Abstract

Collapse phenomenon behavior has been broadly studied in the petroleum industry but few changes have been implemented in this subject. Since the early 1960's when the American Petroleum Institute (API) published the Recommended Practice 5C3 for casing design equations under burst, collapse and tension, these equations have remained unchanged until the present.

These equations ignore the effects of the cement sheath on collapse resistance and assume uniform collapse loading of the casing. Incorporating the effects of a cemented wellbore improves collapse casing design. The study presented in this paper describes the effects on the collapse loading conditions of the mechanical properties of cement and rock formation surrounding the casing strings. In order to investigate these effects, the Finite Element Analysis technique was used. Three separate cases are studied: unsupported casing; casing with a cement sheath bonded outside of it; and casing, cement sheath, and consideration of the surrounded rock formation. For all of them, the same constant value of stress was applied at the outer boundary of each model. According to the results yielded by this study, improvements of up to 62% in casing collapse resistance can be achieved, depending on the mechanical properties of the cement systems and rock formation properties.

Introduction

Casing strings serve one of the most important functions in a well. Casing isolates the wellbore fluids from the subsurface fluids and formation fluids. Casing also provides a high-strength flow conduit to the surface, permits safe control of formation pressure, and prevents collapse of the borehole during drilling.

Traditionally, casing string design has been accomplished in accordance with the string type and function. Generally, there are four types of casing strings: conductor, surface, intermediate drilling, and production casing. For each one, a combination of loads is applied to the pipe body in order to select the best tubular characteristic that agrees with burst, collapse, tension/compression, and triaxial resistance. The traditional procedure takes into account the worst isolated load for burst, collapse and tension. A relatively new methodology, presented by Klementich and Jellison in 1983 (1), treats each drilling, completion and production event as a load case, and depending on the situation and severity, could be considered as burst, collapse, or a combination of both, with the presence of tension or compression. This last methodology is known as Service Life Model.

Studies have shown that cemented pipe under burst loads have better performance than uncemented casing. Fleckenstein, et al. (2) demonstrated in their studies that a casing string constrained with a hard cement sheath supports 58.4% less triaxial stress than an uncemented pipe for the 5–1/2", 23# casing studied.

The collapse of steel pipe from external pressure is a much more complex phenomenon than pipe burst from internal pressure. A simplified free-body diagram analysis does not lead to useful results; however, a more complex, classical elasticity could be used to establish the radial stress and tangential hoop stress in the pipe wall. For both collapse and burst conditions, the resultant induced tangential stress will be the maximum (Bourgoyne et al. 1991) (3).

The main objective of this paper is to model several collapse loading conditions for different cement and rock formation mechanical properties in order to simulate the stress transmissibility through different media in the wellbore. This will allow an estimate of the resultant induced stresses in the casing under a variety of collapse loading conditions.

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