Resistance of Cemented Concentric Casing Strings Under Nonuniform Loading

Abstract

Summary This paper discusses the case of two opposing unidirectional collapse loads uniformly distributed along the diameter of a single casing string and two cemented concentric casing strings. The stress distribution in the casing and cement and the deformation of the pipe are determined by adapting known solutions in the mathematical theory of elasticity to the problem of single- and dual-casing strings subjected to this type of loading. Results were obtained for the case of uniform hydrostatic radial collapse loads for the two casing sizes (9% and 7 in.) and their cemented structures. The resistance of the single pipe to this type of nonuniform loading may drop to 11 % of its design resistance to uniform hydrostatic collapse pressure. The resistance of the cemented concentric strings under pressure. The resistance of the cemented concentric strings under uniform and nonuniform loads is about the sum of the steel pipe resistances under the same conditions. The paper also includes the superposition of uniform and nonuniform collapse loads and the practical implications of these results. We conclude that nonuniform loading should be considered in casing design, especially opposite salt formations. Introduction Nonuniform loading is the major cause of casing collapse in most oil wells penetrating viscoplastic evaporite formations, Nonuniform loading can occur without regional movement of the evaporites. This type of loading can exist in a borehole with an irregular shape that resulted from the drilling fluid washing out the salt. An adequate fill of the annulus with cement may be difficult to obtain because of this washout, and bending of the casing may occur as a result of salt creep. Nonuniform loading is divided into three categories :a uniform unidirectional load applied on the outer surface of the casing superimposed to a possible hydrostatic pressure on the inner and/or outer surfaces;opposing line loads on the outer surface with hydrostatic pressure on the inner and/or outer surfaces; andopposing outside line loads on opposite ends of a diameter combined with complete restriction to radial deformation perpendicular to the line of loading in the presence of hydrostatic pressure on the inner and/or outer surfaces. These three loading conditions are special cases; more complex combinations may result from the superposition of two or more load types. The first condition could be considered a predominant one in most oil and gas wells penetrating salt formations. The second and third conditions may occur at the beginning of salt flow. Bending could result from the second and third conditions. API casing specifications were set for the effect of uniform hydro-static loading. The effect of nonuniform loading on tubular goods has scarcely been studied or specified. This paper highlights the problem of casing collapse under nonuniform loading and examines problem of casing collapse under nonuniform loading and examines the effect of nonuniformity of the load on API tubular goods with a mathematical model to estimate the resistances and stresses on those pipes. Moreover, this paper compares the resistance and radial deformation of the single pipe and the dual strings under uniform and nonuniform conditions. Also, it examines the case of combined loading (simultaneous action of uniform and nonuniform loads) on the resistance and radial deformation of single and dual strings. Mathematical Analysis Lame's formulas for thick-wall pipe are used to calculate the stresses in a single- and dual-casing string under uniform/nonuniform combined loading. The results of the dual string under nonuniform loading are related to those of uniform loads. For dual strings, the casings are always considered concentric.