Casing- and Screen-Failure Analysis in Highly Compacting Sandstone Fields
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SummaryMany casing- and screen-damage incidents have been reported in deepwater 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 caprock at intervals approximately 1,000- to 1,600-ft true vertical depth (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 geopressured, 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 nonlinear finite-element-method (FEM) 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 the 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 because of 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 caused by 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. On the basis of the study findings, we also present some completion-design guidelines to avoid or mitigate compaction-induced casing damage in both the overburden and reservoir intervals.
