Durability of Polymer-Based Wellbore Sealants in a Corrosive Environment

Abstract

Abstract Development of wellbore sealant alternatives to cements is a topic of high relevance for production and injection wells, permanent plugging and abandonment, and remediation of leakage behind the casing. Some examples of alternative sealants are epoxy-based sealants, geopolymers, and bismuth-based alloys. Depending on the application, sealing materials are expected to encounter challenging downhole conditions, such as corrosive environments (e.g., CO2, H2S) and pressure or thermal cycling. This is especially significant for permanent plugs, where long-term perspective needs to be considered. In this work, we conducted long-term exposure of three polymer-based wellbore sealants (labelled as A, B and C) to an artificial seawater water environment with dissolved H2S gas. The polymer-based sealants are compared to each other and to a Portland G cement blend that was subjected to the same testing procedure. The experiments reported here are a part of a more extensive campaign that aims to study the behaviour of these materials after up to 12 months exposure to H2S. The aging tests were performed as batch-exposure conditions in a pressure cell at 100 °C and 10 bar. Cylindrical (core) samples of the same material were submerged together in artificial seawater in a glass beaker, and a mixture of H2S and nitrogen was dispersed into the seawater. We characterized mechanical properties before and after H2S exposure by unconfined compressive strength (UCS) tests. X-ray micro-computed tomography (CT) was performed to visualize changes potentially induced by the reaction with H2S. After H2S exposure, sealants A and B displayed significant axial and radial deformation during UCS tests (ductile behaviour), which is a very different behaviour compared to a typical wellbore cement. Sealant C exhibited ductile behaviour during compression but without considerable deformation. For comparison, strain at the peak stress was in the order of 200-300 mm/m for sealants A and B, whereas for C it was approximately 60 mm/m, after one month of H2S aging. For all three materials, a decrease of UCS and Young's modulus was observed after H2S exposure. For sealant C, the UCS was still relatively high after three months of aging, at around 70 MPa, which was about 50 % decrease from the reference value. CT results revealed no obvious progression of a reaction front for sealants A, B and C, however, different effects (e.g. change of porosity, precipitation, cracking) were observed throughout the volumes. The unique behaviour of these materials under stress and the possibility of tuning the physical and chemical properties hold vast potential for different applications. One of the long-term goals is to optimize the material properties to make them more suitable for the permanent plugging of both petroleum and CO2 wells.