A New Frontier for Oilwell Tubular Materials
Authors: Sewell Frederic D. 
Published: December 2007

SEWELL, FREDERIC D., HUMBLE OIL and REFINING CO., HOUSTON, TEX. JUNIOR MEMBER AIME

Introduction

The petroleum industry's developing need to expand its reserves by drilling 15,000- to 20,000-ft wells opens up a new frontier for oilwell tubular materials. These wells require tubular goods which far exceed in physical strength the tubular goods upon which the petroleum industry has depended for years.Drill pipe transmits power to the drilling bit and provides a conduit to direct the flow of drilling fluid to the bit for removal of formation cuttings. Here one may think of it as a hollow, rotating, vertical power transmission shaft, 15,000- to 20,000-ft long, under tension or compression and bending without the lateral support of roller bearings, and subject to more than 10,000-psi internal pressure.Casing supports the formations drilled through and seals the wellbore from these formations except in Ne completion interval. In these wells, think of the casing as a vertical pressure vessel designed for 10,000- to 15,000-psi internal burst pressures, something only slightly less than that in external collapse pressure, and tension loads up to 100 per cent of its own weight in air. Installed with threaded field connections, the vessel must, in addition to withstanding stresses imposed by loading, be leak-proof.Tubing transmits petroleum to the surface where the Christmas tree controls the flow. It also is an underground pressure vessel designed for the same internal pressure as the casing; it needs to be leak-proof and sealed off from communication with the casing at fixed ends. Nevertheless, it is subjected to added stresses generated by temperature fluctuations of as much as 300F.Thus, despite great care in design and handling, dangerous and costly tubular-goods failures still occur. A 12,000-ft, 365-ton string of protective casing recently designed for 8,000-psi service in a 20,000-ft wildcat well illustrates this situation.After one of the leading steel manufacturers produced the casing, several inspections attempted to cull out defects. Every joint of special high-pressure threads tested satisfactorily at 10,000 to 13,000 psi. Special practices prevented unloading damage, but during the unloading a visual inspection of the casing located some joints with overlooked mill defects and damage caused in shipment. Repair of the joints included recutting damaged threads, gauging the threads and pressure-testing. Casing running practice included the use of special tools and procedures to minimize notching and other damage, continuous observation by engineers on the job and setting carefully to design conditions.But only a few hours after installation, the casing string parted.This failure of high-strength material and numerous previous ones of various descriptions, plus the promise that well completion depths from 15,000 to 20,000 ft will become frequent, dare scientists and engineers to make many technological advances, not only to prevent similar failures to these depths, but also to permit future well completions at depths of at least 25,000 ft. The fantastically complicated problems presently being encountered in deep wells appear to involve engineering mechanics, mechanical engineering, metallurgy, chemical engineering, civil engineering, electrical engineering and petroleum engineering, and challenge engineering disciplines to unite in producing practical solutions to a myriad of problems. In the following discussion of engineering problems in deep wells, it is hoped that you will hear ringing in your ears the question "How would I solve that problem?".

Connections for Tubular Goods

The current method of running tubular goods in the hole requires screwing sections together using threaded connections such as those shown in Fig. 1.

JPT

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