This paper presents the development and application of a process that is characterized by the injection of a rock adsorbing cationic polymer of selective behavior with regard to the flow of reservoir fluids. The polymer adsorbed to the rock surface forms a hydrophilic film that swells in the presence of water, and strongly reduces the water relative permeability. In the presence of oil, the film maintains its original thickness, only slightly reducing the oil permeability. Concurrently it was developed a simple and precise method for the diagnosis and selection of wells, mainly baed in the graphic analysis of the production data. The process has been tested in 17 oil wells of Fazenda Pocinho field, with water salinity around 3,000 ppm and temperatures up to 50 C. Water cuts between 70 and 90% decreased 10 to 20 percentile points in most of the treated wells. Furthermore, most of them also showed oil production increase. From an economic point of view, the selective polymer injection process has proven to be cost effective with both low chemical and operational costs.
Water production in association with crude oil has been one of the major difficulties for the petroleum companies around the world. The breakthrough of either formation or injection water, results in accelerated oil production decline, increasing operational costs of pumping, treatment and disposal of large volumes of water.
Several techniques are available to reduce water production, or to prevent its early breakthrough, and the choice of the adequate technique for each case, at first, depends on the source of the produced water. The reservoir and well analysis, therefore, must accurately define its origin. Water coning and injection water channeling frequently cause high water/oil ratios, and the use of flow barriers is frequently successful. These barriers can be either of the plugging or permeability reducing type, characterizing the blocking and selective systems, respectively. The plugging type barriers, which block simultaneously gas, water and oil flow, can be formed by cement, resins, solid particles suspensions, silicates, paraffin or water soluble polymers crosslinked "in situ" in the reservoir. On the other hand, selective systems mainly block the water flow, causing little or no restrictions at all to hydrocarbon flow. These selective barriers are generally formed by water soluble copolymers, that adsorb to the rock surface, forming a hydrophilic film that reduces the water mobility.
The design of selective polymer systems depends on the rock mineralogy and permeability, reservoir temperature and formation water salinity. The rock type. sandstone or limestone, defines the polymer ionic character. The predominantly negatively charged adsorbing sites at the surf of sandstones calls for the cationic type of polyacrilamides. Conversely, limestones are flavored by the anionic of polymers to enhance the adsorbing capacity and thus the formation of the hydrophilic film responsible for the permeability reduction to water. Concerning the reservoir temperature and water salinity, polymer tolerance can be improved by adding products as AMPS (2-acrylamide-2-methylpropanesulfonic acid) and/or vinylpyrrolidone to the molecular structure of the base polymer. The salinity, however, can inhibit the hydrophilic film swelling and so impair the water phase permeability reduction effect. With respect to the rock permeability, the higher the rock permeability, the higher the hydrodynamic volume required for the polymer in the case of monolayer adsorption. It is also possible to improve the film thickness by adding successive alternate layers of anionic and cationic polymers. In the same way, it is possible to inject successive layers of selective polymer and crosslinking agents (e.g. trivalent metallic ions) to obtain a similar effect.
