Preventing Instability in Partially-Cemented Intermediate Casing Strings
Authors: Dellinger Thomas B. , McLean J.C. 
Published: December 2007

This paper was prepared for the 48th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Las Vegas, Nev., Sept. 30-Oct. 3, 1973. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Casing instability is troublesome in partially cemented intermediate casing strings. partially cemented intermediate casing strings. This is especially true if drilling proceeds for several thousand feet into geopressured intervals with the attendant higher mud weights and circulating temperatures. Unstable casing will buckle and deflect laterally, producing doglegs within the pipe. Rotating and tripping drill string rapidly wears the casing wall in the dogleg area. Instability can occur above the cement top or in long, large-diameter, washed-out sections of hole within the cemented interval.
In 1970, Mobil Oil Corp. lost one 21,000-ft well without reaching the secondary objective, and in 1971, a 19,000-ft well was lost without reaching the primary objective. Both losses were attributed to holes worn by the drill string through the walls of the intermediate pipe. The wear of the casing in a third well pipe. The wear of the casing in a third well was monitored by successive casing-caliper surveys. The caliper surveys verified that casing wear progressed rapidly after conditions that would cause casing instability were reached. Calculations and analysis verified that instability occurred.
A computer program and an analysis technique have been developed to analyze conditions leading to casing instability. The program calculates and presents corrections to prevent instability for all expected future conditions. The corrections may be applied during the setting and cementing of the casing.
Since initiation of this analysis in 1971, Mobil has drilled 22 wells on the Texas Coast with long strings of intermediate casing and with corrective measures taken to prevent instability. No casing failures due to instability have been experienced on these 22 wells. On offshore wells with ocean-bottom suspension, special procedures are required.

Introduction
Temperature and pressure changes can cause unstable casing after the pipe is cemented and suspended. This concept is difficult to visualize and generally has not been considered in planning well programs. However, experience has shown that under certain conditions unstable suspended casing will move laterally (buckle) both above and below the cement top.
Casing instability, due to temperature and pressure changes, has been discussed for many pressure changes, has been discussed for many years. Oberg and Masters reported on physical stress-strain measurements on casing in 1947. Klinkenberg presented a good theoretical development in 1951.