Asphaltene Stabilization in Alkyl Solvents Using Oil-Soluble Amphiphiles

Abstract

Abstract Asphaltene plugging is a known cause of near well-bore formation damage. The stability of asphaltenes is predominantly controlled by the thermodynamic state of crude oil. Changes of temperature, pressure and compositions of crude oil can result in the precipitation of asphaltene components. In this study, the stability of asphaltenes in alkane solvents was investigated using a series of alkyl benzene-derived amphiphiles. The results indicate that the effectiveness of amphiphiles on asphaltene stabilization are strongly influenced by their chemical structure, including the polarity of head groups, the length of alkyl tails and the existence of extra polar groups. These results also provide the information about the interactions of asphaltenes and resin molecules in the crude oil. The small-angle X-ray scattering technique was used to investigate the physical structure of asphaltenes in the amphiphile/alkane solutions. The results indicate that solubilized asphaltenes can interact with each other or with amphiphiles to form associated colloids. The phenomena of precipitation and dissolution of asphaltenes in the porous media was investigated using visual micromodel experiments. In a dissolution experiment, the oil-soluble amphiphile/alkane solutions were proved to be capable of removing asphaltene deposits from porous media. Introduction Crude oil produced from sub-terranean formations is usually composed of a distribution of molecules with different chemical structures and molecular weights. The heaviest and most polar molecules, called asphaltenes, usually aggregate together through aromatic orbital association, hydrogen bonding, and acid-base interactions to form onion-like micellar particles with an extremely polar core and a slightly polar shell. Asphaltic particles are more polar and heavier than the aliphatic components in the crude oil and hence, cause a variety of problems during oil production. They may cause formation damage as a result of precipitation and deposition on the pore throats, or they can alter the wettability of rock surfaces (from water-wet to oil-wet) by the adsorption onto mineral surfaces. These variations of oil-water and oil-solid interfaces change the placement and flow of reservoir fluids and may reduce the efficiency of oil recovery. Asphaltenes are defined classically as the fraction of crude oil that is soluble in benzene but insoluble in pentane or heptane. Asphaltenes obtained by the above method include the most polar and the highest molecular weight species of the crude oil. The solubilities of asphaltenes has been studied extensively by using the concept of the solubility parameter. The solubility of asphaltenes was measured by dissolving them in the organic solvents with different solubility parameters and was found to be very closely related to the solubility parameters of solvents. This parameter usually defined as () (where U, V are heat of vaporization and molar volume of the solvent, respectively) is an indication of the cohesive energy density of the solvent. The higher the solvent solubility parameter, the better the solvent can prevent asphaltene molecules from aggregation. The onset of asphaltene flocculation in crude oil is primarily determined by the changes in the composition, temperature and pressure of crude oil in the reservoir during oil recovery. The most significant examples of asphaltene precipitation occur under conditions of gas flooding or miscible flooding of light hydrocarbons (LHG) or CO2. LHG or CO2 dissolved in the crude oil can decrease the solubility parameter of crude oil and induce the precipitation of asphaltenes. The amount of asphaltene precipitation caused by LHG and CO2 increases with increasing pressure but decreases with increasing temperature. The precipitation of asphaltenes in the reservoir by miscible or gas flooding may cause serious near well-bore asphaltene plugging. Since massive volumes of crude oil flow toward well-bore regions, even tiny amounts of unstable asphaltenes can be extremely detrimental. Previous studies found that the pressure change has a significant effect on asphaltene flocculation. When the pressure is reduced, the solubility parameter of crude oil will decrease due to the decrease of the oil density until the bubble point pressure is reached. Below the bubble point pressure, light gases evolve from the oil. The remaining liquid phase becomes more dense and makes asphaltenes soluble again. The effect of temperature on the asphaltene precipitation is even more complex. P. 339^