Vapor extraction (VAPEX) process is solvent analogue of SAGD process in which low molecular weight solvent are used to reduce the viscosity of heavy oil and bitumen. This process may be a good candidate to be applied in problematic reservoirs in which thermal methods are not efficient due to severe heat losses.
Asphaltene precipitation as a result of compositional alteration of solvent/bitumen has substantial effects on performance of VAPEX. In this process if operating pressure is very close to dew point pressure, asphaltene precipitation occurs. In-situ deasphalting of heavy oil and bitumen reduces the viscosity of the produced oil and creates the oil that is more easily refined, but the main concern is whether this advantage can outweigh reduced permeability and plugging of formation caused by adsorption and precipitation of asphaltenes.
In current work experiments have been conducted in low permeability sandpacks in both dry and non-dry systems to assess the performance of Vapex process when asphaltene precipitation occurs. Experiments are carried out in a 2D visual cell using highly asphaltenic heavy oil from one of Iranian reservoirs (Kuh-e-Mond) and propane as a solvent.
The results show that performance of Vapex process increases when asphaltene precipitation occurs provided that optimal pressure has been designed to prevent extensive asphaltene deposition and subsequent plugging of porous media. It has been observed that adsorption of asphaltenes has considerably decreased in non-dry systems. Movement of precipitated asphaltene and less adsorption in non-dry experiments results in a more stable and efficient Vapex process while in dry experiments higher adsorption of asphaltenes resulted in plugging of low permeability porous medium.
Introduction
Concurrent with increase in oil demand, there is marked decline in conventional oil production. Huge heavy oil and bitumen resources are increasingly recognized as a strategic resource and potential contributor to worldwide energy security. Thermal processes like SAGD and CSS have increased tremendously worldwide reserves especially in Canada and probably SAGD could be considered as one of the most effective EOR techniques applied in petroleum industry. But, thermal processes cannot be applied in cases with severe heat losses like thin formations and heavy oil in deep reservoirs and offshore. Solvent-based methods like VAPEX process may be a suitable option for the recovery of heavy oil in problematic cases.
The Vapour extraction is basically an analogue of the steam assisted gravity drainage (SAGD) process. In VAPEX process, vaporized solvents are used instead of high temperature steam and the oil viscosity is lowered by in-situ dilution instead of heating. A vaporized light hydrocarbon or a mixture of solvents, such as propane or butane, is injected through a horizontal well into the formation containing the viscous oil or bitumen (Butler & Mokrys 1991, Das 1997).
It has been experimentally shown that the optimum solvent injection condition is near solvent dew point where the vapour phase has the maximum solubility and diffusivity in the heavy liquid phase (Butler & Mokrys, 1993).Carrying out experiments with pure solvents in these conditions leads to asphaltene precipitation as a result of high solvent concentration in boundary layer (Das & Butler 1998).
Asphaltene flocculation and deposition during natural depletion and/or miscible gas injection in enhanced oil recovery (EOR) processes is a common problem in oilfields throughout the world. Changes in some environmental parameters, such as pressure and composition (addition of solvent or dispersant injection), can change the stable condition of an oil mixture to another condition where the mixture will be unstable and heavy organics, such as asphaltenes or waxes, flocculate and deposit (Mofidi & Edalat, 2006).
