Improvements in Polymer Flooding: The Programmed Slug and the Polymer-Conserving Agent

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Abstract
In the investigations described here, both the programmed slug technique and the use of the polymer-conserving agent lead to greater oil recovery in the early life of the flood, thus effecting an over-all economic improvement over the use of the conventional square slug. Introduction The concept of programming a slug to reduce viscous fingering is not a new one. Slobod and Lestz proposed the use of a transition zone graduated in proposed the use of a transition zone graduated in viscosity between that of the displaced phase and that of the displacing phase in miscible flooding applications. Their laboratory experiments showed an improved recovery over the constant-viscosity case. Gogarty has proposed the use of a graded, thickened fluid bank following a soluble oil slug in which the mobility of the thickened fluid bank matches that of the soluble oil slug at the interface and is then increased incrementally to match the mobility of the displacing brine. Mungan has performed laboratory experiments with a partially hydrolyzed polyacrylamide (PUSHER 700 polymer, a product of polyacrylamide (PUSHER 700 polymer, a product of The Dow Chemical Co.) in which he compared the recovery efficiency of a constant-concentration slug with that of a slug of exponentially declining concentration, both containing an equal mass of polymer. His experiments showed both increased and more rapid oil recovery in the latter case. However, the amount of polymer used in his experiments was much more than is economically feasible for practical field applications. The first purpose of our study, therefore, was to investigate the economic significance implied by Mungan's work. A specially designed reservoir simulator was developed to study the effects upon both recovery and economics primarily for variations in slug design, adsorption level, permeability variation, and oil viscosity.The second purpose was to observe the effects of using a recently developed chemical called a polymer-conserving agent (PCA) which, laboratory polymer-conserving agent (PCA) which, laboratory experiments have shown, adsorbs at approximately the same level as conventional PUSHER polymers but costs the operator considerably less. Injected ahead of the main polymer slug, PCA satisfies the adsorption demand in those portions of the reservoir that it contacts, thus reducing the attrition of the main polymer bank to a very low level. Results are presented polymer bank to a very low level. Results are presented for a high-concentration PCA spearhead slug followed by a polymer slug of linearly declined concentration.During the course of this study more than 100 computer runs were made in an attempt to gain some insight into the implications of these concepts under a variety of reservoir conditions. The examples presented here are intended to illustrate the concepts and presented here are intended to illustrate the concepts and should not be construed as representing either the upper limits or the lower limits of improved performance. performance. The Programmed Slug In any displacement process involving the use of a slug of displacing chemical followed by brine, there is a tendency for viscous fingering to occur at either or both of the slug interfaces because of adverse mobility ratio contrasts. JPT P. 33

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