Development of Experimental Phase Equilibria For Miscible Hydrocarbon Enhanced Oil Recovery
Authors: Connor H.J. , Pope A.E. 
Published: March 2010

Abstract
A major factor in the success of a miscible hydrocarbon enhanced oil recovery process is the phase behaviour relationship between the injection and reservoir fluids.
This paper describes the experimental procedures developed at Gulf Canada R&D to generate phase equilibrium data for specific reservoirs. The data have been applied in evaluating the potential for enhanced recovery in a number of Western Canadian carbonate reservoirs. The over-all procedure for a complete phase equilibrium study is described. This comprises field sampling, reservoir fluid characterization, injection fluid preparation, development of phase envelopes and correlation of phase behaviour.
An integral part of the program involved the development Of a combination of analytical and high-pressure techniques to facilitate the rapid accumulation of vapour-liquid equilibrium data for reservoir-injection fluid systems. The Peng-Robinson equation of state and a new K value correlation were used successfully to rationalize the measured data.
Experimental phase behaviour measurements in conjunction with appropriate correlation methods have proved valuable in tailoring the design of injection fluids to meet both miscibility requirements and economic constraints.

Introduction
Recent reports(1,2) have indicated that the use of enhanced recovery methods can increase Canada's available conventional oil reserves by approximately 2 to 3 billion barrels. Up to 95% of the additional oil with an API gravity greater than 25º could feasibly, and economically, be recovered using currently available miscible displacement techniques involving injection into the reservoir of either CO2 or hydrocarbon fluids.
Realizing this potential, Gulf Canada Resources Inc. have been actively pursuing a program of initiating limited full-scale enhanced recovery projects and implementing pilot-scale floods(3) at various locations in Western Canada. To date, because of the types of reservoirs considered primarily carbonate and the local availability of suitable injection fluid components, attention has centred on the process of hydrocarbon miscible displacement.
In support of these projects, the Gulf Canada R&D Department has developed an extensive experimental program to provide data both for preliminary screening of candidate reservoir and injection fluids and for detailed design of optimum injection fluids in specific reservoirs. This paper describes the experimental program that has evolved during application to such projects over the past five years.

Miscible Displacement
The residual oil saturation, after primary and secondary production of a reservoir, is a function of the formation geometry and the interfacial tensions of the fluids occupying the formation. The purpose of the injection fluid in enhanced recovery is to access the residual oil, and, by modifying the balance of interfacial tensions, to mobilize the residual oil to production. In miscible displacement processes, as the name implies, this purpose is achieved when miscibility is attained between the displacing fluid (injection fluid) and the residual oil, thereby eliminating the interfacial tension between the two.
The miscibility of an injection fluid and reservoir oil is governed by the phase behaviour which is unique to each specific combination. The relationship between phase behaviour, miscibility and composition at reservoir temperature and pressure is illustrated on the pseudo-ternary Point R is the reservoir fluid composition and X represents the critical composition or plait point.