Analysis of Tensile Strength Test Methodologies For Evaluating Oil and Gas Well Cement Systems

Abstract

Efforts to produce better annular isolation for oil and gas wells resulted in the development of methods to quantify the induced stresses that may occur within the cement sheath of a well. Along with the ability to calculate induced stresses in a cemented annulus, is the realization that typically, if there are sufficiently induced stresses to cause a mechanical failure of the set cement, failure will likely be of a tensile nature. While the ability to predict the compressive and the tensile stresses likely to be induced in a cemented annulus is certainly a step forward for the ability to provide better fit for purpose cementing designs, it also tends to highlight a larger challenge. Specifically, even though the American Petroleum Institute (API) and the International Organization for Standardization (ISO) established recommended methods for the testing of oil and gas well cement compressive strengths, no similar standards currently exist for the testing of the tensile strength of oil and gas well cements. Without the ability to test an oil or gas well cement tensile strength in an accurate and repeatable manner, it becomes very difficult for design engineers to utilize induced stress data to determine if a given cement system possesses sufficient tensile strength to resist the induced tensile stresses. This lack of standardized tensile strength testing has led the oil and gas industry to adopt various tensile strength test methods that were originally developed for the construction concrete industry. The authors have used many of these different tensile strength test methods and devices in their own work. They discovered that very often, a cement system can yield widely different tensile strengths when tested with different procedures. In this paper, the authors review the basic construction concrete tensile tests most commonly used in the oil and gas cementing industry, and then analyze the tensile strength results obtained with the different testing methodologies. Correlations are developed between the tests that allow design engineers to better compare cement systems when the subject slurries have been tested with different tensile strength test methods. Introduction Given no current API test procedure for the determination of cement tensile strength, the authors, like most others involved in the pursuit of cements with enhanced mechanical properties, relied on test methods outlined by the American Society for Testing and Materials (ASTM), which has already developed tensile strength tests for construction concrete. In the course of evaluating various cement compositions and additives for the enhancement of ultimate tensile strengths, the authors had the opportunity to view tensile strength test data from a third - party lab on a particular organic cement additive that was similar to a product tested in their own laboratory. The representative of the company that produced the organic additive was very excited with the data obtained from the third-party lab, because it showed significantly higher tensile strengths than for similar materials tested by the authors. At first the authors were somewhat confused by the tensile strength data from the third-party lab, because it tended to show much higher tensile strengths than their own test results. Even though the physical properties of the two materials were slightly different, and perhaps could have accounted for the differing results, the authors decided to look further into the tensile strength test methodology used in the two laboratories, in an effort to determine if anything obvious could account for the widely varying test results.

Effects of Long-Term Exposure to Ultrahigh Temperature on the Mechanical Parameters of Cement

“…Tensile failure occurs in lieu of failure in compression because the load is applied in an area of the specimen that is in a state of triaxial compression, allowing it to survive higher compressive stresses than would be achievable with a uniaxial compressive test. The indirect tensile strength is then calculated as follows: 13 have shown that, depending upon specimen size and aspect ratio, the STS method tends to give consistently higher tensile strength results than those measured with the DTS method. Since the cement sheath in the wellbore is exposed to a combination of compressive and tensile stresses it was decided for the purpose of this study the use of the STS results for modeling cement sheath failure would be reasonable.…”

Effect of using Austrian pine cones powder as an additive on oil well cement properties

“…The tensile strength and flexural strength tests of oil well cement is not yet standardized at atmospheric or downhole conditions. For this reason, it was carried out following the standards designated for concrete testing in the construction industry [ 34 ]. Firstly, the inner faces of the cement molds were cleaned and should be lubricated with a non-reactive releasing agent to make samples removal after curing easier.…”

“…For each tested age, the average test results on three samples from each concentration shall constitute the test result. After measuring the maximum axial load required to crush the cement sample, Equation (10) can be applied to calculate unconfined compressive strength, Equation (11) can be used to calculate the tensile strength [ 31 , 34 , 35 ] and Equation (12) can be used to calculate the flexural strength. where, is the unconfined compressive strength (MPa), Fmax is the maximum axial load (N), and D is the cement sample diameter (mm).…”

Characterizing Casing-Cement-Formation Interactions Under Stress Conditions: Impact on Long-Term Zonal Isolation

“…al. 12 reported the results of splitting tensile strength testing (ASTM Standard C 496-90) using various aspect ratios of samples and results from a direct unixial tensile strength testing method (ASTM Standard C-190-85). The work concluded that, dependent on the respective aspect ratios of the samples, the splitting tensile test results were on average 1.5 to 2.5 times higher than those generated using the direct unixial tensile strength method.…”