Improved ECD Prediction and Management in Horizontal and Extended Reach Wells with Eccentric Drillstrings
Authors: Erge Oney , Vajargah Ali Karimi , Ozbayoglu Mehmet Evren , van Oort Eric 
Published: January 2016

Abstract
Accurate prediction of Equivalent Circulating Density (ECD) plays a vital role in hydraulic preplanning and real-time management of drilling operations. Such accuracy is often not available in case of drilling horizontal and extended reach wells, when there is a partially or fully eccentric annulus in a large section of the wellbore. Many hydraulic models, including those widely used in the industry based on e.g. the Narrow Slot approximation, simply ignore the effects of drillpipe eccentricity on annular pressure loss and equivalent circulating density (ECD), thereby generating unreliable estimates for these important quantities.
In this study, we discuss the experimental, analytical, and numerical results of a comprehensive investigation into the impact of drillpipe eccentricity on ECD in horizontal and extended reach wells while circulating non-Newtonian drilling fluids. A 91 ft. long flow loop with a drillpipe to hole/casing ratio of 0.5 (fairly common in conventional drilling applications) was used for the experimental investigation. Drillpipe was placed at the bottom of the hole/casing, thereby simulating a fully eccentric annulus. Four non-Newtonian drilling fluids, which their rheological behavior best described by the Yield Power Law (YPL) model, were used as test fluids. Moreover, a novel numerical model based on a Computational Fluid Dynamics (CFD) approach was developed. Subsequently, the experimental data was compared with both the new proposed model as well as with several other widely used analytical and numerical models previously reported in the literature. Field data analysis was also conducted to investigate the impact of eccentricity on ECD in a deviated well.
The experimental results confirm and add to the historical observations that the annular frictional pressure loss in a fully eccentric annulus is significantly less than the values predicted by the Narrow Slot approach, sometimes by more than 50%. In general, all the evaluated models underestimated the effect of drillpipe eccentricity on annular frictional pressure loss. However, by comparison, the newly proposed model in this study exhibited by far the least discrepancy with the experimental results. The difference between the estimated and experimental results was found to be proportional to the magnitude of the fluid yield stress and consistency index, and attributed to the complexity associated with simulating YPL fluids.
This study provides valuable insight towards superior hydraulic planning and real-time ECD management in horizontal and extended-reach wells. Its beneficial impact on managing the annular pressure between the fracture and pore pressure in the field to prevent drilling problems such as kicks, mud loss and wellbore instability events is expected to be immediate. Its use will also be key to successful implementation of innovative drilling techniques such as managed pressure drilling and dual gradient drilling, which require highly accurate annular pressure prediction and management.