Comparative studies of compact triaxial perforation stability tests and thick wall cylinder tests
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Copyright 2014, Society of Petroleum Engineers. Comparative studies of perforation stability with a standard TWC tester (Thick Wall Cylinder tester) and a new compact TPS tester (Triaxial Perforation Stability tester) were performed using sandstone cores with UCS (Unconfined Compressive Rock Strength) ranging from 500 to 7000 psi. This paper aims two objectives: one for advocating the merits of a new technology and the other for improving the interpretation of the conventional TWC test results. TWC testers are commonly used for evaluating perforation stability to predict the onset of sand production (Geertsma, J. 1985). However, most users are confused how to interpret the results since the TWC testers use cylindrical cores where only uniform confining pressure can be applied, while the real in-situ stress is directional around perforations. The core-cut directions are parallel or perpendicular to the whole core axis and are different between companies. The new triaxial perforation tunnel stability tester uses conditions similar to the real perforations. Note that the TPS tester uses cubic rocks with a 0.5 inch hole and the ratio of the horizontal to vertical stresses in perpendicular to the hole axis can be varied between 0.3 to 1.0. Using these two devices, various rock samples were tested varying the ratio of horizontal and vertical stresses. The results showed that unlike the standard TWC tests, the perforation stability tests with the same in-situ stress ratio as the field one did not require corrections of directional in-situ stress effect. Note that depending on rock non-linearity, complex correction factors from hydrostatic confining stress to directional stress is required for the TWC tests since pre-mature failures are commonly observed for hydrostatic loading. A guideline of interpretation of the TWC results is given based on the comparison of TWC and TPS tests for the correction factor of the TWC tests if the results are applied for directional stresses. Using the new compact perforation stability tester is also advocated since the new tester is as compact as the standard TWC tester and the interpretation of the results is simple and accurate since the directional stress is applied with the same stress ratio as the field stress condition.
