Abstract

This study aimed to analyze the evolution of the porosity and permeability of fault rocks under non-Darcy flow-stress coupling. The permeability of broken coal, mudstone, and sandstone with different particle size gradations under different stresses was investigated using an experimental system for the permeability evolution of broken rocks. Experimental results show that broken coal exhibits the best compressibility, followed by broken mudstone, and broken sandstone presents the worst compressibility. The porosity and permeability of the broken rocks decrease rapidly with the increase in confining pressure and then decrease slowly until they stabilize. The crushing of large particles into fine particles under stress is the main reason behind the decreasing porosity and permeability of mixed broken rocks. The smaller the fractal dimension of the particle size gradation during loading, the more serious the particle breakage, the greater the mass loss of large particles, and the greater the decrease in porosity and permeability.

1 Introduction

Coalbed methane, a common associated resource in coal mining, is stored in coal seam voids. Hydraulic fracturing, drilling, and coal mining can change the stress distribution in the coal seam, resulting in the large-scale release of coalbed methane and leading to gas protrusion accidents[1]. However, this phenomenon is obvious for broken coal rock masses, which are likely to cause gas outburst[2]. Therefore, characterizing the gas permeability of broken coal is important for the safe exploitation of coalbed methane.

Chilton et al.[3] summarized the relationship between effective grain size and critical pore pressure from the permeability tests of broken rocks with different grain sizes. The permeability of porous media is determined by the number of pores. Liu et al.[4, 5] and Ma et al.[6] studied the permeability of broken sandstone, coal, and shale by using axial displacement control and analyzed the relationship between porosity and permeability parameters. Pore structure is an important factor affecting the permeability of porous media, and the pore structure of porous media is determined by its particle size and particle arrangement. Yamada[7], Li[8], and Ma[9-11] studied the effect of particle size and particle ratio on the permeability of porous media and concluded that small particles in crushed rocks can fill pores and block seepage channels.

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