The use of a pH-triggered polymer gelant to seal cement fractures in wells
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The potential leakage of hydrocarbon fluids or CO2 out of subsurface formations through wells with fractured cement or debonded microannuli is a primary concern in oil and gas production and CO2 storage. A novel application using pH-sensitive microgel dispersion is introduced as a solution to remediation workovers using oilfield cement, which often fails to provide effective seal in smaller fractures. The application is based on the reaction of a low-pH poly(acrylic acid) polymer that can develop substantial yield stress when passing through strongly alkaline cement fractures. While the pH–trigger mechanism and rheology show promising results, an unexpected phenomenon, known as polymer syneresis, produced a byproduct that was proven to compromise the seal of the injected gel in place. This study focuses on understanding the development of polymer viscosity and identifying the main components of syneresis. Several chemicals were studied to inhibit syneresis and tested as either polymer additives or cement pre-treatment in cement fracture corefloods. The chemical inhibitors and its applications were then selected based on not only their ability to eliminate syneresis, but also their reaction with polymer during injection and subsequent development of gel yield stress in cement fractures. Cores pre-treated with a chelating agent, known as sodium triphosphate (Na5P3O10), showed good injectivity during polymer placement and significant improved sealing performance during water breakthrough tests. The resulting gel-in-place provided longer periods of effective seal and held pressure gradients orders of magnitude higher than just a few psi/ft from gel placed in un-treated cores. Furthermore, the comparison of holdback pressure gradients between designed corefloods indicated improvement in gel strength as fracture aperture is decreased and polymer shut-in time is increased. With proper cement pretreatment, the pH-triggered polymer-gel system has been seen to effectively plug small fractures and have valuable applications for long-term robust seal in leaky wellbores.