As aggregation proceeds, accumulations of asphaltene macro particles appear. Bubble point effects are important because they represent the chemical stripping mechanism of destabilization. As the pressure drops below the bubble point, light ends encounter the aliphatic tails of the resins and maltenes causing a momentary imbalance in the character of the surrounding media. This momentary imbalance is sufficient to cause a stripping of the resins and maltenes and cause destabilization. Mechanical processes facilitate this action in several ways, but the most important of these is the movement of the crude from one pressure zone to another lower pressure zone.
