ABSTRACT: Using Indentation (Axial Point-Load) Tests to Evaluate Strength Anisotropy and Stress Directions in Low-Permeability Gas Reservoir Sandstones

LAUBACH, S. E., S. J. CLIFT, and J. HOLDER, University of Texas at Austin, Austin, TX

Knowledge of azimuth of maximum horizontal stress (SH[max]) is a prerequisite for effective placement, completion, and stimulation of many wells in low-permeability reservoirs because open natural and hydraulically induced fractures tend to be aligned in this direction. Indentation (axial point load) tests of rock strength involve breaking samples with small, diametrically opposed anvils. Direction of sample fracture can reflect strength anisotropy, which may correspond to a principal direction of in situ stress if fracture growth is guided by residual stresses or preferentially oriented microfractures that exist owing to differential core expansion. Indentation tests on low-permeability sandstone core from Wyoming and Texas show marked strength anisotropy.

Comparing results of more than 700 indentation tests with other stress-direction indicators (acoustic velocity anisotropy, anelastic strain recovery, well-bore breakouts, and hydraulically induced fractures), we found that preferred sample breakage direction parallels maximum horizontal P-wave velocity and is approximately normal to azimuth of well-bore breakouts, consistent with indentation fracture growth parallel to in situ SHmax and strikes of hydraulically induced fractures. Samples we tested are macroscopically homogeneous. In these rocks, grain alignments do not control induced fracture alignment. Degree of indentation-fracture preferred orientation varies drastically with cement type and porosity. Strength anisotropy is pronounced in soft, clay-rich Frontier Formation sandston (Green River basin, Wyoming) and moderate to weak in hard, quartz-cemented Travis Peak Formation sandstone (East Texas basin). Point-load tests are a potentially useful supplement to expensive in situ measurements for inferring stress directions. With oriented core, tests could be done in the field, with simple, inexpensive equipment. Because we use centimeter-scale samples, we avoided sedimentary heterogeneities that may interfere with results. As with other core-based methods for inferring stress directions, results need to be compared with in situ measurements, such as breakout azimuth.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)