During a solvent-based heavy oil recovery process, such as vapor extraction (VAPEX), a condensable solvent is injectedinto a heavy oil reservoir. Solvent dissolution into heavy oil and possible asphaltene precipitation drastically reduce its viscosity so that the diluted heavy oil can flow towards a production well. In the past, several physical modeling studies have shown that the produced heavy oil has much less amount of heavy components than the original heavy oil. This phenomenon is often referred to as in-situ upgrading. In this paper, a series of laboratory experiments is conducted under reservoir conditions to quantify the in-situ upgrading of heavy oil due to the solventdissolution and asphaltene precipitation by using a pure solvent (propane) and a mixture solvent (70 mol% methane + 25 mol% propane + 3.5 mol% n-butane + 1.5 mol% iso-butane), respectively. It is found that after a solvent is made in contact with heavy oil at a relatively high pressure for a sufficiently long time, the solvent-heavy oil system at equilibrium state can be roughly divided into three different layers. The top layer is a solvent-enriched liquid phase, the middle layer comprises heavyoil with the dissolved solvent and the bottom layer mainly consists of heavy components. The solvent-heavy oil mixtures inthese three layers show rather different chemical and physical properties, such as solvent concentration, carbon number distribution and viscosity. The top layer has the highest concentrations of solvent and light components and the lowest viscosity of heavy oil even after its dissolved solvent is flashed off. The heavy oil in the middle layer has similar carbon number distribution to the original heavy oil. The bottom layerhas the lowest solvent concentration and the highest concentration of heavy components. The heavy oil in the bottom layer after its dissolved solvent is flashed off has much higherviscosity than the original heavy oil. These experimental results indicate that in a solvent-based heavy oil recovery process, the solvent-heavy oil mixture in the top and middle layers can berecovered because of its lower viscosity, whereas the heavy oil in the bottom layer may be left behind in the heavy oil reservoir because of its higher viscosity. In this way, the produced heavy oil is in-situ upgraded during the solvent-based heavy oil recovery process.
Western Canada has tremendous heavy oil and bitumen Deposit[1, 2]. Approximately 70% to 80% of the original-oil-inplace (OOIP) remains unrecovered at the economic limit after the cold production[3]. Heavy oil contains a large portion of heavy components, which are the major reason for its high viscosity (>1,000 mPa?s) and low API gravity (<20 ° API gravity) [4]. Heavy oils and bitumen are highly viscous so that they cannot be recovered by using some conventional recovery techniques for medium and/or light oils. In practice, thermal methods are often used because they can dramatically reduce heavy oil viscosity. However, the majority of Canadian heavy oil reservoirs cannot be exploited economically by using thermal methods alone due to thin pay zones and/or bottom water aquifer[5].
