LAUSR

Abstract We study the effect of borehole ovalization and large-scale irregularities during primary cementing of a horizontal section of well. We use a two-dimensional gap-averaged model that can compute the displacement within varying annular geometries: both eccentric and elliptic. We study displacement of a drilling mud by a spacer within both, elliptical-uniform cross-sections and those which vary irregularly in the axial direction, based on field caliper measurements. The combination of eccentricity and ellipticity adjusts the azimuthal positioning of the wide and narrow gaps producing a variety of characteristic behaviours, e.g. the front may advance in the form of 2 peaks at off-centre positions. The irregularities can induce additional azimuthal flow when compared to uniformly circular wellbores. In cases when the displaced mud has a significant yield stress, the displacement can leave patches of residual mud at the top of eccentric horizontal wellbores: not usually found without ellipticity.