Special Issues in the Stress Analysis of Casing Strings in Steam-Injection Wells: Mathematical Development and Design

Abstract

The design and development of steam-injection fields is a mature subject. Because the tubulars in these wells invariably experience inelastic loading, issues such as the effects of temperature on the static and cyclic (fatigue) material properties become important. The original papers that address steam-injection casing-design issues date back to the 1960s. These studies include temperature prediction (Leutwyler and Bigelow 1965), casing-stress analysis (Willhite and Dietrich 1967), and development of design guidelines (Holliday 1969). Though these works acknowledge the role of temperature on the static and cyclic material properties, data on the cyclic thermal properties of oil-country tubular-goods steels rarely appear in oilfield literature (Placido et al. 1997; Maruyama et al. 1990). This paper presents a mathematical model of casing strings subjected to thermal loads in steam-injection wells. The model includes the effects of temperature on material properties and the effects of wellbore curvature and prestress during the heating cycle. Several counterintuitive aspects of the casing-stress state during cooling/unloading are examined. Further, the general equations are shown, with appropriate simplifications, to reproduce the earlier work (Willhite and Dietrich 1967; Holliday 1969). Example calculations are used throughout to illustrate key insights.