Summary Tests performed in the laboratory have shown that there exist two types of mechanisms that could lead to loss of cement-sheath integrity: mechanical degradation, when cement is submitted to compressive or tensile loadings that are too high compared with its strength, and chemical degradation. The worst case is when both mechanisms occur at the same time or one after the other. For example, a cement sheath that is damaged before entering into contact with a degrading fluid will allow this fluid to penetrate deeper into the cement sheath, hence accelerating cement chemical degradation. As a consequence, it is of paramount importance to understand the mechanisms that could lead to loss of cement-sheath integrity before any chemical degradation occurs. It is with this objective that a mechanistic model was developed to simulate the various modes of loss of cement-sheath integrity after cement has been placed: (a) cement volume variations during hydration owing to chemical shrinkage/expansion; (b) cement volume-variations during hydration owing to cement heat-production; (c) contraction (dilation) of the casing owing to a decrease (an increase) in mud density/temperature; (d) cement volume decrease owing to pore collapse; and (e) thermal cycling. This paper has two objectives: (1) present the mechanistic model and (2) on the basis thereof, show that loss of cement-sheath integrity depends not only on cement properties but also on the well architecture and well history.

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Summary Microannuli at the well cement-sheath interfaces may result in loss of zonal isolation, which is the source of many problems, such as sustainable annular pressures, crossflows between reservoirs, and undesirable flow behind the casing. The microannuli are commonly explained by variations in cement volume during hydration (chemical shrinkage/expansion) or by contraction of the casing because of a decrease in mud density/temperature because these could create a gap if the cement is unable to follow the induced deformations. However, these two modes are not sufficient to predict all possible types of microannuli encountered in oil and gas wells, meaning that other modes have been missed. This paper presents a comprehensive mechanistic analysis of microannulus formation to highlight and explain other modes and to detail the conditions under which they can appear. It is grounded in both theoretical and experimental evidence and takes into account most of the features that characterize cement after it has been placed, including cement volume variations and heat production during hydration, mud-density and temperature variations, cement thermo-poro-elasto-plastic behavior during and after hydration, thermo-poro-elasto-plastic behavior of the formation, and initial state of stress in the formation.

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