Effect of Eccentric Annulus, Washouts and Breakouts on Well Cementing Quality: Laminar Regime

Annular cementing is an essential step in well construction in underground CO2 storage projects. The quality of cementing depends on the quality of mud displacement by the fluids injected into the annulus during the cementing job. An engineering numerical model of annular fluid displacement in a vertical well was used in this study to investigate the effect of well profile on the displacement quality. Annular displacement of the thinner fluid in place by the injected thicker fluid was modelled. The effect of the following factors on the displacement was studied: slightly irregular shape of the well cross-section; eccentric positioning of the casing in a circular well; a local washout (enlargement of the well cross-section); throughgoing breakouts. Random irregularities of the well cross-section were found to have only a minor effect on the advancement of the displacement front and on the injection pressure. Eccentric positioning of the casing pipe was found to cause flow channelization due to the yield-stress rheology of the fluids. The eccentricity was found to affect the injection pressure as well. An isolated washout was found to have only a minor influence on the injection pressure and the displacement front propagation. Borehole breakouts were found to cause severe channelization of the injected fluid, and a substantial reduction in the injection pressure. Continuous enlargements of the annulus caused e.g. by borehole breakouts or eccentric positioning of the casing were thus shown to have a more detrimental effect on the annular displacement than a local, isolated enlargement caused e.g. by a washout. Abnormally low injection pressure during a cementing job might serve as an indication of flow channelization through such a throughgoing enlargement.