Through the combination of at least two polymer families, and the optimization of other components, a rubber compound has been developed for use in downhole applications that will swell in water-based fluids. A cellulose component, such as carboxy methyl cellulose (CMC), is used together with an acrylate copolymer (AC) that can increase the swelling capacity of an acrylonitrile butadiene rubber (NBR) in water. The amount of swelling achieved depends on physical boundaries and limitations, the salinity of the water, and the temperature.