Zetian Zhang

The relationships among stress, effective porosity and permeability of coal considering the distribution of natural fractures: theoretical and experimental analyses

Although the relationships among stress, effective porosity and permeability of coal are a fundamental research topic that has been studied for decades and are widely used in analyzing the mechanical behavior of coal seams and predicting coalbed methane production, most relevant studies are based on idealized models and do not consider the influence of natural fracture distributions. To obtain a comprehensive understanding of the interrelationships among stress, effective porosity and permeability of coal, a series of effective porosity and permeability determinations have been conducted under different overburden stresses using an automated permeameter–porosimeter considering the directional distribution of natural fractures in coal. The experimental results show that the directional distribution of natural fractures provides a substantial contribution to the anisotropy of the effective porosity and absolute permeability of coal, which exponentially decrease with increasing overburden stress. An existing permeability model was modified to reflect the influence of the natural fracture distribution on the power law relationship between effective porosity and permeability, i.e., the exponent is not constant, but a variable related to the natural fracture distribution. The anisotropic effective porosity sensitivity and stress sensitivity of coal are also discussed, and the coal mass is shown to have the highest effective porosity sensitivity and lowest stress sensitivity in the direction perpendicular to the bedding planes compared to those in other directions.

Fractal and volume characteristics of 3D mining-induced fractures under typical mining layouts

Mining-induced fractures, which can release natural gas and enhance gas migration, are indispensable in the co-extraction of coal and methane (CECM). Investigations into mining-induced fracture systems ahead of the coalface are beneficial in elucidating the complex differences between mining processes and their related static and dynamic strata stress fields. Triaxial tests were conducted on coal samples to simulate the mining-induced mechanical behaviors of the coal seam under three widely used mining processes, i.e., top-coal caving mining (TCCM), non-pillar mining (NPM) and protected coal seam mining (PCSM). Mining-induced fractures were created at the laboratory scale. An industrial computed tomography scanning system was employed to scan the ruptured coal samples and measure the geometric characteristics of the fractures associated with each simulated process. Coronal and sagittal views as well as a precise 3D solid geometrical model were reconstructed for each process. It is shown that the mining layout influences the spatial morphology of mining-induced fractures. Using the box-counting method, a quantitative fractal characterization of the fracture system was estimated for each mining method. The estimated average fractal dimensions of the fracture systems generated in the simulated TCCM, NPM, and PCSM processes were 2.0557, 2.0362 and 2.0129, respectively. Additionally, a scale-independent fracture intensity, FI, was defined to further characterize the volume features of the mining-induced fracture systems. The fracture intensity was 0.6 for the NPM process, 0.5 for TCCM and 0.21 for PCSM.

The Effect of Bedding Structure on Mechanical Property of Coal

The mechanical property of coal, influencing mining activity considerably, is significantly determined by the natural fracture distributed within coal mass. In order to study the effecting mechanism of bedding structure on mechanical property of coal, a series of uniaxial compression tests and mesoscopic tests have been conducted. The experimental results show that the distribution characteristic of calcite particles, which significantly influences the growth of cracks and the macroscopic mechanical properties of coal, is obviously affected by the bedding structure. Specifically, the uniaxial compression strength of coal sample is mainly controlled by bedding structure, and the average peak stress of specimens with axes perpendicular to the bedding planes is 20.00 MPa, which is 2.88 times the average amount of parallel ones. The test results also show a close relationship between the bedding structure and the whole deformation process under uniaxial loading.

A Multiscale Simulation Method and Its Application to Determine the Mechanical Behavior of Heterogeneous Geomaterials

To study the micro/mesomechanical behaviors of heterogeneous geomaterials, a multiscale simulation method that combines molecular simulation at the microscale, a mesoscale analysis of polished slices, and finite element numerical simulation is proposed. By processing the mesostructure images obtained from analyzing the polished slices of heterogeneous geomaterials and mapping them onto finite element meshes, a numerical model that more accurately reflects the mesostructures of heterogeneous geomaterials was established by combining the results with the microscale mechanical properties of geomaterials obtained from the molecular simulation. This model was then used to analyze the mechanical behaviors of heterogeneous materials. Because kernstone is a typical heterogeneous material that comprises many types of mineral crystals, it was used for the micro/mesoscale mechanical behavior analysis in this paper using the proposed method. The results suggest that the proposed method can be used to accurately and effectively study the mechanical behaviors of heterogeneous geomaterials at the micro/mesoscales.