Oilwell cements:NMR contribution to establish the phase diagram pressure/temperature of the mixture H_2O/Ca_3SiO_5

摘要

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 betweenthe 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 1000 bar and 250 deg C at the bottom of the well.Hydration of synthetic tricalcium silicate Ca_3SiO_5-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 - SiO_2,CaO,H_2O- 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 deg C under 400 bar and 6 days at 200 deg C under 600 bar).Machanical tests were performed to evaluated the compression strength of those silicate hydrates synthesized under such drastic conditions and to verify that they were still efficient binders.