Abstract
Reliable prediction of fracture process in shale-gas rocks remains one of the most significant challenges for establishing sustained economic oil and gas production. This paper presents a modeling framework for simulation of crack propagation in heterogeneous shale rocks. The framework is on the basis of a variational approach, consistent with Griffith’s theory. The modeling framework is used to reproduce the fracture propagation process in shale rock samples under standard Brazilian disk test conditions. Data collected from the experiments are employed to determine the testing specimens’ tensile strength and fracture toughness. To incorporate the effects of shale formation heterogeneity in the simulation of crack paths, fracture properties of the specimens are defined as spatially random fields. A computational strategy on the basis of stochastic finite element theory is developed that allows to incorporate the effects of heterogeneity of shale rocks on the fracture evolution. A parametric study has been carried out to better understand how anisotropy and heterogeneity of the mechanical properties affect both direction of cracks and rock strength.
Highlights
Hydraulic fracturing, commonly known as fracking, is an important process in extracting gas from shale formations
This paper focuses on geomechanical characterization of shales and investigates the anisotropic mechanical behavior of shale specimens under Brazilian test conditions, as well
Mechanical characterization of geomaterials such as shale rocks is still an open challenge, especially as their real behavior is affected by intrinsic heterogeneities and natural laminations
Summary
Commonly known as fracking, is an important process in extracting gas from shale formations. The experimental observations showed that the direction of stress-induced cracks which are generally perpendicular to the maximum principal stress changes toward the bedding orientations These tests are crucial to understand the effect of material composition and local heterogeneities on the mechanical response of rocks, which, because of their natural genesis, significantly differ from one to another (Bobko and Ulm 2008; Veytskin et al 2017). A new computational modeling framework for simulation of crack propagation in heterogeneous layered materials is developed It combines random field and continuum damage theories, and Weibull distribution and its spatial autocorrelation length information are used for generating samples of stochastic fields representing the layering structure as well as local heterogeneity of the shales. The numerical data are discussed to better understand how anisotropy of the properties affects the morphology and direction of cracks
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