Field Experience With Floodwater Diversion by Complexed Biopolymers

Abstract

Abstract Due to preferential flow of the injected water through the most permeable zones, waterflooding of stratified reservoirs is generally inefficient. A process for improving the performance of waterfloods process for improving the performance of waterfloods in such reservoirs has been developed; it is based on complexing biopolymers with multivalent cations to form gels for selective blocking of water-thief zones, thereby diverting the trailing floodwater to previously under-invaded reservoir regions to previously under-invaded reservoir regions to recover by-passed oil. This polymeric modification of stratification and of water injection profile leads to increased volumetric sweep of the reservoir by the floodwater and, in turn, to improved oil production. This paper summarizes Mobil's experience production. This paper summarizes Mobil's experience in its first seven field projects in Oklahoma using this process. A total of two hundred and five injection wells were treated with complexed biopolymers, resulting in substantial alteration of water flow patterns and in significant incremental oil recovery. Introduction Treatment of injectors and producers as a means of improving waterflooding efficiency has been studied in both the laboratory and the field since the late 1950's. The treatments have encompassed a host of techniques to modify the fluid flow characteristics in the neighborhood of the affected wells. Specifically, these methods, which are often described as stratification control or profile control, have included the selective reduction of the formation matrix relative permeability to water and the blocking of fractures and "excessively" permeable strata in the reservoir. Thus, physical plugging of zones of high flow capacity by cements and solid slurries has been attempted; (1)–(5) such efforts, however, have met with only a limited degree of success. Similarly, oil/water emulsions,(6) lignosulfonates,(7) and polymeric solutions an gels (8)–(26) have also been considered and, in some cases, field tested. Among the polymeric systems examined for profile control are polyacrylamides, polysaccharides, carboxymethylcellulose, furfural-alcohol and acrylic/epoxy resins, WORCONR, and polyisocyanurate. Most of the work to date has been focused on the polyacrylamides because of their relatively low cost. polyacrylamides because of their relatively low cost. These polymers have been used in their normal as well as cross-linked forms. In either state, their effectiveness is severely hampered by susceptibility to shear degradation during injection and by sensitivity to reservoir brines. To mitigate these problems, dilute solutions of these polymers have problems, dilute solutions of these polymers have sometimes been injected first into the reservoir and then crosslinked in situ. For example, the Phillips process employs a sequential injection of cationic process employs a sequential injection of cationic polyacrylamides crosslinking agent (aluminum citrate polyacrylamides crosslinking agent (aluminum citrate or chromium ions generated in situ by the reduction of Cr2O7= (26)), and anionic polyacrylamides. Such a scheme is said to reduce the shearing of the polymers and to provide a more enduring and more deeply penetrating treatment. Available results on the penetrating treatment. Available results on the field use of these processes are inconclusive; the performance appears generally to be rather erratic. performance appears generally to be rather erratic. Like the polyacrylamides, polysaccharides have also been extensively evaluated for enhanced oil recovery, albeit for mobility control rather than for profile control. Compared to the polyacrylamides, profile control. Compared to the polyacrylamides, these biopolymers possess significantly greater shear stability and salinity tolerance; they too can be cross-linked with multivalent cations to form viscous gels for modification of reservoir stratification. Despite these desirable qualities, cross-linked biopolymers have not received serious attention for floodwater diversion until recently; no report of such usage has appeared in the literature. In 1981, we embarked on a program to evaluate a chromium complexed polysaccharide process for profile control and improved oil recovery. The present paper describes our experience with the application of this process in seven Oklahoma waterflood projects. process in seven Oklahoma waterflood projects. P. 137