New Interpretation Of Recovery Mechanisms In Enriched Gas Drives
Authors: Novosad Zdenka , Costain Terry 
Published: March 2010

Abstract
The importance of understanding the mechanism of miscibility development when designing solvents for enriched gas drives cannot be underestimated. Failure to correctly identify the process mechanism could lead to misinterpretarion of experimental data such as slim lube displacements, and result in solvents designed either too lean or rich.
This paper provides a new interpretation of multiple-contact miscible oil recovery proce5Ses by enriched gas drives. Using an example of salvent design for an Alberta reservoir it is shown that the low-pressure enriched gas drive is not always a condensing process as traditionally assumed by the industry. Often the process is a vaporizing drive qualitatively different from a high-pressure lean gas drive.
This interpretation of process mechanism is supported by equation of state calculations which offer further insight into factors determining tile process type for a given combination of reservoir fluid, solvent, reservoir temperature and operating pressures. The relationship between solvent pressure temperature phase envelope and reservoir conditions is shown to be the most importance factor. The vaporizing mechanism is favoured when the reservoir temperature is high and the solvent is lean in components heavier than propane.
A limited review of the literature dealing with condensing gas drives was conducted and the recovery mechanisms of several ongoing or planned miscible projects in Alberta were examined. The analysis indicates that many of these are vaporizing gas drives. The evidence is presented indicating that incorrect representation of system phase behaviour in a pseudoternary diagram and the failure to recognize the diagram limitations often lead to erroneous interpretation of process mechanisms.

Introduction
Several hydrocarbon miscible projects are currently under way in Alberta. As injection of solvents miscible with crude oil in any proportion may be uneconomic, the solvent is often designed to form a miscible bank in a dynamic, multiple-contacting process.
The focus of this study is on rich gas floods, first described in the literature some 30 years ago(1–3) as condensing gas drives. The process is considered to be well understood(4). The in-situ generation of miscibility is thought to be a result of gradual enrichment of reservoir fluid in intermediate components of solvent to a point where it becomes fully miscible with the injected solvent. Analogy with the CJ-C4-C10 system(2) and similar true ternary systems is often invoked to describe the condensing gas drive mechanism.
This study presents the evidence indicating that in many enriched gas drives miscibility does not develop via the condensing process, but instead, via vaporization or (upper) liquid extraction of reservoir fluid components into the solvent. The process is qualitatively different from the conventional condensing gas drive but also from the vaporizing (high pressure) lean gas drive. This finding has important implications for the interpretation of experiments conducted in order to determine the minimum solvent enrichment, the slim tube displacements, as is demonstrated with an example of so(vent design for an Alberta reservoir.
The equation of state (EOS) simulations are presented which support this new interpretation of miscibility development in some rich gas drives and offer further insight into factors controlling the process mechanism.