Abstract

The Vapor Extraction (VAPEX) process has been proposed as a viable alternative to steam-based heavy oil recovery methods. In this process, a vaporized solvent is injected into a horizontal well placed higher in the formation, and the diluted heavy oil is produced by gravity drainage from a horizontal production well situated below. One shortcoming of this process is the slow diffusion of the solvent into the bulk of the oil.

The mass transfer mechanism in the VAPEX process involves molecular diffusion and convective dispersion within a porous medium at a microscopic scale - phenomena that are not well understood. In this study, we are proposing an alternative method to VAPEX. Instead of directly injecting the vaporized solvent and producing oil, cyclic solvent soaking is applied to a heavy oil reservoir in order to maximize the mixing time between the solvent and the heavy oil.

In this paper, the most effective solvent mixture, which must be in its gaseous phase and also close to its dew point pressure under the prevailing reservoir conditions, is determined using a swelling test, built in the Winprop(1) model. The solvent mixture is then verified by comparing different solvent compositions using thermodynamic simulations. The optimal soaking time for a certain amount of injected solvent is also examined by analyzing the effect of viscosity and the produced gas-oil ratio (GOR) on the performance of the process. Consequently, optimal values for the number of cycles of solvent injection, the soak time and the amount of back production based on the optimal soaking time, is determined.

The influence of well geometry on oil recovery is also considered here. Properly positioning the horizontal injectors and producers can enhance significantly the overall oil production rate.

Introduction

Currently, the economic extraction of viscous heavy oil is a major challenge in the petroleum industry. To deal with this problem, Butler and Mokrys (2) have proposed the concept of injecting light hydrocarbon solvent into the reservoir (VAPEX) as an alternative to Steam-Assisted Gravity Drainage (SAGD). This approach is especially beneficial for thin pay zone reservoirs where the heat losses to the over- and under-burden have a negative impact on the economics of such processes. The VAPEX process does not require water recycling and treatment, yields much lower carbon dioxide emissions and can be operated at reservoir temperature. The capital and operational costs are estimated to be much less than those of a SAGD project (3).

The concept of VAPEX is very similar to that of SAGD. In VAPEX, a vapor chamber, rather than a steam chamber, is formed around the horizontal injector, and the diluted oil flows by gravity towards the producer. The principal mechanisms in this process are quite complex. Toluene, as a solvent, has been observed, in an experiment (4), to extract Athabasca and Suncor coker feed bitumen from a Hele-Shaw cell. In those experiments, the solvent-bitumen contact region is composed of a frontal layer bounded by two different, sharp solvent concentration layers.

This content is only available via PDF.
You can access this article if you purchase or spend a download.