Mechanics of swellable elastomeric seals

Abstract

This thesis investigates the mechanics of swellable elastomeric seals for the purpose of hydraulic fracturing in oil and gas applications. The first component of the thesis is the development of a laboratory-scale apparatus for the visualization of swellable seals in situ up to the point of leakage. Experiments using this apparatus show that leakage is a result of large, nonuniform deformation that stretches the seal material tangential to the sealing surfaces and leads to a corresponding loss of traction normal to the sealing surfaces due to Poisson contraction. This phenomenon was investigated in two analogous seal systems - an O-ring and a rectangular swellable elastomer used to seal a rectangular channel. Both analog systems exhibit leakage due to the same mechanism. Corresponding finite element simulations predict a fluid leakage path that agrees qualitatively with experiments. The second part of this thesis consists of an experimental investigation of the eect of geometry and metal support rings on the performance of swellable seal systems. Although this work is highly applied, it reveals two interesting results. The first is that mechanical supports, in the form of rigid metal support rings, provide most of the support for the applied differential pressure. Secondly, in some seals, changing the length of the rubber part of the seal does not significantly affect the maximum differential pressure that the seal can support. Motivated by the experiments showing no dependence of critical leakage pressure on seal length, we conduct an analytic investigation of the combined effects of compressibility and aspect ratio on the performance of the seal system. We nd an approximate, linear elastic Saint-Venant type solution that agrees well with nonlinear (finite deformation neo-Hookean) finite element simulations, indicating nonlinear effects are unimportant in the bulk of the seal, and only important at the high-pressure and low-pressure ends. Using finite element simulations, we characterize the energy release rates for the growth of cracks in the regions of high stress concentration at the ends of the seal. We show that, despite the linear Saint-Venant solution not being valid at the ends, it correlates the energy release rates obtained in the nonlinear finite element solutions. Although the Saint-Venant solution enables understanding of the location where fracture will first occur, experimental observations indicate that fracture often happens on both ends of the seal. In order to understand this, we implement a user subroutine within the finite element software Abaqus to predict fracture initiation and propagation. Results indicate that, despite fracture initially occurring on either end, the growth of cracks leads to fracture on both ends of the seal, consistent with experimental observations.