Wellbore integrity issues, including casing leaks and poor zonal isolation behind the casing, present a tremendous challenge to conventional cement slurries when dealing with low injectivity, pressure restrictions, pin-hole leaks, and/or a primary cementing microannulus, among others. If the conditions of these near-wellbore (NWB) issues exceed the capabilities of conventional cement slurries, alternative methods might be necessary. This paper presents the field implementation of an organically crosslinked conformance polymer sealant (CPS) system used successfully to address these wellbore integrity challenges. Although it is not a replacement for cement slurries in all situations, the CPS system provides an alternative chemical and engineering solution for treating NWB problems in special circumstances.
The CPS system uses a copolymer of acrylamide and t-butyl acrylate (PAtBA) crosslinked with polyethyleneimine (PEI). The CPS system is injected into the formation and void spaces as a low viscosity solution that activates at a predicted time to form a hydrogel to completely shut off matrix permeability, fractures, fissures, and/or channels. This system has been successfully tested to withstand a differential pressure of at least 2,600 psi, has a working temperature range of 40 to 400°F, and is resistant to acid, CO2, and H2S environments. The CPS system does not develop any compressive strength, which simplifies the cleanup stage by easily jetting it out of the wellbore, opposed to cement that has to be milled out, which becomes increasingly challenging at shallow depths and/or in highly deviated wells.
To date, more than 1,000 CPS system treatments have been performed globally to address conformance problems, such as water coning/cresting, high-permeability streaks, gravel-pack isolation, and/or fracture shutoff. In this paper, case histories related to casing-leak repair, annular flow between pipes, and poor zonal isolation behind the casing are presented. Additionally, lessons learned from the diagnostic stages of both offshore and inland case histories are highlighted.