The Use of a pH-Triggered Polymer Gelant to Seal Cement Fractures in Wells

Abstract

Summary The potential leakage of hydrocarbon fluids or carbon dioxide (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. The presence of fractures in a cement annulus with apertures on the order of 10–300 µm can pose a significant leakage danger with effective permeability in the range of 0.1–1.0 md. Leakage pathways with small apertures are often difficult for conventional oilfield cement to repair; thus, a low-viscosity sealant that can be placed into these fractures easily while providing a long-term robust seal is desired. The development of a novel application with pH-triggered polymeric sealants could potentially be the solution to plugging these fractures. The application is based on the transport and reaction of a low-pH poly(acrylic acid) polymer through fractures in strongly alkaline cement. The pH-sensitive microgels viscosify after neutralization with cement to become highly swollen gels with substantial yield stress that can block fluid flow. Experiments in a cement fracture determined the effects of the viscosification and gel deposition with real-time visual observation and measurements of pressure gradient and effluent pH. Although the pH-triggered gelling mechanism and rheology measurements of the polymer gel show promising results, the polymer solution undergoes a reaction caused by the release of calcium cations from cement that collapses the polymer network (syneresis). It produces an undesirable calcium-precipitation byproduct that is detrimental to the strength and stability of the gel in place. As a result, gel-sealed leakage pathways that were subjected to various degrees of syneresis often failed to hold backpressures. Multiple chemicals were tested for pretreatment of cement cores to remove calcium from the cement surface zone to inhibit syneresis during polymer placement. A chelating agent, sodium triphosphate (Na5P3O10), was found to successfully eliminate syneresis without compromising the injectivity of polymer solution during placement. Polymer-gel strength is determined by recording the maximum-holdback pressure gradients during liquid-breakthrough tests after various periods of pretreatment and polymer shut-in time. Cores pretreated with Na5P3O10 successfully held up to an average of 70 psi/ft, which is significantly greater than the range of pressure gradients expected in CO2-storage applications. The use of such inexpensive, pH-triggered polyacrylic acid polymer allows the sealing of leakage pathways effectively under high-pH conditions.

Functional Gels and Chemicals Used in Oil and Gas Drilling Engineering: A Status and Prospective

“…To address issues such as the potential loss of carbon dioxide through cement-fractured wells in subsurface formations, an aqueous dispersion of an insoluble smart gel was developed by Ho et al [40]. This was prepared from pH-sensitive polyacrylic acid polymers and could be injected into cement fractures at low pH, such as water.…”

Comprehensive Review of Sealant Materials for Leakage Remediation Technology in Geological CO2 Capture and Storage Process

“…In addition, another microgel system has also been reported . The pH-triggered polymer gel system injects polymer (i.e., microgel) into the formation to be treated in the form of a high pH alkaline solution. , First, the reactive polymer solution enters the cement fractures flushing the alkaline brine that was initially in the fracture. Second, a neutralization reaction between the polymer solution and cement surface causes the pH to increase.…”

Leakage Mitigation During CO2 Geological Storage Process Using CO2 Triggered Gelation

“…Whatever the pressure and permeability/porosity, the relative foam viscosity can be described as a power law vs the shear rate evaluated from an empirical law established for polymer systems in which the interstitial velocity, permeability, and porosity are the main variables. Ho et al studied the effectiveness of a kind of pH-triggered polymer gelant to block CO 2 leakage through cement fractures. The blocking effect of this pH-triggered gelation was demonstrated under high-pH conditions.…”