Conduction Calorimetry and X-Ray Diffraction Investigation of Cement Retardation at 70 – 120 ° C

Abstract

Studies on cement retardation in the temperature range of 70 – 120°C using conventional well cementing techniques indicate discontinuities with respect to thickening time. In the temperature range of 70-100°C unexpectedly long thickening times can occur that are not linear but exponential in relation to retarder concentration. At 100-120°C thickening times become shorter again with some retarders showing a notable threshold concentration above which there is minimal increase in thickening time. These discontinuity effects have serious implications in formulating slurries for use in this temperature range. Conduction calorimetry was used to determine the effects of various retarders on a thermally stable Class G system (Class G cement with 40% silica flour) at a water to cement ratio of 0.4 over the temperature range 70 - 120°C. Retarders included sucrose, dextrose, trehalose, lignosulphonate and glucoheptonate and were chosen for their different retardation effects on cement. Studies were performed in a Setaram C80 conduction calorimeter with a high pressure gas controller. This system allowed the temperature to be ramped from ambient to the test temperature under pressure and more closely simulates well conditions. At 70 - 90°C conduction calorimetry shows heat of hydration curves similar to that obtained at ambient temperatures, though in this case the initial heat flow peak was not determined as cement and water could not be mixed in-situ. The shape of the second peak varies considerably depending on the retarder used and in some cases has a notable shoulder on the decline. The shoulder under certain test conditions can appear as a third peak. It is considered that this peak relates to reaction of the ferrite phase which is around 15-18% in the Class G cement. At 100 – 120°C differences between retarders are even greater. Moderate retarders give three distinct peaks where the second peak is considered to relate to reaction of the ferrite phase forming hydrogarnet whereas the third peak is attributed to the silicate hydration. Increasing retarder concentration does not significantly affect the time at which the second peak is observed but does show a linear increase in retardation of the third peak. Powerful retarders show only two peaks with retardation of the second peak directly related to retarder concentration. X-ray diffraction was performed on samples removed from the calorimeter at selected times in an attempt to identify phase changes associated with the observed heat flow peaks.