A new compact triaxial perforation tunnel stability tester
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Thick wall cylinder testers (TWC testers) are commonly used for evaluating perforation stability. However, the results are ambiguous since the standard TWC testers use cylindrical cores where only uniform confining pressure can be applied, while the real in-situ stress is directional around perforations. A new triaxial perforation tunnel stability tester (TPS tester) is developed to simulate perforation stability under more realistic conditions. Normally, it requires a relatively large pressure cell and three independent pressure pumps to create a triaxial stress state in a cubic sample. However, the equipment developed in this paper is as simple and as small as the standard TWC testers with the following configurations: (A) Sample is a cubic rock with a 0.5 inch hole as the standard TWC test. The dimension of the cubic sample is 1.5 in. x 1.5 in. and the thickness is adjusted by core availability (normally varies from 0.75 in. to 1.5 in.). (B) The stress ratio of the horizontal to vertical stresses in perpendicular to the hole axis can be varied between 0.33 to 1.0. The pressurization system requires only one pump by increasing the two confining stresses proportionally. A square jacket has a device to induce the directional loading. (C) The pressure cell is as small as the standard TWC tester and the test procedure is also as simple as the standard TWC test. Using the above device, various rock samples are tested varying the ratio of horizontal and vertical stresses. The results show that unlike the standard TWC tests, the perforation stability tests with the same in-situ stress ratio as the field one do 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 standard TWC tests since pre-mature failures are commonly observed for hydrostatic loading. The equipment is handy and portable so that it can be used in a small field laboratory. Copyright 2011, Society of Petroleum Engineers.
