Thickening Time Test Apparatus Provides Method of Simulating Actual Shear History of Oilwell Cements

Abstract

Abstract A reliable prediction of cement thickening time is central to the success of oilwell cementing operations. The amount of time that a cement remains "pumpable" is dependent on several factors, including shear rate history. Current industry testing methods, however, allow for only a single rotational speed of the consistometer slurry cup during a thickening time simulation. This, in turn, relegates the range of shear rates found within the cup to those due only to the changes in the Theological properties of the cement. Yet, oilwell cements encounter a much wider range of shear rate conditions during placement due to changing surface pump rates and variable wellbore geometries. The physical attributes of oilwell cements, including thickening time, are influenced by this shear rate history. A conventional pressurized consistometer, fitted with a variable speed motor, is combined with a standard slurry cup containing a helical screw paddle (HSP). This device provides the basis for comparative thickening time testing. Computer modeling allows for an actual depiction of the shear rate history to be integrated into a thickening time evaluation. This information was used to program a modified consistometer to replicate the shear rate, temperature, and pressure conditions found during placement. Presented are the comparisons of standard API thickening times, thickening times with the HSP rotating at 150 revolutions per minute (RPM), and thickening times with the HSP in actual shear rate history simulations. The results indicate that thickening times are shortened in a low shear rate environment. At higher shear rates, the differences in thickening time among the three testing methodologies are minimal. Knowledge of the actual shear rate encountered during a cementing operation and a slurry’s susceptibility to changes in thickening time due to shear rate history, is vital to predicting the true thickening time of the slurry under downhole conditions.