A research team from the University of Wisconsin-Madison and Northwestern University is combining experimental observations and shear band analysis in order to investigate the failure properties of high porosity rock. Shear band analysis is a mathematical description of the observed behavior of materials such as rocks by which their failure under applied stresses is concentrated in a narrow band rather than being uniformly distributed. True triaxial tests, in which test samples are subjected to three independent and typically unequal principal stresses simulating realistic field conditions, are being conducted on high-porosity compactive sandstones. The tests employ novel loading paths for each of the principal stresses. These paths, unprecedented in laboratory experiments, are designed to facilitate formulation of constitutive relations and comparison with the theoretical predictions of shear band analysis. The results of this research will provide the first systematic test of the applicability of shear band theory for the full range of stress conditions.
A better understanding of rock mechanical behavior and its relation to failure of compactive porous sandstones is essential for interpreting field observations and as input for numerical simulations of complex geomechanical and geophysical problems. These include earthquake mechanics, design of underground storage facilities, energy recovery and sequestration of carbon dioxide. Failure in these materials tends to occur in narrow bands and therefore can drastically alter the ability of the rock formation to trap or transmit fluids. These narrow zones of failure may reduce flow across them and interfere with fluid circulation in applications involving fluid injection (e.g. for carbon dioxide sequestration or liquid waste isolation) and fluid extraction (e.g. for oil field operations). These failure zones can also provide conduits for enhanced flow along them. Such flow can disrupt rock formations intended to trap fluids and prevent them from reaching the surface or interacting with shallow groundwater.
We conducted laboratory experiments on two porous sandstones, which are representative of formations associated with oil reservoirs, or are being considered as potential sites for carbon sequestration. The experiments consisted of subjecting these rocks to forces simulating realistic conditions in the earth’s crust. The outcomes of these experiments show that previous research in which the conditions simulated only special cases of natural conditions resulted in underestimating the strength of these rocks (failure of which would be detrimental to maximizing oil or gas production or to carbon dioxide storage). Our experiments also showed that failure in these rocks occurs by the development of a planar fracture, or crack, which occurs at azimuth and inclination, depending on the acting forces and their directions. Moreover, an existing failure theory was found to reasonably predict the failure strength and failure-plane attitude, provided sufficient information about the sandstone properties and prevailing forces is available. This research has trained one PhD student in rock mechanics laboratory testing using the ‘state of the art’ equipment and testing procedure. His thesis resulting from this research granted him a PhD degree, and he was immediately hired by Stanford University as a post-doctorate student. The project also provided basic training to several undergraduate students in experimental rock mechanics, and should serve them well in their careers. Key outcomes from this project were disseminated to the geoscience and rock mechanics communities through presentations at the American Geophysical Union (AGU) annual meetings in 2011, 2012, 2013, and the forthcoming 2014, The European Geosciences Union (EGU) annual assemblies in 2012, 2013, and 2014, the American Rock Mechanics/Geomechanics annual meetings in 2012, 2013, and 2014, as well as "Sino Rock 2013". Journal papers to be submitted to Journal of Geophysical Research and the International Journal of Rock Mechanics and Mining Sciences are in preparation. We conducted laboratory experiments on two porous sandstones, which are representative of formations associated with oil reservoirs, or are being considered as potential sites for carbon sequestration. The experiments consisted of subjecting these rocks to forces simulating realistic conditions in the earth’s crust. We conducted laboratory experiments on two porous sandstones, which are representative of formations associated with oil reservoirs, or are being considered as potential sites for carbon sequestration. The experiments consisted of subjecting these rocks to forces simulating realistic conditions in the earth’s crust.
