Effects of Metal Thickness and Temperature On Casing and Tubing Design For Deep, Sour Wells
Authors: Greer J. Brison 
Published: December 2007

Studies show that increasing temperature alleviates the effects of sulfide stress cracking in high-strength steel. This means that higher strength steels can be used in the hotter portion of the hole as a means of lowering stresses in the more critical, cooler portion.

Introduction

The production of deep, sour wells places severe demands on tubular goods. Below about 15,000 ft it becomes necessary either to use strength grades higher than C-75 or to employ special equipment such as tapered strings or down-hole hangers because of stresses generated by the hanging weight of the steel. In addition, it is not uncommon to have a 20,000-psi BHP in a 20,000-ft well and this creates a high burst requirement. As a result, special steels manufactured by the quench-and-temper process have been developed' 2 for use in these wells. This paper is concerned with the effects of temperature, paper is concerned with the effects of temperature, wall thickness, and defect size on the sulfide stress cracking (SSC) performance of commercial steel used for casing and tubing and how these factors affect manufacturing, design, and completion practices. There has been considerable interest lately in the effect of high temperature in reducing the SSC potential. Studies involving hydrogen embrittlement from potential. Studies involving hydrogen embrittlement from cathodic charging show that embrittlement is most severe at about room temperature or slightly below and becomes less severe for both lower and higher temperatures; a study of embrittlement due to hydrochloric acid revealed the same general temperature dependence These results are helpful in that a recent study involving embrittlement from hydrogen sulfide suggested that the temperature effect might be due to a change in corrosion products and should not be counted on for field application. Although the corrosion reaction is an important factor governing the embrittlement potential, once the hydrogen is in the steel its effect seems to be controlled to a large extent by the temperature, probably through a diffusion mechanism; therefore, temperature could be used as a design parameter. Practically all stress corrosion experiments measure stress as a variable; however, this may be one of the most poorly understood parameters used. In recent years, testing concepts have changed dramatically through the use of fracture mechanics. In fracture mechanics, materials are assumed to have flaws due to inadequacies in manufacturing and inspection, and a relationship is obtained between permissible stress, flaw size, and a property that is characteristic of the material. The state of stress can greatly affect the load to failure of a flawed material. This does not normally pose a problem for steels of interest in oil production pose a problem for steels of interest in oil production because the steel strength is relatively low, ductility is high, and walls are not thick enough to produce other than plane stress conditions at fracture. Even so, ductility can be decreased considerably by changing the stress from uniaxial to biaxial. Where embrittlement is involved, however, the state of stress may approach plane strain and become important in the lower-strength steels used in oil production. The advent of deeper wells and the use of stronger steels having thicker walls would also aggravate the state-of-stress problem.

JPT

P. 499