Today's analytical models can calculate stresses imposed upon the annular cement sheaths of wells, and it has become apparent that induced stresses can often be tensile in nature. These models often predict the cement failing in tension as well. This understanding has caused the industry to step beyond API tests that only recognize methods for testing cement under compressional loads. Instead, the determination of cement tensile mechanical parameters should be considered as important as cement compressive strength. Unfortunately, with no API guidelines, most oilfield cement tensile behavior testing typically uses ASTM construction concrete test methodology. With oilfield cements, most ASTM tests suffer from various shortcomings. These can be traced to the fact that they are designed primarily to test construction cements that are not usually placed more than a few meters underground. These tests do not incorporate procedures to replicate the curing of the cement in downhole environments, and do not conduct the actual static tensile testing under conditions similar to those occurring in oil and gas wells. When oilfield cements are prepared and cured using API procedure in HTHP curing chambers, they must still be subjected to significant induced stresses as cooling and de-pressurization back to ambient conditions occur before they can be subjected to ASTM tests. Samples prepared and tested in such a manner may undergo sufficiently induced stress to exhibit physical signs of initial mechanical failure, prior to even being placed in ASTM testing fixtures.
To produce cement tensile behavior data that is more reflective of actual downhole performance, the authors developed an automated, microprocessor controlled testing device that cures and mechanically tests the cement under simulated downhole conditions. Once slurry is placed in the testing device, and the temperature and pressure is ramped up to simulate downhole curing conditions, the samples never see ambient conditions again until the conclusion of testing. The microprocessor controls and automated data acquisition unit also allow for the determination of tensile stress-strain relationships, prior to testing the sample to ultimate (mechanical) failure in tension.
In this work, the authors present detailed descriptions of the operational capabilities of the new testing device, along with data developed with the device. Using the tensile data developed under more realistic downhole conditions, induced stress models can now generate even more accurate predictions about fit-for-purpose cement designs for wells of all depths and applications.