The VAPEX process has attracted much attention as a potential recovery process for heavy oil, yet VAPEX mechanisms are still poorly quantified. The laboratory work described here was undertaken to explain the roles of different mechanisms in VAPEX, and to test the relative magnitudes of the mechanisms by means of numerical simulations. In particular, the paper focuses on capillary effects.
We describe an experiment that was carried out in a 2D rectangular cell, using a 4.3 darcy sandpack partially saturated with heavy oil, with n-butane as the VAPEX solvent. The sandpack was oriented with an initially vertical gas-oil interface, and n-butane was supplied to the interface at a constant pressure. Aside from the butane injection, the cell was closed; unlike most VAPEX experiments, there was no production. Rather, the oil was diluted by the solvent and drained into the lower part of the pack, while being monitored by a CT scanner. Separate experiments measured capillary pressure, butane solubility, and viscosity reduction due to butane dilution at the conditions of the experiment.
Numerical simulations were carried out with a commercial simulator, using all of the measured data for the system. The major unknown was the diffusion coefficient for the system, which was found by history-matching. A method is shown by which physical diffusion/dispersion can be separated from numerical diffusion. Capillary pressure is shown to play a significant role in the process. If capillary effects are ignored, the CT saturation profiles cannot be matched, and one infers an artificially higher dispersion coefficient.
Solvent-based processes for the recovery of heavy oil have attracted increasing attention in recent years. The most promising of such processes that has been proposed is the Vapour Extraction or "VAPEX" process first proposed by Butler and Mokrys1. VAPEX is a solvent analogue of Steam-Assisted Gravity Drainage (SAGD) in which the viscosity-reducing effect of heat is replaced by the viscosity-reducing effect of solvent dilution. A pure solvent or solvent mixture is injected from the upper of two horizontal wells (a similar well arrangement to SAGD), typically at a pressure close to its dewpoint, and dissolves in the oil, reducing its viscosity enough to allow it to drain by gravity to the lower producer well. The produced oil creates a growing vapour chamber.
There are no VAPEX field results presently available in the public domain. However, there have been many laboratory studies, and these have been extrapolated in various ways (usually via scaling arguments) to field predictions. There have been few simulation studies published and even fewer combined experimental/simulation studies. A goal of the present study is to make a contribution to the understanding of the process and the development of appropriate simulation methods for VAPEX. The philosophy of this work is that simulation methodology is best tested against relatively simple and well-controlled experiments, which permit a minimum of parameter adjustment in history-matching.
Previously reported mechanistic studies by the present authors2,3 have been directed more toward viscous fingering phenomena, but those efforts revealed certain features of VAPEX and VAPEX simulation as well.
