Prediction of oilwell cement thickening times using a cementing model

Oilwell cement thickening times are measured by mixing cement and additives and placing them in a consistometer in the laboratory to simulate downhole behaviour. This procedure is often unrepresentative of the practical operation on the rig. There will normally be variations in temperature, additive concentration and slurry density. All of these affect the thickening time in some way. However, unless each slurry is tested in the laboratory it is impossible to predict the result. Failure to identify excessively sensitive slurries has resulted in instances of lost rig time in practice. In an attempt to prevent such sensitive slurries from being pumped in the future, work has been undertaken on the feasibility of a model to predict thickening time based upon the cement hydration chemistry. Although the model described is still in the early stages of development, the indications are encouraging that such a model could be put to practical use. Correlation between experimental and predicted thickening times for three different class G cements and two different types of lignosulphonate retarder over the temperature range 27-120°C was shown to be generally good.