Surface casing cement behavior during rig testing scenarios: Lab testing and structurally modeling the integrity of the shallow casing-cement system.

摘要

The structural integrity of casing set in mechanically anisotropic shale is simulated to evaluate the effect of rig testing on annular cement performance. The structural models incorporate cement mechanical properties at varying hydration times. Especially considered is the behavior of the interfacial bond between casing and cement.;An experiment, the Cohesive Zone Experiment (CZE), captures the effect of hydration time on bonding behavior for two common oilwell cements. The CZE reveals cement at early hydration time (4-6 hours) has a compliant bond. The adhesive bond of this "immature" cement does not fail acutely when stressed to failure; the bond retains some strength through displacement past the initial fracture. However, cements at late hydration time (22-24 hours) do not show this compliance. The bond at late times fails acutely; displacement sufficient to initiate fracture is sufficient to fail the bond. The compliant bond behavior should be modeled with a specific energy value: the critical fracture energy. The non-compliant bond should be modeled with a specific force value: the critical (or ultimate) shear strength.;The models are verified against familiar D-J Basin surface casing scenarios; however, they are designed to be general and germane to other basins and completion scenarios. The models reveal the significance of geometric and mechanical variables on the potential risk of downhole fluid segregation failures, undesirable "backside" gas migration, and contamination risks to protected drinking water aquifers.;The models support the conventional wisdom around surface casing operations: casing centralization has a significant effect on the response of stressed annular cement; "typical" waiting-on-cement (WOC) times are adequate to allow cement to develop sufficient strength; and rig testing does not excessively stress the competently completed wellbore.