Paper: Remedial Control of Injection Water Improved Sweep Efficiency

The economics of secondary recovery is certainly important to today's domestic oil industry. There is, however, another important consideration which must be taken, in view of present shortages of proven reserves-ultimate recovery of secondary oil. In other words, can more of these known reserves now in place in reservoirs already drilled and under waterflood be produced economically? Obviously, no oil will be banked to a producer if no water is injected into the section containing the moveable oil saturation which remains after primary production is accomplished. There are three common problems encountered in waterflooding which make it difficult even impossible-to inject water into and through all of the reservoir containing the residual moveable oil: 1. Little or no response to injection due to lack of confinement to the section of interest 2. Premature water breakthrough due to zones of high water saturation or extreme variations of permeability within the section of interest 3. Water breakthrough due to fingering of injection fluid caused by over-injection and/or directional permeability within the section of interest. This paper will consider only problems 2 and 3. Problem 1 was discussed by the author in a previous short course (1971). Water breakthrough occurs rapidly in zones of high water saturation since injection always seeks the path of least resistance. High water saturations offer less resistance to the flood water because the relative permeability to water is greater. Water breakthrough occurs rapidly in thin zones of high permeability for two reasons: 1. Most of the primary oil comes from the higher permeability, leaving high water saturation. 2. The extremely high permeability zones offer less resistance to the injection water anyway. Water breakthrough comes soon when over-injection causes water to finger through to producers. Over-injection occurs when an optimum rate is calculated for a total section to be flooded and only a small zone accepts the injected fluid-again the path of least resistance. Directional permeability simply offers the shortest distance between two points-in this case, between the injection and producing wells. The paths of injection flow after breakthrough become even easier since they exhibit higher relative permeability to water.