Shale Poroelastic Behavior Determination by a Novel Tensile Strength Test

Abstract This research proposes to answer the question, what are the directional dependencies of the poroelastic properties of shale? To fully answer this question, the following tasks will be completed. First, determine shale properties: strength, elastic and permeability. Second, determine what effective stress(es), or range, exceeds the shale's tensile strength. Lastly, determine a theoretical model for fracture propagation in shale. A large-scale laboratory experiment has been designed to subject shale to pore over-pressurization and induce tensile fractures bringing the sample to failure. It is anticipated that the pressure differential, elevated pore pressure in a low permeability rock sample, and concomitant bleeding off of confining pressure will induce tensile fractures affected by poroelastic properties of the rock. The results from this experiment will be used to calibrate a forward finite element numerical model, which can then simulate the tensile fracturing process and answer the above question. With rare exception, tensile strength is determined using the indirect method, a method that is industry standard, but by definition cannot describe uniform internal failure as it is intended to fail a sample diametrically. This project will use laboratory experiments to simulate the physical processes of initiating a tensile fracture with excess pore pressure. A numerical model will be constructed that simulate the laboratory experiments to illustrate the coupling of pore pressure, matrix stress and fracture initiation. The laboratory experiment is designed to provide data to with which to calibrate the model and validate the model's output. This model can then be used on the one hand to better design completions programs for hydraulic fracturing and, on the other hand, how to design safer storage caverns in shales. This research will make use of the coupling between a shale's matrix properties and the fluid properties contained within the pore space in order to evaluate the shale's usefulness for various applications: 1) completions engineers in the petroleum industry who endeavor to hydraulically fracture shales, and 2) those who do not seek to fracture shales, such as engineers designing shale caverns to store natural gas or carbon dioxide.