Detailed Modelling of Drilling Fluid Flow in a Wellbore Annulus While Drilling
Volume 1, Number 5
To produce a well safely, the wellbore pressure during drilling must be in a range that prevents collapse yet avoids fracturing. This range is often called “the operating window.” Exceeding the limits of this range can trigger wellbore instability or initiate well control incidents. At the same time, accurate pressure prediction requires an understanding of the hydrodynamics processes in a borehole while drilling. Describing these processes is complicated by many factors: the mud rheology is usually non-Newtonian, the flow mode can be laminar or turbulent and the drillstring can rotate and be positioned eccentrically. Known semi-analytical approaches cannot account for the full range of fluid flows that can arise during drilling. These techniques do not take into account all factors, whereas accurate numerical simulation of the flow of drilling fluids is a means to describe the fluid behaviour in detail.
For numerical solutions of hydrodynamics equations a unique algorithm was developed. It was based on a finite-volume method and a new model of turbulence for non-Newtonian fluids. The algorithm considers rotation and eccentricity of the drillstring. Newtonian and non-Newtonian fluids, as described by the Herschel–Bulkley rheological model, have been implemented.
Data obtained via systematic parameter studies of the flow in a borehole can help in rapid determination of parameters like pressure drop, velocity field, and wall stresses corresponding to any drilling condition.
Applying the new model for the annulus flow and comparing the results to the parallel plate flow approximation enabled us to quantify the error made due to the approximated solution for non-Newtonian fluid rheology. The difference between the solutions grows as the annular gap increases. Such growth is a function of the rheological parameters. At the same time, some effects like secondary flow appearance can only be observed when applying the new solution method.
© 2012 Canadian Energy Technology & Innovation (CETI) Group
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