Smart expandable fiber additive to prevent formation of microannuli

One of the critical issues that occur in many oil and gas wells is the failure of the cement sheath because of debonding from the casing string or from the formation. This results in the formation of microannuli, which can become pathways for fluid migration. Cement shrinkage during setting is regarded as one of the main causes of the formation of microannuli. In this paper, a new class of polymer-based expandable additives in the form of fibers is incorporated into the cement to compensate for shrinkage and thereby help prevent the formation of microannuli in oil and gas wells. The proposed fiber additives are made from shape-memory polymers (SMPs) and expand when exposed to temperatures above a specific value that is, by design, below the downhole temperature of the cemented zone. Fiber expansion occurs after the placement of the cement slurry but before its setting to avoid the inducement of any microfractures. As a result of the expansion of the cement paste, flow channels and fluid migration may significantly decrease while preserving the mechanical properties required for the mechanical integrity of the cement sheath. The bridging effect of fibers across individual microcracks helps control the propagation and coalescence of small fractures. Considering the inert property of the proposed additive, the water-cement ratio and its chemical properties do not need to be revisited. The measured increase in cement ductility makes the cement system more resistant to cracking. The cement expansion, fluid loss, gel strength, compressive strength, ductility, and tensile strength of the samples containing these fibers are examined using destructive and nondestructive methods, as reported here. The proposed class of expandable additives can help operators reach sustainable well integrity by increasing the contact stress at the cement–casing and the cement–formation interfaces to prevent fluid migration and the propagation of cracks.