PUBLICATION RIGHTS RESERVED This paper is to be presented at the 36th Annual Fall Meeting of the Society of Petroleum Engineers of AIME in Dallas, October 8–11, 1961, and is considered the property of the Society of Petroleum Engineers. Permission to publish is hereby restricted to an abstract of not more than 300 words, with no illustrations, unless the paper is specifically released to the press by the Editor of JOURNAL OF PETROLEUM TECHNOLOGY or the Executive Secretary. Such abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in JOURNAL OF PETROLEUM TECHNOLOGY or SOCIETY OF PETROLEUM ENGINEERS JOURNAL is granted on request, providing proper credit is given that publication and the original presentation of the paper. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper. Summary In order to develop an experimental technique for determining the pressure of miscibility, the mechanisms of "high pressure gas drive" and "condensing gas drive" have been reviewed. When the fluids used contain more than three components, the mixing of the fluids as a result of the displacement imposes an additional condition on the laws governing the variation of composition in the mixing zone. In the displacement of one fluid by another at a given temperature, the pressure of miscibility is independent of the characteristics of the porous medium and of the velocity displacement. The experimental method developed uses comparatively high velocities in a porous medium more permeable than that found in the field. It has been checked by tests made on pure compounds, and then applied to the measurement of miscibility pressure of a crude oil with several gases. Other experiments have been made to check the conclusions of the theoretical study. Introduction When one intends to produce an oil field by miscible displacement, one of the first questions which must be answered is: Under what conditions will the reservoir oil be miscible with the gas available for injection. Or, since pressure is the only parameter which the producer can control, within what range should the pressure be maintained so that miscible displacement can be achieved by each of the available gas streams? Upon the answer to these questions depends,the time at which the injection of the miscible gas is initiated, andthe choice of the gas to be used. The mechanisms of "high pressure gas drive" and "condensing gas drive" have been frequently described for the simple case of three pure components. Certain authors (1), (2) have endeavored to use this description to determine quantitatively the conditions under which miscibility will be obtained for the case of crude oil and natural gas. This requires that the several components of the fluids be divided into three groups, each of which behaves like a pure substance. It is believed that this method cannot give accurate results, and we prefer, as some others do (3), to determine the conditions of miscibility by experimental displacement. However, this method can only be employed conveniently if each experiment can be made in a relatively short time, which, in turn, requires a core made from a porous medium different from the reservoir rock. Indeed, one can seldom obtain a sample of reservoir rock long enough. Furthermore, it is often desirable to study several gases which might be suitable for injection.