J.E. Griffith* and S. Osisanya**
This paper describes recently observed problems of cement sheath deposition on the inside circumference of a casing string. Actual job parameters and several proposed causes of the cement sheath are reviewed. The probable hypothesis for the cement sheath deposition is a combination of temperature, cement retardation, large cement volumes, and the surfactant contained in the slurry. This hypothesis was explored through the use of large-scale, physical models to duplicate the actual well problem in the laboratory.
Based on test results, the principal cause of cement sheath development is surfactant in the cement slurry. Once the surfactant is removed or retarding agent is added to the slurry, the cement slurry can be successfully placed. This paper also includes a laboratory procedure that provides for timely screening of potential cement slurries to determine sheathing tendencies.
Portland cement is widely used to help ensure zonal isolation of economic formations and to extend the life of the casing string. Since the beginning of oilwell cementing, the industry has encountered and solved many problems associated with cementing hostile wells. Cementing conditions such as gas migration, high temperatures, high pressures, and deep-water cementing continue to hold challenges for the industry to solve. However, a number of cement jobs have been successfully executed in these hostile environments.
One area of oilwell cementing not completely understood is the deposition of cement sheaths inside the casing. Operators know that cased-hole tools such as packers and bridge plugs sometimes fail to operate properly, because pieces of cement have been found in and around the downhole tools after the job. For one operator alone, this problem resulted in remedial repairs costing several million dollars.
Cement sheath inside the casing was detected on five 9 5/8-in. intermediate casing strings where the top 9 5/8-in. plug was not displaced to the casing shoe. The location where the plug stopped varied widely, as did the amount of pressure needed to displace the plug to its final location. Whenever attempts were made to drill ahead, excessive bit-drag was encountered above and below the plug.