Improved Model for Predicting Annulus Pressure Drop during Underbalanced Drilling

Abstract

Most time during underbalanced drilling or cleaning of well, formation fluid (oil and water) influx is usually experienced as soon as productive zone is penetrated because formation pore pressure gradient are higher than hydrostatic pressure gradient. Successful flow of fine drilling cuttings and formation fluid (gas, oil and water) to the surface flow facilities therefore demands for accurate prediction of pressure drop along the well bore during underbalanced drilling. Previous studies on pressure profile prediction in the wellbore during underbalanced drilling have not taken into account the effect of cuttings volume. The purpose of this study is therefore to develop an improved model that predicts the effect of cuttings volume on annulus pressure drop in the wellbore during underbalanced drilling. This is an extension of Guo and Fadairo model for investigating the effect of cutting volume pressure drop during underbalanced drilling. The result of the study shows that the cutting flow rate increases with small pressure differential while it decreases with large pressure differential. This depicts that cutting retards fluid flow and increases bottom-hole pressure and fluid density, resulting in low fluid velocity and cutting lifting capacity of the drilling fluid. This may result in cutting accumulation, ultimately reducing the ability of the drilling fluid to effectively lift drill cuttings from bottom-hole to the surface and eventually lead into stuck drilling string.

Investigations into the coiled tubing partial underbalanced drilling (CT-PUBD) technique for drilling hard formations

“…The main problem associated with underbalanced drilling is the poor wellbore stability (Di Meglio et al 2014;Udegbunam et al 2015;Cheng et al 2013). It is also difficult to drill underbalanced in abnormal pressure formations owing to drilling safety and well control issues (Samson et al 2015).…”