Effects of Heavy Oil-Solvent Interfacial Tension on Gravity Drainage in the vapor extraction (VAPEX) Process

Abstract

Abstract The vapor extraction (VAPEX) process is a promising technology for recovering heavy oil and bitumen resources in an economically viable and environmentally friendly way. Although a number of laboratory experiments have been conducted to study the VAPEX process, the oil recovery mechanisms by gravity drainage in this process are not well understood yet. In this paper, both experimental and theoretical approaches are adopted to study the effects of gravity and the capillary force on gravity drainage in the VAPEX process. First, the interfacial tensions between a heavy oil sample and four different solvents (methane, ethane, propane, and carbon dioxide) are measured at different pressures below their respective vapor pressures by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. Then, the Bond number, which is defined as the ratio of gravity to the capillary force, is calculated for the VAPEX process in a heavy oil reservoir and in a physical model of sand pack, respectively. It is found that the measured interfacial tension between the heavy oil and a solvent is reduced almost linearly with pressure for the four heavy oil-solvent systems tested. Correspondingly, the Bond number increases with pressure. An increased Bond number indicates relatively large effect of gravity on the VAPEX process and thus enhanced oil recovery. Introduction The vapor extraction (VAPEX) process[1, 2] is a non-thermal oil recovery process, in which a vaporized solvent is injected into a heavy oil or bitumen reservoir. The solvent dissolves into the heavy oil so that the oil viscosity is significantly reduced. The diluted oil is then drained by gravity to a lower horizontal production well. Interfacial phenomena play an important role in the gravity drainage flow in a porous medium because gravity, capillary and viscous forces control fluid flow through a porous medium[3]. It has been found that the interfacial tension of a heavy oil-solvent system is reduced when a solvent is injected into an oil reservoir[4, 5]. The reduced interfacial tension alters the gravity-capillary force balance and thus reduces the residual oil saturation. The effects of the heavy oil-solvent interfacial tension on the enhanced oil recovery in the VAPEX process are considered to be significant[4]. Therefore, it is of fundamental and practical importance to study the interfacial tension phenomena in various heavy oil-solvent systems so that a better understanding of the VAPEX process can be achieved. Among many existing methods for determining the interfacial tension, the pendant drop method is probably the most suitable for measuring the interfacial tension between a heavy oil sample and a solvent at high pressures and elevated temperatures[5]. Traditionally, the pendant drop method was used to determine the interfacial tension by photographing a pendant drop and then measuring the drop dimensions from the negative films[6]. This method was significantly improved and utilized to determine the interfacial tensions of the pure hydrocarbon + CO2 systems[7, 8], the synthetic oil + CO2 system, and the crude oil + hydrocarbon gases + CO2 system[9]. More recently, a new sophisticated technique for determining the interfacial tension from the liquid drop shape analysis, known as the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case, has been developed by Rotenberg et al.[10] and further improved by Cheng et al.[11]. In comparison with the other existing methods, the ADSA technique for the pendant drop case is accurate for the interfacial tension measurement (±0.05 mN/m), fully automatic, and completely free of the operator's subjectivity. At present, this technique has become a standard method for measuring the interfacial tension.