Contributory Influence of Drill Cuttings on Equivalent Circulation Density Model in Deviated Wellbores

Abstract

The estimation of equivalent circulation density (ECD) in oil well drilling and completion is rather of crucial importance and much care must be given in its calculation. This is because ECD being so sensitive, errors in its value estimation could lead to severe drilling and completion problems like kicks, loss circulation etc. especially when drilling in horizontal well sections, deepwater, depleted reservoirs, and wells with narrow pressure window. Traditional ECD calculation have only focused on the annular frictional pressure loss (AFPL) as the only contributory pressure loss (PL) in the ECD calculations, and most literatures have given more attention to this concept. Other factors aside AFPL contributes to the total PL at the bottom of the wellbore and these affect the value of ECD during drilling and completion operations. One of these factors is PL due to drill cuttings in the wellbore. In this work, the additional effect of drill cuttings to the annular frictional loss (AFL) in the wellbore has been considered. Awah BX2 well in the Niger Delta was used as case study. The Awah BX2 well is a deviated well that starts its inclination at 5000ft depth and having a measured depth of 14000ft. Mud of 8.8 ppg was used in the study. Matlab software was used in the model simulation. Emphasis was made on results from the mud with and without drill cuttings. Effects of ROP, mud flowrate, concentration of cuttings have been investigated to determine the ECD values and pressure losses in the wellbore. From the results, it was observed that the presence of cuttings in the mud increased pressure loss and hence the ECD. It was also observed from the analysis that increasing value of bit rate of penetration (ROP) increases the concentration of cuttings and hence the pressure loss and ECD for a particular mud flowrate but when the concentration of cuttings increases so much, the ROP decreased. Increase in mud flowrate decreases the effective solids density and hence decreases the ECD and pressure loss. The model utilized in the study more accurately predicts pressure losses and ECD than that traditionally used in ECD calculations.