Petroleum exploration and development industry is witnessing a rapid growth in the use of swelling elastomers. They are being used in new applications aimed at enhanced oil recovery through slimming of well design, zonal isolation, water shutoff, etc. Initially developed as a problem-solving strategy (for repair of damaged or deteriorating wells), swelling elastomers are now targeting major savings in cost and time through reduction in borehole diameter, reduced casing clearance, cementless completions, etc. Due to material and geometric nonlinearity, modeling and simulation of swelling elastomer applications becomes quite complex. In this work, finite element simulation has been carried out to study swelling elastomer seal performance in downhole petroleum applications using the software ABAQUS. A hyperelastic model (that most closely resembles swelling elastomer behavior) is used for simulation of seal behavior. A series of experiments have been designed and performed to determine necessary material properties of a water-swelling elastomer as it gradually swells when exposed to saline water of two different concentrations at 50C (to emulate field conditions of medium-depth oil wells). A large number of simulations are carried out to investigate sealing behavior against water salinity and swelling time. Sealing pressure at the contact surface between elastomer and formation (or outer casing) is studied for variations in seal length, seal thickness, compression ratio, water salinity, and swelling period. Results show that seal contact pressure increases with amount of swelling, seal length, and compression ratio; higher salinity environment results in lower sealing pressure; and more contact pressure is generated in the case of rock formation as compared to steel outer casing.