Effect of Capillarity and Surface Tension on the performance of VAPEX Process

Abstract

Abstract Vapour Extraction process is one of the newest emerging technology for heavy oil reservoirs which is based on the injection of hydrocarbon solvents into viscous oil. Despite the fact that no promising field results have been reported so far about the successfulness of VAPEX process, talking about the application of this method in low permeable rocks seems to be controversial. Although the potential of this process has long been practiced by previous researchers, their predictions are just based on their high permeable systems and there is no attempt to apply this process for so-called tight reservoirs. The scope of this study is to do a profound investigation for finding and understanding the interplay of the involved capillary, gravitational and viscous forces for fluid flow in low permeable systems. A simulation study was carried out with different models of varying capillary pressures. Effect of solvent concentration on gas-oil surface tension and subsequently on residual oil saturation was investigated. Capillary hold-up phenomenon as a problematic issue was studied through block height and capillary number alteration. The results showed that in higher capillary pressure values, the shape of vapor chamber differs from conventional model and it is more piston-like. Residual oil saturation declines as a result of gas-oil interfacial tension reduction by means of solvent concentration alteration. Diluted oil may not be totally produced as a result of capillary hold up. Reservoir height and solvent injection velocity are the key factors which are contributing to this deficiency. Capillary hold up level decreases as the block height and capillary number increase. Introduction There are a number of processes to extract heavy oil from reservoir, including steam injection, cold production, In Situ Combustion (ISC) and Vapour extraction (VAPEX). In VAPEX process vaporized hydrocarbon solvent are injected into heavy oil reservoir. The solvent diffuses into the heavy oil, dilutes and upgrades it and finally the diluted oil drains by gravity to a horizontal production well. In comparison with thermal processes, VAPEX is a more efficient method to produce heavy oil as it is consuming less energy to produce the same amount of the oil. Vapex process is governed by three main forces including capillary, viscous and gravity force. Capillary force in VAPEX process will be varied by concentration of gas solvent. Capillary pressure can play a dual role in VAPEX process, at the beginning of process when gas is injected into heavy oil reservoir, a higher capillary pressure causes a delay in breakthrough time, wells communication and gas production. At the same time, a higher capillary pressure in the finer pores causes thicker saturation zone around the edge of VAPEX chamber that leads to enhancement of mass transfer. Capillary imbibition in the finer pores of the transition zone causes the partially diluted oil to be drawn out from the bitumen body. This leads to exposure of a new surface of the bitumen into the maximum solvent concentration (surface renewal) and also can extend the mass transfer area.