The use of polymers to improve areal and volumetric sweep efficiency of waterfloods in limestone and dolomite reservoirs has been very limited and has seldom achieved the desired results. Anhydrous polymers, used in calcareous formations, have been generally ineffective. A relatively new process has been developed and field tested whereby a monomer solution is mixed and pumped into the formation. Once in place, the well is shut in and the monomer allowed to polymerize in situ. The resultant polymer may polymerize in situ. The resultant polymer may (within limits) vary in concentration and viscosity as desired.
Successful treatments have been performed in carbonate reservoirs in three areas of West Texas and New Mexico, and results have been evaluated and studied over several months. Laboratory work has been done on actual formation rock to determine adsorption characteristics and length of time retained.
Various type treatments have been used in injection wells in an attempt to increase the volumetric and areal sweep efficiency of waterfloods in limestone and dolomite reservoirs. Polymers, slurries, and gelatinous Polymers, slurries, and gelatinous materials are just a few of the more common agents used in this application.
Polymers used to increase the viscosity of the injection water may improve the areal sweep efficiency by producing additional oil from streamlines that have been developed between the injection and production wells.
Craig defines areal sweep efficiency as follows.
"In water-flooding, water is injected into some wells and produced from other wells. In an areal sense, injection and production take place at points. As a result, pressure distributions and corresponding streamlines are developed between injection and production wells. In symmetrical well patterns, a straight line connecting the injector and producer is the shortest streamline producer is the shortest streamline between these two wells, and as a result, the pressure gradient along this line is the highest. So injected water moving areally along this shortest streamline reaches the producing well before the water moving along any other streamline.