Reservoir Engineering Design of A Low-Pressure Rich Gas Miscible Slug Flood
Authors: Ballard J.R. , Smith L.R. 
Published: December 2007

The rich gas miscible slug process is generally thought to be applicable only where reservoir pressures are above 1,400 psi. However, the process may be operable at lower pressures under certain conditions. This paper examines criteria for evaluating the applicability of the process and describes the design of one project for a West Texas reservoir having an average pressure of 925 psi.

Introduction
The various miscible displacement processes hold considerable potential for economically recovering oil otherwise unrecoverable by more conventional processes such as waterflooding. This potential processes such as waterflooding. This potential stems from the fact that displacement efficiencies approaching 100 percent are achieved and thus all oil is recovered from the swept portion of the reservoir. Three factors that have historically limited realization of this potential are (1) poor sweep efficiency resulting from extremely unfavorable mobility ratios, (2) high costs associated with the expensive fluids that are injected, and (3) limited applicability due to the operating pressure requirements that are imposed.
Several approaches to the problem of poor sweep efficiency have been proposed, including presolvent water injection, cosolvent water injection, and gaswater injection following solvent placement. The rich gas slug process, with which this paper is concerned, is one possible means of reducing fluid costs, since a mixture of liquefied petroleum gas (LPG) and dry gas is less expensive than pure LPG. The operating pressure limitation, however, is more severe than for a pure LPG slug process. In general, rich gas displacement has been considered applicable only to reservoirs at 1,400 psi or greater. The purposes of this paper are to (1) review some of the purposes of this paper are to (1) review some of the theory of miscible rich gas displacement, (2) examine the limitations imposed on operating pressure, and (3) present engineering calculations and approaches adopted present engineering calculations and approaches adopted in the design of one low-pressure rich gas project in West Texas. An actual field project based on this design is currently being installed.

Description of the Process Displacement Mechanism
The rich, or condensing, gas drive process utilizes a gas that is rich in intermediate-molecular-weight (C2 through C4) hydrocarbons. Such a gas need not be miscible directly upon contact with the reservoir oil. It is well known that if the gas contains enough intermediates, miscibility will develop after sufficient contact between the gas and the crude. This requires the gas to move for some distance into the reservoir. The mechanism of miscibility development involves transferring (or "condensing") intermediates from the gas to the oil. Upon contact with liquid, the gas is stripped of intermediates and moves on ahead, allowing multiple contacts to occur between incoming gas and the liquid phase. Thus, through a chromatographic type of separation and concentration of components, a liquid transition zone is formed that consists of original reservoir oil at the outward edge, with mixtures gradually richer in intermediates toward the wellbore. If the injected gas is rich enough in intermediates, it eventually contacts a modified liquid phase with which it is directly miscible and an efficient displacement will result. Whether such a miscible zone will develop depends upon the composition of the rich gas and of the reservoir oil, the reservoir temperature, and the operating pressure.
JPT
P. 599