Discussion of a Method for Evaluating Cementing Quality with Low-Density Cement Slurries

With the application of low density cement slurries, the existing cementing quality evaluation criteria are no longer applicable, so it is necessary to amend the criteria after sufficiently studying the various influencing factors. A downhole acoustic field model for cased wells is established in this paper through analysis of acoustic field. Combining the effect of different factors on acoustic characteristics of set cement with the basis of evaluation criteria for cementing quality, the improvement methods for the criteria was proposed. Furthermore, the operation procedure of improved evaluation criteria for cementing quality was provided.The logging evaluation index for improved cementing quality was obtained by comparing the logging response after improving the conditions with that in conventional conditions.The fitting functional equations of acoustic amplitudes were made by using different densities of cement slurry, the correlation coefficients for all equations are higher than 0.93. Through the comparison of linear coefficients of matching equations, it is shown that the matching coefficient of micro-silica low density cement slurry is higher than that of floating bead micro-silica low density cement slurry, and the lowest one is from the low-density coal ash cement slurry. After waiting on the cement for 48 hours, the upper coefficient limits of their acoustic amplitudes are 20.74, 19.15 and 18.82, and the lower coefficient limits are 20.85, 14.81 and 12.96 respectively. The cementing quality evaluation indices of all low-density cement slurries decrease with increasing density, and they have a very strong linear relationship between them. In light of the improved evaluation criteria, the evaluation indices can be calculated accurately by density of various cement slurries. This method can play an important role for the improvement of cementing quality evaluation criteria.