Casing Failure Study - Cedar Creek Anticline
Authors: Clegg J.D. 
Published: December 2007

Eighteen years of experience in completion and remedial operations to deter casing failure on the Cedar Creek Anticline has proved that the chance of failure is low if a high-collapse-strength casing is used and if the casing opposite any salt section is cemented properly.

Introduction
Casing failures opposite salt formations are common in many fields throughout the world. Many papers have been written about this problem mostly concerning the completion aspect and salt flow loading. This paper covers more than 18 years of experience by Shell on the Cedar Creek Anticline in Montana and North Dakota. In this field a serious casing failure problem exists opposite the Spearfish and Opeche formations at depths of 5,000 to 6,000 ft. Not only is there a completion problem but there is also a producing problem in wells incorrectly completed. producing problem in wells incorrectly completed. To date, of 671 completed wells, some 37 have been lost, and in 31 of them the casing has deformed. (The area and fields involved are shown in Fig. 1.) Sufficient held data are available to draw conclusions about various completion and remedial techniques. As a result of this study, a casing protection program was started that has successfully stopped loss of wells.

History
The Cedar Creek Anticline discovery well, Pine 32-30, was completed in Dec., 1951, and the 7-in. casing failed in Aug., 1953. In the next 8 months two more wells failed opposite the salt sections. Various actions were taken to solve this problem. Until July, 1955, most wells were:
drilled with fresh-water mud, which permitted large washout in the salt sections; completed with casing that would withstand about 2,500 psi differential pressure in collapse at 5,500 ft; and cemented with indifference to cementing through the salt intervals.
In July, 1955, the following changes, similar to those recommended by Texter, were made in the drilling and completion program:
Salt-saturated mud was used to prevent large washouts in the salt sections. Casing of stronger collapse strength generally 5 1/2 in., 17 lb/ft., N-80 was run opposite the salt sections. Salt-saturated pozzolan cement was placed completely through the Spearfish and Opeche salt sections.
In the ensuing years the "July 1955" program was followed with good success. Fig. 2 shows the casing programs generally used. programs generally used. A program was begun in Jan., 1958, to protect the completed wells (drilled primarily before July, 1955) that were considered most likely to fail. Some of these wells were subsequently equipped with heavy tubing opposite the salt sections and with permanent completion equipment (PCE). Following two failures in wells equipped this way, the program was stopped. Of some 50 listed wells in which a hazard of casing failure existed, 8 were subsequently lost and 17 experienced casing deformation.
In March, 1960, the "July 1955" drilling and completion program was changed. It was decided that standard casing design practices could be used without increasing the risk of casing failure where a good cement job is anticipated.
JPT
P. 676