Steels for Oilwell Casing and Tubing - Past, Present and Future
Authors: Thomas P.D. 
Published: December 2007

THOMAS, P.D., ASIATIC PETROLEUM CORP., NEW YORK, N.Y.
As the result of the continuing trend to deeper wells and higher pressures, the oil industry has regularly required casing and tubing with higher physical properties and better surface quality, while retaining sufficient ductility to meet the increasingly severe service conditions. This has caused the mills to develop new grades of steel, improved heat treatments and equipment and, currently, new procedures utilizing more efficient methods for inspection of casing and tubing. Likewise, users have had to review and improve their field practices.
The API Div. of Production Committee for the Standardization of Oil Country Tubular Goods has been alert to these changing and tightening requirements and to future developments reflecting the still continuing trend to deeper wells and higher pressures, with its related increase in stringency of specifications for oil country tubular materials.

Introduction
In preparing this paper on steel for casing and tubing, there was a choice of points from which to view the problem-from that of the steel alone or, more broadly, from that of the casing and tubing as they go through the mill, arrive on site, go down the hole and perform their duties in keeping the hole open and the oil and gas coming up. This broader viewpoint-which includes not only the pipe itself, but also the threads, couplings and the made-up joints-was selected because there are metallurgical problems at every step of the way. Current developments with respect to specifications also form an integral part of this picture and are included in the discussions.
Back in 1930, Grade D casing and tubing were included in API Standard 5A and, with minimums of 55,000-psi yield and 95,000-psi tensile strengths, became the "high-strength grade" of that day. Manufacturers, operators and engineers marveled at how deep it would set; when the first well was drilled past the magic mark of 10,000 ft, a knowledgeable opinion of that day stated firmly that wells would not go much deeper because of the limitations imposed by the strength of drilling equipment and tubular products. However, this sage had overlooked the aggressiveness of the drillers and producers, and the willingness of pipe and tool manufacturers to assist them. As a result of the cooperation between these groups in mutually working out their operating, materials and equipment problems, wells now have been drilled as deep as 25,340 ft, and oil has been produced from 21,443 ft. These achievements illustrate the determination of the oil industry and its suppliers to progress with their work.The long-range trend since 1930 has been toward drilling deeper wells, both in terms of individual wells and in the annual average depth of wells drilled. N-80 casing and tubing were first included in API Standard 5A in 1939, and P-110 casing in 1956. In 1960, P-110 casing was taken from 5A and, along with P-105 tubing, was incorporated into the new API Tentative Standard 5AX; in turn, this tentative document promoted to full "standard" status on an action started in June, 1962, and completed in 1963.Also in June, 1962, at the API Div. of Production meeting in New Orleans, La., a group of Gulf Coast engineers served notice on both the tubular manufacturers and the users that in their future plans there would be need for tubing and casing with minimum yield strengths of about 140,000 to 150,000 psi. As with N-80 and P-105/ P-110 casing in the early stages of development, the pipe manufacturers are already well along in their development work on this super high-strength grade and have several strings of casing already in service.Tubing will present a more difficult problem, at least until quenching and tempering facilities are available for its production, because the maximum limits of physical properties obtainable by normalizing and tempering were just about reached with P-105. With each move to a higher grade there have been sighs and misgivings, but the parade continues.These progressively higher-strength steels for casing and tubing have provided the higher joint strengths in tension, collapse resistance and yield strengths under internal pressure needed to meet the requirements of the deeper wells and the accompanying higher pressures.

Stresses in Casing and Tubing
Steels in the API casing and tubing standards (API Standards 5A and 5AX) are graded on their minimum yield strengths, ultimate tensiles and enlongations.
JPT
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