Casing is cemented into the wellbore to provide zonal isolation of producing zones, protect fresh water zones from contamination, prevent casing collapse caused by moving salts or sloughing clays and isolate the casing from corrosive brines. In other words, casing is cemented to create annular isolation. Historically, the major physical property of the cement used to determine whether these results would be attained for the life of the well, was the unconfined compressive strength of the set cement.
Recent studies have shown that there are other "strengths or mechanical properties" that are even more important to consider. These strengths include tensile and flexural.
This paper discusses why tensile and flexural strengths must be considered in slurry design, along with the interrelationship of compressive, tensile and flexural stresses that occur in wells, and provides Appalachian Basin field examples, where reduced, unconfined compressive strength cement, with enhanced tensile and flexural strengths has been successfully used.
The main purpose of any primary cementing job is to provide zonal isolation and hold the casing in place. There are many factors that influence the cement's ability to achieve these objectives. Hole conditioning, flow regime and pipe reciprocation are a few of the mechanical techniques which can be employed. These and others have been investigated and reported in various studies1–4.
Physical and chemical properties of the unset cement slurry also have a role in the success of the primary job. Christian, et al5 and Beirute, et al6 showed how cement dehydration and its control effect the primary cementing. Sutton, et al7 demonstrated the relationship between gel-strength transition time and gas migration. The American Petroleum Institute (API) has established standards for cement slurry properties8.
Historically, the only physical property of set cement that was tested was the unconfined compressive strength. Originally, it was felt that the set cement required a compressive strength at least equal to that of the producing formation9. In 1957 Craft10 published a study of west Texas and east New Mexico producing formations. He found that compressive strengths ranged from 8,215 psi to 22,500 psi. However, most set cements will only exhibit an ultimate compressive strength in the range of 5,000 to 9,000 psi. Since primary cementing jobs had been reasonably successful, the comparative compressive strength theory was dispelled. However, it was still generally felt that more was better.
It had also been demonstrated that the tensile strength of the setting or set cement is of primary importance. Tensile strength relates directly to the ability of the cement sheath to hold tubulars in place. Farris11 showed that as little as 8 psi tensile strength is adequate to accomplish this requirement. Since the ratio of compressive strength to tensile strength in most cements ranges from 8 to 12, a compressive strength in the range of 100 psi is all that is needed to hold many casing strings in place.