Annular displacement in a highly inclined irregular wellbore: Experimental and three-dimensional numerical simulations

Abstract Primary cementing of well casings is the operation where drilling fluid is displaced from the annulus between the casing and the drilled rock formation and replaced by a cement slurry. The annulus to be cemented is normally narrow and eccentric, and the formation wall may exhibit irregular geometric features such as local enlargements or washouts, originating from the drilling process and varying geological properties. Eccentricity and the presence of wellbore irregularities can lead to residual, non-displaced drilling fluid or mixing of the cement slurry with other wellbore fluids, causing contamination and failure to hydraulically isolate the annulus outside the casing. We study non-Newtonian fluid displacement in an irregular annulus experimentally and numerically for concentric and eccentric configurations of the inner pipe, and for horizontal and slanted orientations of the annulus. An overgauge section simulates a washed out zone in the wellbore. Eccentricity promotes fluid flow in the wide sector of the annulus, leading to an axial elongation of the interface between the two fluids. Depending on eccentricity and inclination, residual non-displaced fluid remains in different parts of the hole enlargement, while inclination from the horizontal is found to improve the fluid displacement in the enlarged section. The experiments and simulations agree well for the tests where the annulus is slanted, with simulations replicating displacement trends and arrival times measured in experiments. The quantitative difference between experiment and simulation is larger for the horizontal and concentric annulus tests. We discuss modelling assumptions and experiment uncertainties that can explain the observed differences.