Summary Cementing plays a critical role in ensuring zonal isolation and long-term well performance, motivating continued evaluation of supplementary cementitious materials (SCMs) to partially replace Portland cement while maintaining oilwell operability and sealing-relevant properties. With this study, we examine the effect of partial replacement of Class G oilwell cement with fly ash (FA) across a range of blend compositions under controlled laboratory conditions. Five cement systems containing 0%, 20%, 40%, 60%, and 80% FA by weight of binder (BWOB) were prepared at a constant water content of 44% BWOB and conditioned at 150°F. An integrated experimental framework was adopted in which all blends were first screened using unconfined compressive strength and splitting tensile strength measurements after 24 hours curing, followed by a detailed characterization of the selected formulation using fresh-state performance, elastic response, and microstructural indicators. Fresh-state testing included rheology, thickening time, free water, and high-pressure, high-temperature (HPHT) fluid loss to assess placement-related behavior. The 60:40 ordinary Portland cement (OPC)/FA blend exhibited mechanical performance comparable to neat cement, with compressive and tensile strengths of 4,873 psi and 587 psi, respectively, while providing improved slurry stability, extended thickening time, and reduced fluid loss. Density profiling and X-ray computed tomography (CT) indicated reduced segregation and improved internal homogeneity, accompanied by a decrease in porosity from 32.3% to 29.8%. Elastic wave measurements showed modest increases in dynamic Young’s modulus and Poisson’s ratio relative to neat cement. The results demonstrate that moderate FA substitution can maintain key operational and mechanical criteria while enhancing slurry stability and microstructural uniformity, supporting its consideration in laboratory-scale formulation screening for oilwell cementing.
Shah et al. (Sun,) studied this question.