A Multiphase Model for the Early Stages of the Hydration of Retarded Oilwell Cement

Abstract

Cement is used in the oil industry to line oil wells. The major com- ponents of oilwell cement are tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and calcium sulphate (CaSO4). With the exception of C2S, each of these plays an important role in the initial thickening of cement slurry. It is important to control the time that it takes for a slurry to thicken, and this is achieved by the addition of chemical retarders, which delay the onset of thickening. In this paper, the action of a retarder whose efiects are flrstly, to form a com- plex with calcium ions, and secondly, to inhibit the growth of ettringite crystals is investigated. Ettringite is a product of the hydration of C3A and the subsequent reaction of the products with calcium sulphate. A modifled version of a model for the hydration of C3S previously investigated by Salhan, Billingham and King (J. Eng. Math. 45 (2003) 367), along with the chemical kinetic scheme for the action of a retarder on ettringite proposed by Billingham and Coveney (J. Chem. Soc. Faraday Trans. 89 (1993) 3021) is used. The model distinguishes between liquid and solid phases, and treats water, which is signiflcantly depleted by the formation of ettringite, as one of the chemical constituents. It is found that both of the chemical actions of the retarder contribute to slowing the initial reaction rate, and that the sudden crystallization of ettringite as the efiect of the retarder is overcome, investigated by Billingham and Coveney, occurs in successive layers around the surface of the cement grain.