Casing and Tubing Design Considerations for Deep Sour-Gas Wells

Abstract

Summary Tubular metallurgy is usually considered to be the dominant design requirement associated with deep sour-gas wells and as such has received extensive treatment in the industry's literature. Other requirements vital to the assurance of a successful well design, however, have not been treated adequately in the literature. This paper addresses some of the nonmetallurgical requirements incorporated in our drilling operations in the deep, sour, geopressured areas south of Jackson, MS, and in the Tuscaloosa trend in Louisiana. The following topics are included:the need and use of the maximum distortion strain energy - von Mises-technique for designing production tubulars;a testing technique used to evaluate connections for use with these tubulars;unusual design and operational features of these wells; andthe inspection and handling requirements to ensure the integrity of the wellbore tubular Introduction The design of the wellbore tubulars for deep sour-gas service is an application of quality assurance. First, materials that are resistive to sulfide stress-cracking are selected. Operating procedures and limits are specified for the design that might include as a design parameter the maximum anticipated bullheading pressure of the well or the need for continuous corrosion-inhibitor circulation. Then the size, weight, and connection for each wellbore tubular string are selected so that the stress in these tubulars is limited to some value below the minimum yield strength of the material, thus giving added assurance of resistance to sulfide stress-cracking. Finally, special inspection and handling procedures to ensure the tubular's physical integrity at the time of running are specified. If the design fails to address any one of these items adequately, a catastrophic failure of the tubulars and blowout of the well may result. The first of these design items-the material's metallurgical requirement-has received extensive discussion in the literature, but discussion of the other three has been much more limited. To provide additional information on these items, we summarize those less-conventional design aspects associated with our exploration activities in the deep, sour, geopressured area south of Jackson MS, and in the Tuscaloosa trend. The first design task is the selection of the wall thickness, weight, and connection for a tubular string, assuming the size has already been selected. As many engineers with experience in designing deep, sour-gas service wells already know, the reality of the situation is that the tube and connection are selected separately-i.e., the tube's size and wall thickness are selected so that the stress is limited to some specified value and then the connection is selected to satisfy the same or possibly some other stress requirement. This separation is necessary as many connections do not limit the maximum stress to below the minimum yield strength of the material. And for very-high-pressure wells, the needed connection for the selected tubular may not exist and has to be custom-designed by one of the thread manufacturers. For this reason, tube selection and connection selection will be treated separately. The next topics are some of the unique operational limits and procedures required to control the axial load on the long casing and tubing strings typical of deep sour-gas wells. And finally, the special inspection and handling procedures we use to ensure physical quality of the procedures we use to ensure physical quality of the tubulars are described.