Chemical structure of cement aged at normal and elevated temperatures and pressures, Part II: Low permeability class G oilwell cement

Recently, Low Permeability Cement formulation has been developed for oilwell cementing. 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 Low Permeability Class G oilwell Cement immersed for one year in brine at T = 293 K, p = 105 Pa and T = 353 K, p = 7 × 106 Pa using 29Si, 27Al NMR and XRD techniques. Elevated temperature and pressure conditions increase the rate of the pozzolanic reaction and have significant effects on the polymerisation of C–S–H and on the incorporation of Al in the C–S–H structure. Leaching resulted in the formation of calcite and a more polymerised C–S–H with the appearance of tobermorite in the sample cured at elevated temperature and pressure.