Casing failure probability is high in Ann Mag Field, South Texas due to the high pressure high temperature operational environment. The formation sands are over-pressured where the pore pressure ranges from 0.85 to 0.93 psi/ft. Casing damage has been experienced in over 11 wells from 18 that have been drilled in the area, near 61% of total wells were damaged during their production life. Casing failure may be caused by formation shear failure, formation compressive failure, casing tension failure, casing collapse or fault activation. Casing buckling is not considered in the study because cement bond logging shows that the cement sheath is good. Triaxial tests were carried out to measure formation mechanical properties that were used for reservoir compaction, fault activation and formation failure analysis. The Mohr-Coulomb failure criterion was applied to study the formation failure and the minimum pore pressure required to activate the fault. The finite element methods were used to analyze the casing tension failure caused by reservoir compaction. According to the formation shear failure analysis, the minimum allowable pore pressure around the casing is around 2,000 psi. In the study of fault activation, for shale formation, at the internal friction coefficient of 0.51, the probability that the normal fault can be activated is very high. The maximum drawdown and depletion were calculated based on potential casing failure types. In the plot of workability operational limits, shale formation has narrower safe zone than the sand formation. Recommendations for drilling and production were made to increase the well service life and improve the gas recovery. This paper presents the casing failure mechanism and characterization under HPHT conditions in south Texas that can be prevented in the future wells and provides workability operational limits for different formations.


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