The primary objective of a cementing job is to create a stable mechanical link between the casing and formation. This link provides a strong support for the casing and isolates different zones to prevent any contamination to aquifers during the lifecycle of the well. The damage of the cement sheath may result in contamination of drinking water resource and negative environmental impact. Hence, the loss isolation of the cement has received much attention from researchers and the public.

The possibility of losing the isolation of the cement sheath during the lifecycle of the well is summarized and the potential risk in contaminating underground water formations is discussed in this paper. A mathematical model for predicting the failure of the cement sheath in an anisotropic stress field is presented. The model combines the effect of temperature, the variation of stress around the wellbore, internal pressure, and the integrity of the casing, cement, and rock formation. The objective of the model is to obtain the maximum internal pressure that we can apply on the casing without causing cement sheath failure. A numerical simulator was developed to calculate this maximum pressure for any inclination and azimuth angle. Computational results show that cement sheath failure depends strongly on the in-situ stress field of the surrounding formation. With stronger uniform formation support, the casing and cement can withstand larger internal stress. The failure of the cement sheath is more severe in a highly anisotropic stress field than in an isotropic field. Hence, the assumption of uniform formation stress around the wellbore may cause significant error in predicting failure of the cement sheath.

The results of this study provide petroleum engineers a tool to determine the maximum internal pressure during drilling, production and stimulation to avoid losing cement isolation. It is recommended to use this model to check the integrity of the wellbore during any operations during the lifecycle of the well.

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