A Novel Experimental Technique for Studying Solvent Mass Transfer and Oil Swelling Effect in the Vapour Extraction (VAPEX) Process

Abstract In this paper, a new experimental technique is developed tostudy the solvent mass transfer in heavy oil and the resultant oil swelling effect by applying the dynamic pendant drop volume analysis (DPDVA). In the experiment, a pendant drop of heavy oil is formed inside a visual high-pressure cell, which is initially filled with a solvent (e.g., propane) at the desired pressure and temperature. As the solvent gradually dissolves into heavy oil, the volume of the pendant oil drop keeps increasing due to the oil swelling effect. The sequential digital images of the dynamicpendant oil drop are acquired by applying computer digital image acquisition technique. Such acquired digital drop images are analyzed to determine the interfacial profiles and the volumes of the dynamic pendant oil drop at different times. Theoretically, a mathematical model is formulated to describe the solvent mass transfer and the oil swelling effect. This model shows that the volume change of the dynamic pendant oil drop solely depends on the diffusion coefficient of the solvent in heavy oil and the oil swelling factor. Therefore, the diffusion coefficient of the solvent in heavy oil and the oil swelling factor can be determined by finding the best fit of the theoretically predicted volumes of the dynamic pendant oil drop to the experimentally measured data. Experimental tests are conducted for propane-heavy oil system at constant temperatureof T=23.9 ° C and constant pressures of P=0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 MPa. It is found that both the diffusion coefficient and the oil swelling factor of propane-heavy oil system increasewith pressure. The major advantage of this newly developed DPDVA technique is that simultaneous measurements of solvent diffusivity in heavy oil and oil swelling factor can be completed within two hours at a pre-specified constant pressureand temperature. Introduction In the vapor extraction (VAPEX) process, a solvent (e.g., methane, ethane, propane, butane, carbon dioxide, or their mixtures) at a pressure close to its dew point is injected into a heavy oil reservoir[1]. Previous studies have already shown that molecular diffusion of the injected solvent in heavy oil plays a vital role in the VAPEX process[2, 3, 4]. Thus the diffusioncoefficient of the solvent in heavy oil under the actual reservoir conditions becomes an important parameter in the reservoir simulation and field design of the VAPEX process. In the literature, there are several experimental methods formeasuring solvent diffusivity in heavy oil. These experimentalmethods can be roughly categorized into conventional and nonconventional methods. Conventional methods involve compositional analysis of liquid samples taken from the solvent-heavy oil mixture at different times and locations during a diffusion test[5, 6, 7]. These methods are expensive, intrusiveand time-consuming, especially if the diffusion test is conducted at high pressures. In addition, compositional analysis of solventheavy oil mixture is prone to large experimental error. Nonconventional methods measure the change of a property of the solvent-heavy oil system during the molecular diffusion process.

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