Mechanical and structural behavior of a swelling elastomer under compressive loading

Swelling elastomers are a new breed of advanced polymers, and over the last two decades they have found increasing use in drilling of difficult oil and gas wells, remediation of damaged wells, and rejuvenation of abandoned wells. It is important to know whether an elastomer type or a certain seal design will function properly and reliably under a given set of oil or gas well conditions. This paper reports the results of an experimental and numerical study conducted to analyze how compressive and bulk behavior of an actual oilfield elastomer changes due to swelling. Tests were carried out on ASTM-standard compression and bulk samples (discs) before swelling and after different swelling periods. Elastic and bulk modulii were experimentally determined under different swelling conditions. Shear modulus and Poisson's ratio were estimated using derived isotropic relations. Cross-link chain density and number average molecular weight were obtained using predictive equations of polymer physics. Mechanical testing was also modeled and simulated using the nonlinear finite element package ABAQUS, material model being Ogden hyperelastic model with second strain energy potential.Values of elastic and shear modulus dropped by more than 90% in the first few days, and then remained almost constant during the rest of the 1-month period. Poisson's ratio, as expected, showed a mirror behavior of a sharp increase in the first few days. Bulk modulus exhibited a fluctuating pattern; rapid initial decrease, then a slightly slower increase, followed by a much slower decrease. Salinity shows some notable effect in the first 5 or 6. days, but has almost no influence in the later days. As swelling progresses, chain density decreases, much more sharply in the first week and then showing almost a steady-state behavior. In contrast, cross-link average molecular weight increases with swelling (as expected), but in a slightly fluctuating manner. Very interestingly, Poisson's ratio approaches the limiting value of 0.5 within the first 10. days of swelling, justifying the assumption of incompressibility used in most analytical and numerical models. In general, simulations results are in good agreement with experimental ones. Results presented here can find utility in selection of swelling elastomers suitable for a given set of field conditions, in improvement of elastomer-seal and swell-packer design, and in modeling and simulation of seal performance.