Casing and screen failure analysis in highly compacting sandstone fields
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Many casing and screen damage incidents have been reported in deep-water oil and gas fields in the Gulf of Mexico and other locations around the world. We reviewed historical casing/well failure events in a highly compacting sandstone field and performed a comprehensive geomechanics analysis of various casing damage mechanisms (tension, axial compression, shear, and bending) related to large reservoir depletion. Among five wells that experienced mechanical well integrity issues, two of them showed casing restrictions in the cap rock at intervals around 1,000 to 1,600 ft-TVD above the top of the depleting (main) reservoir. A multi-finger caliper log obtained from one of the wells indicates that the overburden casing failure occurred at a highly geo-pressured, thin sand layer approximately 1,100 ft-TVD above the top of the compacting reservoir. The remaining casing failure events occurred near (less than 200 ft-TVD) or within the compacting reservoir interval. A 3D non-linear finite element model has been developed for simulating stress changes in the overburden and the reservoir intervals and evaluating the effect of lithological anomalies on casing stability. The simulation results indicate that large tensile and shear strains could develop within a thin, weak-strength layer in the overburden and at the interface between cap rock and depleting reservoir interval. Casing damage by bending/shear could also occur at these thin-layered sands saturated with overpressured gas. In the reservoir interval, shear stresses acting on the screens can be relatively high due to the difference of the movements between the internal base pipe and the external shroud and gravel. Screen failure may also occur at the welded points. If casing failure occurs in the unperforated sand layer just above the compacting reservoir, it induces localized high velocity flow on the upper part of the screen causing potential screen erosion. Casing failure due to fault slip near the reservoir occurs only if a fault has sealing capability while maintaining a large pressure differential across the fault plane. The numerical analysis results presented in this work help engineers understand possible casing and screen deformation and failure mechanisms experienced in highly compacting sandstone fields. Based on the study findings, we also present completion design guidelines to avoid or mitigate compaction-induced casing damage in both the overburden and reservoir intervals. Copyright 2011, Society of Petroleum Engineers.
