Correlation of Gelation Times for Polymer Solutions Used as Sweep Improvement Agents

Gelled polymers are being used increasingly to redirect or modify reservoir fluid movement in the vicinity of injection or production wells for the purpose of improving water/oil ratios. To date, little has been reported about the chemistry of the in-situ gelling process that involves a multivalent metal ion, a reducing agent, and a polymer. This paper reports results of a study of the relationship between process parameters and gelation time, which is defined as the time required for the solution to reach a specified viscosity. The gelation time is determined by continuous monitoring of the viscosity following addition of the chemical reactants. The varied parameters in the investigation include polymer type and concentration, chromate concentration, and reducing-agent type and concentration. Five different polyacrylamide polymers were used including polyacrylamides that were hydrolyzed to different degrees, an anionic copolymer, and a cationic copolymer. Sodium bisulfite and thiourea were used as reducing agents. It was determined that for a given polymer-reducing agent system, the reciprocal of gelation time is a linear function of the reciprocal of the polymer concentration for a fixed metal ion concentration. The slope of the straight line is a function of the degree of polymer hydrolysis, the existence of the anion or cation functional groups, and type of reducing agent used. Introduction Waterflooding generally has proved to be a highly successful oil recovery process. In those reservoirs where waterflooding has not been successful, the primary causes have been low sweep efficiencies and the bypassing of oil by water due to viscous fingering. Low sweep efficiencies can be caused by water channeling through fractures or zones of high permeability.The production of a large amount of water relative to the amount of oil produced is often a result of poor sweep efficiency in a waterflood project. Large water/oil ratios (WOR) and the subsequent handling and treating of the produced water lead to high operating costs and often make it uneconomical to continue a waterflooding program.Even where waterflooding has been successful, large volumes of water usually have been required. The economics have been justified by the fact that water is inexpensive and can be recirculated. Since water is relatively inexpensive to inject, there usually has been little concern about controlling the water movement in the reservoir while WOR remained below some predetermined (by economics) value.However, with the implementation of tertiary recovery methods, some of which involve the addition of expensive chemicals to the floodwater, it becomes very important that good fluid control be established. This is necessary so that the maximum volume of the reservoir is contacted by a minimum amount of fluid.There have been many attempts to control fluid movement in reservoirs to improve sweep efficiencies. Methods that have been among the most successful have used polymers, usually polyacrylamides, in some form. Polyacrylamides are thought to be especially effective since they have the ability to reduce formation permeabilities to water while retaining the permeabilities to oil. Early treatment involved simply the injection of polymers. Recent developments have shown that polyacrylamides can be crosslinked in situ to form a three-dimensional gel. SPEJ P. 229^