An in situ synchrotron energy-dispersive diffraction study of the hydration of oilwell cement systems under high temperature/autoclave conditions up to 130 deg C

The technique of synchrotron energy dispersive diffraction has been developed for in situ studies of cement hydration under autoclave conditions. This has been applied to oilwell cements hydrating at typical oilwell temperatures up to 130 deg C. The results show clearly the detailed interplay between 11 detectable phases, from which a phase transformation scheme has been derived; this illustrates the progression of hydration up to 130 deg C for two extreme cases, with and without conservation of water content and autoclave pressure. The monosulphate hydrate phases are found to exhibit different stability bounds, with a surprising sequence of the 14-water, 10-water then 12-water monosulphate as temperature/time increases; the latter form is particularly associated with conditions of water/pressure loss. The effect of retarders on C3S dissolution and CH formation is negligible above 70 deg C, whereas the effect on the calcium sulphoaluminate hydrates is more complex, and possible reasons for this are discussed.