Drilling fluids (DFs) are subjected to forces, downhole and they shear differently according to their rheological behaviour. Fluid could be Newtonian or non-Newtonian. These are the fluid, the hole and the drillstring (DS). Three basic factors affect drilling hydraulics in oil wells. The aspects encountered in drilling hydraulics are downhole circulating pressures, surge and swab, equivalent circulation density (ECD), bit optimization, hole cleaning and volumetric displacement. Of all hydraulics encountered in well operations, drilling hydraulics is perhaps the most important because it covers more aspects in downhole operations than the others. 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). 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. Hydraulics plays vital roles in well operations such as drilling, cementing, completion, and well stimulation. Thus, DS eccentricity must be fully considered during well planning and hydraulics designs. Pressure loss reduction of more than 50% was predicted for the fully eccentric case for Herschel Bulkley fluids. Furthermore, analysis of annular eccentricity reveals that APLs and ECD decrease with an increase in DS eccentricity. 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. Three non-Newtonian fluid models were analyzed, viz: Herschel Bulkley, power law and Bingham plastic models. This study evaluates the effect of drilling fluid rheology types and DS eccentricity on the entire drilling hydraulics. The prevalence of drillstring (DS) eccentricity in the annulus calls for a re-evaluation of existing hydraulic models. Thus, eccentricity exists in practical oil and gas wells especially horizontal and extended reach wells (ERWs) and must be accounted for. This is because of high well inclination angles and different string geometries. In reality, concentricity is almost never achieved even when centralizers are used. ![]() 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”. Drilling Hydraulics has been extensively treated in the literature. Hydraulics characterization requires that all factors are considered as the neglect of any could become potential sources of errors that would be detrimental to the overall well operation. plays profound roles in drilling, cementing and other well operations. Accurate determination of hydraulic parameters such as pressure losses, equivalent circulation density (ECD), etc.
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