Wellbore integrity is a crucial aspect of well construction. The integrity of annular sealing has a bearing on the overall integrity of the well. The integrity of the annular seal is affected by the properties of cement (or sealant, in general), including its permeability, mechanical strength, bonding strength, chemical and thermal stability. Well integrity is also influenced by large defects introduced into the sealant during placement, e.g., gas channels caused by gas migration during setting, or mud pockets left undisplaced in washouts.

The displacement efficiency is a function of the mobility ratio of the fluid in place, e.g., the drilling mud, and the fluids being injected, such as the spacer and the cement. Rheological properties of the fluid in place and the injected fluids, the well inclination, and the geometry of the annulus have an impact on the displacement efficiency.

The effect of rheological properties of the injected fluid and the fluid in place (yield stress and the plastic viscosity) is evaluated in this study by means of numerical modeling. It is shown how a proper combination of the rheological properties can improve the fluid displacement from constrictions in the annulus, e.g., when the casing is not perfectly centralized in the borehole. It is also shown how an unfavorable combination of rheological properties and annular geometry may promote the development of gas channels and mud pockets in the annulus. Unless the borehole cross-section is perfectly circular, an unfavorable combination of rheological properties may lead to viscous instabilities during annular flow, resulting in mud channels left in the annulus. During lifetime of the well, such channels may become conduits for formation fluids. They also may serve as stress concentrators when in-situ stresses or wellbore temperatures are altered. Finite-element computations of thermal stresses near a channel left in the cemented annulus reveal that tensile stresses induced around such a channel by heating may be sufficient to create fractures in cement around the channel.

The methodology elaborated in this study enables prediction of cement integrity from placement to failure and thus spans the entire well lifetime.

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