Investigation of Drainage-Height Effect on Production Rate in the Vapor-Extraction Process
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 101684, "Further Investigation of Drainage-Height Effect on Production Rate in Vapex," by A.J. Yazdani, SPE, and B.B. Maini, SPE, U. of Calgary, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September.
The full-length paper presents the results of a new set of vapor-extraction (vapex) -process experiments in a newly designed very-large physical model. The results are in good agreement with the trend of previous experiments in smaller models. The previously proposed scaleup relationship adequately predicts the results obtained with the new physical model. The new results were used to improve the previously reported empirical scaleup correlation for the vapex process. Experiments with different sand-packs reconfirm square-root functionality of the dead-oil production rate to the permeability.

Introduction
Interest in heavy-oil- and bitumen-recovery methods is growing because of the huge amount of proven resources in the world. Steam-assisted gravity drainage (SAGD), cyclic steam injection, and other steam-injection methods have been implemented successfully in some oil fields. However, there are many situations where economic constraints limit the application of these thermal processes. High costs of steam generation, excessive heat losses in thin reservoirs, produced-CO2 emission problems, water treatment, and many other technical problems are some of the reasons that make it necessary to look for alternatives to thermal methods.
Solvent-based heavy-oil-recovery methods have gained attention recently because of their potential advantages over thermal processes. Solvents, if dissolved in the oil, are able to reduce oil viscosity dramatically. This viscosity reduction is comparable to that obtained by heating. Among the solvent-based methods, vapex has received more interest because of the very encouraging results reported in laboratory studies. Vapex basically is a solvent analog of the SAGD process. Two parallel horizontal wells are drilled one on top of the other in the same configuration as in SAGD. A solvent or a mixture of solvents is injected into the top well near its dewpoint. The solvent-selection criteria depend on reservoir pressure and temperature. A carrier gas usually accompanies the solvent to raise the dewpoint and keep it in vapor form at the prevailing reservoir pressure. Ethane, propane, and butane are considered good solvent candidates. Nitrogen, methane, or CO2 can be used as the carrier gas. The injected solvent vapor displaces the oil and begins to form a vapor chamber around the wells, which then propagates later-ally toward the formation boundaries. The oil production is a result of the dissolution and diffusion of the solvent into the oil zone. The diluted oil moves down to the bottom well through a thin layer near the edge of the oil/solvent interface. In this process, the driving force for the fluid flow primarily is gravity, and the wells are kept at equal pressures. Molecular diffusion and mechanical dispersion are believed to be the controlling mass-transfer mechanisms responsible for the solvent mixing with the oil. The solvent can be extracted at the surface and reinjected to the reservoir. The process continues until the economic limits are exceeded for the oil-production rate when the chamber drainage height decreases.