A Novel Experimental Technique for Studying Solvent Mass Transfer and Oil-Swelling Effect in the Vapour Extraction (VAPEX) Process
Authors: Yang C. , Gu Y. 
Published: March 2010

Abstract
In this paper, a new experimental technique is presented to study the solvent mass transfer in heavy oil and the resultant oil-swelling effect by applying 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). 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 dynamic pendant oil drop are acquired and analyzed to determine its volumes at different times. Theoretically, a previously formulated mathematical model is applied to describe the solvent mass transfer in heavy oil and the oil-swelling effect. The diffusion coefficient of the solvent in heavy oil and the oil-swelling factor are 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 the heavy oil-propane system at constant pressures of P = 0.4 − 0.9 MPa and a constant temperature of T = 23.9 °C. It is found that both the diffusion coefficient and the oil-swelling factor of the heavy oil-propane system increase with pressure. The major advantage of this new technique is that simultaneous measurements of the solvent diffusivity in heavy oil and the oil-swelling factor can be completed within two hours at the pre-specified constant pressure and temperature.

Introduction
In the vapour extraction (VAPEX) process, a solvent (e.g. methane, ethane, propane, butane or carbon dioxide) is injected into a heavy oil reservoir at a pressure close to its vapour pressure(1). Previous studies have already shown that molecular diffusion of the injected solvent in heavy oil plays a vital role in the VAPEX process(2–4). Thus the diffusion coefficient 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 for measuring solvent diffusivity in heavy oil. These experimental methods can be roughly categorized into conventional and non-conventional methods. Conventional methods involve compositional analysis of liquid samples taken from the heavy oil-solvent mixture at different times and locations during a diffusion test(5). These methods are expensive, intrusive and time-consuming, especially if the diffusion test is conducted at a high pressure. In addition, compositional analysis of the heavy oil-solvent mixture is prone to large experimental error. Non-conventional methods measure the change of a property of the heavy oil-solvent system during the molecular diffusion process. This property can be the gas volume(6), oil-gas interface position inside a capillary(7), laser light intensity of the heavy oil-gas mixture(8), gas pressure(9) and shape of a pendant heavy oil drop surrounded by a gaseous solvent(10). In particular, the decaying gas pressure is measured while the molecular diffusion of the solvent into heavy oil proceeds within a closed high-pressure diffusion cell. This is referred to as the pressure decay method, which has been applied to measure the diffusivities of methane, ethane, propane, nitrogen and carbon dioxide in crude oils(11–14).