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 nonuniformload. 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 there sults with conventional collapse equations. Introduction The wellbore diagram in Fig. 1 illustrates the class of wells that inspired this investigation. A prior publication1 presents the context of the design in detail. The wells reside in the Andean foothills in Colombia. Most wells penetrate the Yopal fault and exhibit measurable ovalization in response to tectonic movement. In addition, the pore pressure of an overburden formation (Carbonera sequence) immediately above the reservoir imposes a 12.5 to 16 lb/galequivalent collapse pressure on the 9 5/8-in. production casing. It is conjectured that, first, loss of collapse resistance caused by ovalization of the casing cross section from nonuniform formation loading, or second, loss of collapse backup pressure either caused by a packer leak or associated with awell intervention operation, or, finally, a combination of the first two, can result in a casing collapse failure. Investigation of this loadincrease/resistance decrease is the subject of the present study. Effect of Imposed Ovalization on Conventional Collapse Resistance Ovalization of a casing cross section caused by nonuniform formation loading can cause operational problems in its own right. Deformation of the cross section, however, is limited by the rate of loading induced by the rock. Provided this loading rate is sufficiently slow, years of useable service may exist before the drift of the casing becomes small enough to impinge on interior tubulars or cause problems with tool passage. The situation described above is true, however, only as long as there is no additional collapse mechanism such as a fluid pressure differential. In this latter circumstance, ovalization caused by nonuniform formation loading notonly can in itself lead to unacceptable cross-sectional deformation, but it can also decrease the resistance of a cross section to more conventional loading by fluid pressure. Fortunately, and with particular regard to the early stages of rock-induced ovalization, this coordinated failure mechanism will only occur if the formation-induced ovality that precedes the pressure differential is of sufficient magnitude. Finite Element Model To analyze the phenomenon of nonuniform loading combined with differential pressure, a 2D, plane-strain model of a casing cross section was formulated using the ABAQUS2 general-purpose finite element program. Eight-nodeisoparametric continuum elements with reduced integration were used to modelthe casing cross section (see Fig. 2). Only one-quarter of a cross section was modeled owing to symmetry. Interface elements associate the external surface of the tube with a rigid surface or platen used to emulate formation loading. In each problem considered, the platen is displaced downward a specified distance to impart an initial, nonuniform load to the cross section. Asubsequent load step applies a uniform external fluid pressure until the maximum pressure load that can be carried by the cross section is reached. p. 156–163