Considerations on Solvent Compositions for Hydrocarbon Miscible Flood
Authors: Thomas F.B. , Okazawa T. , Hodgins P. , Zhou X. , Erian A. , Bennion D.B. 
Published: December 2007

Considerations on Solvent Compositions for Hydrocarbon Miscible Flood F.B. Thomas, SPE, T. Okazawa, SPE, P. Hodgins, X. Zhou, A. Erian, D.B. Bennion

Abstract
Gas injection, despite some of the problems encountered in the past, continues to be one of the major candidates for enhanced oil recovery. Many of the reservoirs targeted have shown poor performance under primary and secondary depletion, and therefore tertiary schemes are sought which involve the manipulation of interfacial tension. This one theme however (interfacial tension reduction) has been sometimes overemphasized without accounting for other parameters which enter in to the design of the gas injection process. The main areas of focus in this paper, relevant to designing gas injection schemes are:Fluid characterization and requirements for first- and multiple-contact miscibilityEvaluation of enrichment requirements via explicit interfacial tension measurementsMobility/interfacial tension compromise
With these criteria used in the design of the gas, an analysis is included which comments on the utility of the laboratory work done.

Introduction
Recently, more techniques have been developed for the determination of miscibility but, for the most part, each one has emphasized the importance of achieving zero interfacial tension. It has also been treated as if the achievement of "miscibility" is the panacea of all gas injection schemes and if not a panacea, it is at least a necessary condition. On the basis of experimental work, and some field observations however, it would definitely appear that miscibility is not necessary to achieve high levels of oil recovery. The concept therefore of near miscibility needs to be discussed relative to an objective criterion such as what is the true miscibility pressure or enrichment required for miscibility. This will now be discussed herein.
Evidence suggests that the efficient displacement of oil from porous media is governed by an important compromise reached between two factors which are interfacial tension effects and mobility effects. Within mobility effects for the purpose of this discussion, we include gravity, viscous fingering and the surface area to volume relationship determined from the pore size distribution of the porous media. The interfacial tension influence is represented very well by Equation 1 which shows the relationship between the imposed differential pressure requirement and that of interfacial tension and the diameter of the pore throat of the porous media. This indicates that, in order to access pore throats of diameter D, one must have, for a specific level of interfacial tension, a certain threshold differential pressure P. As the P increases, then access into pore throats of diameter D can be achieved at higher levels of interfacial tension. Conversely, as the pore throat diameter D decreases, then for an applied differential pressure P the interfacial tension must decrease in order to provide accessibility. The relationship shown in Equation 1 is more of nearly a necessary condition. That is, if the interfacial tension is low enough, then accessibility will be achieved into any diameter of pore throat. As interfacial tension approaches zero, then all pore throats by definition will be accessed. The difference however, between access and recovery is explained by the contribution of Equations 2 and 3. Equation 2 includes the standard fractional flow equation where mobility, as is associated with Equation 2, is shown as a proportionality between the viscosity of the oil divided by the viscosity of the gas. It is also known, that as this viscosity ratio increases the tendency to have viscous fingering increases very rapidly. Equation 3 also provides a contribution which is sometimes omitted, but which is becoming more and more apparent in analyses which have been performed by the authors.
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