We report on confined Brazilian and concurrent permeability testing on sandstone. The stress conditions include loading and unloading cycles to aid our understanding of the development of microcracks and the stress dependence of the resulting permeability under mixed stress conditions. The test involves 5 cm diameter disk-shaped, jacketed samples that are subjected to confining stresses while they are diametrically loaded. Specially designed end caps allow for gas flow measurements. The test configuration allows for the samples to be subjected to a range of stresses, with the confining stress as the intermediate principal stress. The permeability of the sandstone sample is shown to increase significantly well before the peak load is reached. The initial permeability increase above that of the intact sandstone occurs after tensile stresses develop at the center of sample that are sufficient to generate a connected microcrack network. During unloading prior to the peak load, the permeability decreases as the tensile stresses in the sample are reduced and eventually become compressive. With each loading-unloading cycle, the permeability at comparable stresses is increased compared to the previous cycle, consistent with increasing damage in the form of an expanding microcrack network. After the peak load is reached, the permeability remains at a relatively great value.
Understanding microcrack development and subsequent permeability changes in rock is important to many applications, from hydraulic fracking to design of nuclear waste storage facilities. When microcrack networks develop in rock, the resulting pore structure can greatly alter the permeability. Microcracks can be formed under compressive, tensile, or mixed stress states. The behavior of rocks under mixed stress state conditions is important and relatively poorly understood (Ramsey and Chester, 2004). We will use a confined Brazilian test configuration to induce microcracking under mixed stress conditions, and concurrently measure the resulting permeability.
The Brazilian test was proposed in 1943, and has become a standard method for measuring tensile properties of brittle rock (Gutierrez-Moizant et al., 2018). In a typical Brazilian test the intermediate principal stress remains zero. Jaeger and Hoskins (1966) proposed a confined Brazilian test where the sample is subjected to confining stress and consequently a non–zero intermediate principal stress. In this way, the state of stress in the sample can be significantly different than the conventional Brazilian test. In particular, this test allows for the role of the intermediate principal stress on microcracking and failure to be evaluated. Recently, a system for measuring permeability during confined Brazilian testing has been developed and used to evaluate microcrack development and permeability changes in concrete (Boyce, 2019). In this study, we report on confined Brazilian and concurrent permeability testing on sandstone. The stress conditions include loading and unloading cycles, to aid our understanding of the development of microcracks and the stress dependence of the resulting permeability.