Comparative Study of the Mechanical and Fracturing Behaviors of Shale from Different Gas Blocks in the Longmaxi Formation.

With the development of shale gas exploration and exploitation, the mechanical and failure characteristics of shale have become one of the focuses. However, few studies have considered the differences in the mechanical properties of shales in different shale gas blocks. In this study, the effects of bedding orientation on the stress-strain curves, elastic modulus, Poisson's ratio, and fracture morphology of shale samples from the Zhaotong block are analyzed by laboratory experiments, first. Then, a standard deviation method is used to characterize the anisotropy degree of the mechanical and fracturing characteristics of shale. The anisotropy ratio of shale samples between the Zhaotong and Chongqing areas has been comparatively studied. Comparison results show that the shale anisotropy induced by the bedding orientations demonstrates apparent regional characteristics. There are significant differences in the mechanical properties of shale obtained from different shale gas blocks. It indicates that the optimization of engineering practices such as the design of hydraulic fracturing should consider the influences of the bedding orientations and shale gas blocks on the shale anisotropy.

Effect of Clay Minerals on Tensile Failure Characteristics of Shale.

The effects of mass fraction, elastic modulus, and the spatial distribution of clay minerals on the failure behaviors of shale under the Brazilian disc split are investigated in this study based on a developed PFC2D model. The feasibility of this established numerical model is validated by the load-displacement curves and fracture morphology of shale in the laboratory test. The influences of the mass fraction, elastic modulus, and spatial distribution of clay minerals on the tensile strength and failure behaviors of shale are comprehensively analyzed using the developed numerical model. The results show that the clay content has the strongest influence on the tensile strength of shale. The number of matrix cracks is sensitive to the clay distribution, and the complexity of the matrix fracture network in shale shows a positive correlation with the clay content. In addition, the variation of the clay modulus has a linear correlation with the macro elastic modulus of shale.

An Estimation Approach for the Effective Elastic Modulus of Lightweight Bulk Filling Material with Compressible Inclusions and Imperfect Interfaces.

In this study, an approach is developed to estimate the density and effective elastic modulus of a lightweight bulk filling material made up of expanded polystyrene (EPS) and cement-reinforced clay (matrix). First, a representative volume element (RVE) is composed of cell A (an EPS and matrix) and cell B (matrix only). Then, an elastic interface is introduced to describe the discontinuity of displacement at the interface between EPS beads and matrix. Third, an Eshelby compliance tensor is modified in cell A to include the effects of imperfect interface and the compressibility of EPS beads. Finally, the approach for the density and effective elastic modulus of the EPS beads mixed cement-reinforced clay is verified with experimental data. The compressibility ratio of lightweight clay is compared under different confining pressures and curing times. It is found that the imperfect interface has salient impacts on the effective elastic modulus with the increase of volume fraction of inclusions. The interface parameters (α and ß) vary with curing time and confining pressure. At the same curing time, the parameter α is almost constant regardless of confining pressure but the parameter ß changes with confining pressure. The compressibility ratio is smaller for longer curing time if the confining pressure is constant.