Oilwell cements: NMR contribution to establish the phase diagram pressure/temperature of the mixture H2O / Ca3SiO5

Abstract

Cement slurry is always used to support the casing of oilwells. This slurry is pumped down the steel casing of the well and then placed in the annular space between the casing and the surrounding rock. Nowadays wells are becoming deeper and deeper so the setting conditions of the cement paste are crucial. The pressure and temperature limit conditions can reach 1 000 bar and 250 °C at the bottom of the well. Hydration of synthetic tricalcium silicate Ca₃SiO₅ – the main component of oil well cements – was performed at high temperature under high pressure to simulate the oilwell conditions. The objective of the study is the establishment of the phase diagram of the ternary system – SiO₂, CaO, H₂O – according to the parameters: pressure, temperature and Ca/Si ratio. In this study, NMR is largely used to identify and to quantify the synthesized silicate hydrates, with two setting conditions (6 days at 120 °C under 400 bar and 6 days at 200 °C under 600 bar). Mechanical tests were performed to evaluate the compression strength of those silicate hydrates synthesized under such drastic conditions and to verify that they were still efficient binders.