Effect of Nonuniform Loading on Conventional Casing Collapse Resistance

Abstract

Summary In various locations worldwide, casing designers are confronted with nonuniform cross-sectional loading caused by contact with a mobile formation. The formation movement may be the result of rock constitution (salt, swelling shale), or it may be tectonic in origin. Typically, well design proceeds with the goal of combating the nonuniform load. If this load occurs slowly, adjustments to the tubular design may be minimal, as the cross-sectional ovalization can only occur at the same rate as the formation movement. Adopting such an approach, however, ignores the fact that ovalization of the cross section because of nonuniform loading may also lower its collapse resistance to more conventional loads such as pressure differential (e.g., external pore pressure minus internal fluid pressure). This paper documents the results of numerical and experimental studies to determine the effect of nonuniform loading on the conventional collapse resistance of casing. The current work focuses on casing exposed to external fluid pressure and extreme point loading, as might be the case in a poorly cemented wellbore. An introductory section sets forth the problem with a specific design scenario for a production casing string loaded simultaneously by imposed ovalization caused by tectonic formation movement and collapse differential from the local fluid environment. The load is then simplified and solved numerically. The numerical results are first justified by physical argument, and then validated by full-scale collapse testing. Finally, the distinction between manufactured and imposed ovality is reinforced by comparison of the results with conventional collapse equations.