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Strength Anisotropy in Low-Permeability Sandstone Gas Reservoir Rocks: Application of the Axial Point-Load Test
S. J. Clift (1), S. E. Laubach (1), J. Holder (2)
Axial point-load tests involve breaking specimen disks of rock with small, diametrically opposed anvils that approximate point loads. Direction of sample fracture can reflect strength anisotropy, which in otherwise homogeneous samples may correspond to the strike of natural microfractures or microfractures that exist owing to processes that reflect in situ stress directions, such as differential core expansion. In low-permeability sandstone reservoir rocks, point-load tests are potentially useful supplements to core-based studies of macrofractures and measurements of stress directions. Point-load tests on low-permeability sandstone core show marked strength anisotropy in soft Frontier Formation sandstone (Green River Basin, Wyoming) and moderate to weak anisotropy in hard Travis Peak Formation sandstone (East Texas Basin). Comparing results of 328 point-load tests with other stress-direction indicators (acoustic velocity anisotropy, anelastic strain recovery, wellbore breakouts, and hydraulically induced fractures), agreement was found between induced fracture strike and inferred maximum horizontal stress direction. The strike of induced fractures is also aligned with natural fracture directions in these rocks, however, and petrographic analysis is necessary in order to use point-load tests to infer stress directions.
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