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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Implementation of CO2 storage in geological media requires a proper assessment of the risk of CO2 leakage from the storage sites. In particular, it is necessary to evaluate the risk that cement sheaths represent leakage pathways as this could occur if cement becomes damaged or when debonding exists at one of the interfaces of the cement sheaths. Tests performed in the laboratory have shown that there exist two types of mechanisms that could lead to cement-sheath loss of integrity: mechanical degradation, when cement is submitted to compressive or tensile loadings that are too high, or 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 in contact with a degraded fluid will allow this fluid to penetrate deeper in the cement sheath, hence accelerating cement chemical-degradation. Hence, it is of paramount importance to understand the mechanisms that could lead to a cement-sheath loss of integrity before CO2 storage. This is with this objective, that we have been working on a mechanistic model that accounts for the various modes of cement-sheath loss of integrity after cement has been placed: a) cement volume-variations during hydration due to chemical shrinkage/expansion; b) cement volume-variations during hydration due to cement heat-production; c) contraction (dilation) of the casing due to a decrease (an increase) in mud density/temperature; d) cement volume-decrease due to pore collapse; e) thermal cycling. This paper has two objectives: presenting the mechanistic model and using this model to show that cement sheath loss of integrity not only depends on cement properties but also on the well architecture and well history.

International audienceThe experimental results of isotropic compression tests performed at 20°C and 90°C on a class G hardened cement paste hydrated at 90°C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424-1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60°C. The time-dependent deformations are more pronounced for hydration at a higher temperature