Selection and Use of CaO-Expanding Cements

Abstract

With the rapid drilling of deep wells, cementing operations encounter new and more complex challenges due to harsh environments. To improve the use and selection of CaO-type expanding cements, a series of measurements have been made. Calcium oxide is used as an expanding agent to overcome contraction as cement slurry sets. Cement systems selection was based on their performances indicated by expansion tendency, consistency behaviour, lime reactivity at 60°C and atmospheric pressure, and autoclave-cured cement paste properties. Slurries were prepared using different mixing methods (API standards and an in-house procedure) to help choose appropriate slurry designs for wide field applications. Expansion and consistency tests were carried out in High Pressure-High Temperature devices at 60, 80, 90 and 120°C and 120 MPa. Mix waters of different salinities and sulphur contents were used to prepare cement slurries. It is shown that the expanding influence of CaO becomes significant when the temperature is higher than 60°C, but less than 80°C; salt content of 20–30% by weight of water; lime concentration of 12.5%. Gel and shear strengths of 150 Pa and 4 MPa, respectively and cement expansion greater than 4 % give sufficient tightness and seal, to stop gas migration in boreholes.