A field study was made to determine the feasibility of changing the injection profile of water-input wells after water breakthrough at producing oil wells. Secondary objectives were to study the particle size and quantity of plugging material required to change the input profile and the methods of applying these materials.fifty-three injection wells were treated with various amounts and types of commercial plugging materials, and the injection profile was definitely changed in most of the wells treated. Increased efficiency of water injection after treatment was indicated by commercial tracer surveys and injection well performance. There were some indications that the rate of water production was reduced and the rate of oil production was increased as a result of selective plugging treatments, but it was found that an immediate change in the rate of oil and water production did not normally occur. Introduction The unending effort to reduce operating costs and increase ultimate oil recovery by waterflooding has required oil operators to evaluate continuously the factors affecting the performance of waterfloods. One of the most detrimental factors is by-passing of oil by the injected water between the injection and producing wells. By-passing (channeling) may be classified aspassage of water through depleted zones of high permeability andpassage of water through fractures or fissures. Oil productive reservoirs are normally composed of strata possessing wide variations in permeability, and the injected water will seek the path of least resistance to the producing well. Therefore, thief zones (zones of high permeability or fractures) will be the main conductors of injected water and will become depleted of producible oil sooner than the less permeable sections. As the more permeable zones become depleted, they will continue to accept the injected fluid at a faster rate because oil saturation is reduced and the effective permeability to water is increased. Unless water injection into the depleted zones can be curtailed and the water directed into the less permeable strata, the depleted zones will continue to transmit water from injection well to producing well during the remaining life of the waterflood. Injecting and producing a large volume of water that is doing little useful work reduces the efficiency of the operation. Various techniques have been used in an effort to isolate or plug the more permeable zones of a reservoir, includingcorrective shooting or selective perforating of injection wells,delayed drilling,multiple packer completion of injection wells,cementing injection and producing wells, andselective plugging. Cementing and selective plugging are the methods most commonly used after water breakthrough into the producing well has occurred. For the purpose of this article a selective plugging treatment is considered to be any treatment whereby a material, liquid or solid is injected into a well to change the injection profile. An ever-increasing number of waterfloods are being operated in the north Texas area. Here the formations have wide variations in permeability and many are fractured, By-passing of injected water is a constant problem, To promote conservation and efficient utilization of the nation's petroleum resources, the U. S. Bureau of Mines, in cooperation with oil companies in the north Texas area, made a field study to determine the feasibility of changing the water injection profile of a water-input well. Secondary objectives were to study the particle size and quantity of material required to change the input profile and the methods of applying these materials. This article presents a discussion of the field work performed, factors to be considered in treating wells, treating methods used and potential benefits that may be derived from selective plugging treatments. Due to the complexity of oil reservoirs and the differences between theoretical and actual performances of injection and producing wells, strict rules cannot be made to assure successful treatments. Definite answers cannot be given to some questions because of the difficulty in evaluating treatments. This article points out some of the thinking from both the theoretical and practical points of view that has developed during the past years and during this investigation. JPT P. 1550ˆ