Chemical structure of cement aged at normal and elevated temperatures and pressures: Part I. Class G oilwell cement

The prime objective of the plug-and-abandon operations is to provide zonal isolation for infinite time. Cement-based materials are generally used as plugging materials. Therefore, it is important to understand the physical and chemical processes causing cement degradation in the downhole environment. In this study, we have characterised a Class G oilwell cement immersed for 1 year in brine at T=293 K, p=105 Pa and T=353 K, p=7×106 Pa using NMR and XRD techniques. In order to have a better understanding of the 27Al NMR spectra, selective dissolution has been performed. The results show that after 1 year of immersion in brine at T=293 K, p=105 Pa, monosulfate is converted to Friedel's salt. Leaching resulted in the disappearance of portlandite and the formation of calcite and a more polymerised calcium silicate hydrates (C–S–H). In the T=353 K, p=7×106 Pa mineralogy, ettringite is converted to hydrogrossular.