Analysis and determination method of addition mud density in casing program design

Abstract The addition mud density is a key part of drilling fluid design to avoid drilling accidents resulting from the pressure surge. However, the current standard of addition mud density is empirical and whether this empirical value can be applied to all wells, especially deep wells, deserves further study. In this paper, in order to quantitatively calculate the variation of downhole pressure during tripping, the transient wellbore surge pressure calculation model during tripping was established and verified by field data fist. Then, through the analysis of the surge pressure and its influencing factors during tripping, it was found that the current standard of addition pressure difference cannot be used as an empirical value for different sections of a well, while the addition mud density was more reasonable for the open hole safety in casing program design. However, the surge pressure was much smaller than the lower limit of current standard addition mud density under the actual working conditions. On the other hand, through the analysis of the wellbore pressure variation and its influencing factors when gas kick occurs, it was found that the variation of bottom hole pressure caused by gas kick was much smaller than that caused by tripping. Therefore, the surge pressure should be the main factor in the determination of addition mud density. Based on this, the determination method of addition mud density for each tripping section was proposed, which can calculate the addition mud density quantitatively and more reasonably for each tripping section based on the comprehensive consideration of factors such as wellbore pressure variation and formation pressure prediction error. The results showed that the current standards of addition mud density are too conservative in medium and shallow formations and may not meet the safety requirements in deep formations.