Detection of the Onset of Asphaltene Precipitation in a Heavy Oil-Solvent System

Abstract When a hydrocarbon solvent is made in contact with a heavy oil under a sufficiently high reservoir pressure, asphaltene precipitation occurs so that the heavy oil is in-situ upgraded during a solvent-based heavy oil recovery process. Some physicochemical properties of this in-situ upgraded heavy oil are rather different from those of the original crude oil in the heavy oil reservoir. In this paper, a series of saturation tests is conducted for a heavy oil-propane system under different saturation pressures in a see-through windowed high-pressure saturation cell with six sampling ports at different vertical locations. The onset of asphaltene precipitation is determined by measuring and comparing several physicochemical properties (e.g., the solubility, oil-swelling factor, density, viscosity, and asphaltene content) of the propane-saturated and flashed-off heavy oils taken from different parts of the propanesaturated heavy oil under each saturation pressure. It is found that when the heavy oil is saturated with propane at P ? 780 kPa, the respective properties of the solvent-saturated and flashed-off heavy oils taken from the upper and lower parts are different to large extents. This may be because asphaltene aggregation occurs in the solvent-saturated heavy oil and some heavy components move downward to the bottom of the saturation cell. If the saturation pressure is increased to P=850 kPa, asphaltene precipitation occurs and some large asphaltene particles are deposited onto the acrylic windows of the saturation cell. Although the physicochemical properties of the solvent-saturated and flashed-off heavy oils measured by using different experimental methods show variable sensitivities to the asphaltene precipitation, its onset can be successfully detected in practice. Introduction Western Canada has tremendous heavy oil and bitumen deposits with estimated original-oil-in-place (OOIP) of 2.5 trillion barrels[1]. They hold great potential to meet the future hydrocarbon fuel demand, while the conventional petroleum reserves are being depleted. Nevertheless, how to effectively and economically recover heavy oil and bitumen remains a technical challenge due to their extremely high viscosities. At present, thermal-based heavy oil recovery methods are often applied because they can dramatically reduce the heavy oil viscosity. However, large heating and water source requirements, heat losses to thin oil formations, and water treatment cost make these tertiary oil recovery methods ineffective and uneconomical. Solvent-based heavy oil recovery processes[2–7] have recently gained more and more attention because of their distinct advantages over the thermal-based heavy oil recovery methods. In a typical solvent-based heavy oil recovery process, such as vapour extraction (VAPEX) process, gaseous condensable solvents[8], together with non-condensable carrier gases[9], are injected and dissolved into the heavy oil to dramatically reduce its viscosity. In some cases, the heavy oil viscosity reduction due to sufficient solvent dissolution may be comparable to that achieved by applying the thermal-based heavy oil recovery methods. Another major advantage of the solvent-based heavy oil recovery processes is asphaltene precipitation through sufficient solvent dissolution so that the heavy oil in the reservoir is insitu upgraded. The precipitated asphaltenes are deposited onto the sand grains and thus left behind in the reservoir. The produced heavy oil has a much lower viscosity and better quality in comparison with the original crude heavy oil[10, 11].

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