However, these works almost entirely rely on the assumption that the drill string lies perfectly at the center of the annulus-the so-called “concentric annulus”. In reality, concentricity is almost never achieved even when centralizers are used. This is because of high well inclination angles and different string geometries. Thus, eccentricity exists in practical oil and gas wells especially horizontal and extended reach wells (ERWs) and must be accounted for. The prevalence of drillstring (DS) eccentricity in the annulus calls for a re-evaluation of existing hydraulic models. This study evaluates the effect of drilling fluid rheology types and DS eccentricity on the entire drilling hydraulics. Three non-Newtonian fluid models were analyzed, viz: Herschel Bulkley, power law and Bingham plastic models. From the results, it was observed that while power law and Bingham plastic models gave the upper and lower hydraulic values, Herschel Bulkley fluid model gave annular pressure loss (APL) and ECD values that fall between the upper and lower values and provide a better fit to the hydraulic data than power law and Bingham plastic fluids. Furthermore, analysis of annular eccentricity reveals that APLs and ECD decrease with an increase in DS eccentricity. Pressure loss reduction of more than 50% was predicted for the fully eccentric case for Herschel Bulkley fluids.

Thus, DS eccentricity must be fully considered during well planning and hydraulics designs. Hydraulics plays vital roles in well operations such as drilling, cementing, completion, and well stimulation. Increasing well depth and complexity in geometry such as horizontal or extended reach wells (ERWs) gives rise to more complicated hydraulics than would be encountered in shallower slightly deviated or vertical wells.

Proper study of hydraulics in a well is crucial as it translates to a reduction of risk, improvement in efficiency, decrease in the overall cost of well operation and reduction in non-productive time (NPT).

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