Asphaltene precipitation and its effects on a solvent-based heavy oil recovery process

During a solvent-based heavy oil recovery process, such as vapour extraction (VAPEX), asphaltene precipitation occurs after a sufficient amount of solvent is dissolved into a heavy oil under certain reservoir conditions. Thus, such an in-situ deasphalted heavy oil has rather different physicochemical properties from those of the original heavy crude oil in the reservoir. In particular, it is much less viscous so that the heavy oil recovery is significantly enhanced.;The experimental results showed that selection of n-alkanes significantly affected the asphaltene yield and characteristics. The viscosity modeling of the heavy oil indicated that the high heavy oil viscosity strongly depends on not only asphaltene content but also on their specific status in the heavy oil. When the heavy oil was saturated with a hydrocarbon solvent at a relatively low pressure, the solubility, oil-swelling factor, and molecular diffusivity of the heavy oil-solvent system increased but its viscosity was dramatically reduced as pressure increased. If the saturation pressure approached or exceeded the solvent vapour pressure, asphaltene precipitation occurred and the solvent dissolution in the deasphalted oil was greatly enhanced. The viscosity, density, and asphaltene content of the flashed-off heavy oil were significantly lower than those of the original heavy crude oil. In the VAPEX experimental tests, asphaltene precipitation occurred when the operating pressure was close to the solvent vapour pressure. The oil production rate was substantially increased in a relatively high-permeability porous medium. However, for a low-permeability porous medium, severe reservoir plugging due to asphaltene deposition drastically reduced or even stopped the oil production.;This Ph.D. dissertation studied the asphaltene precipitation in several heavy oil-solvent systems and examined its detailed effects on their physicochemical properties and on the VAPEX heavy oil recovery process. More specifically, first, four asphaltene samples precipitated with different n-alkanes and four deasphalted oils (i.e., maltenes) were characterized and compared by using various analytical techniques. Secondly, eleven reconstituted heavy oil samples with different asphaltene contents were prepared to model the in-situ deasphalted heavy oils to different extents. Two viscosity models for a colloidal dispersion system were applied to find the best fit to the experimentally measured heavy oil viscosities at six constant temperatures. Thirdly, a heavy oil-propane system in the presence or absence of asphaltene precipitation was characterized by measuring and comparing several physicochemical properties (e.g., the solubility, oil-swelling factor, density, viscosity, asphaltene content, and aromaticity) of the solvent-saturated and flashed-off heavy oils taken from the upper and lower parts of the bulk oil phase. Fourthly, a series of PVT and fluid phase behaviour studies of a heavy oil sample with four respective hydrocarbon solvents was conducted under reservoir conditions. Fifthly, enhanced solvent dissolution into the in-situ deasphalted heavy oil was found by saturating three heavy oil samples that had rather different asphaltene contents with propane at five respective equilibrium pressures. The solubilities, oil-swelling factors, viscosities, and diffusivities of the three heavy oil-propane systems were measured. Also the Peng-Robinson equation of state and the Lederer equation were used to predict the solubility and viscosity of the heavy oil--solvent system, respectively. Finally, a total of eight VAPEX experimental tests were conducted to study the effects of the solvent type, operating pressure, and permeability on the asphaltene precipitation in porous media and on the heavy oil and solvent production.