Damage‐induced permeability model of coal and its application to gas predrainage in combination of soft coal and hard coal

Abstract

Coal permeability is a key parameter that affects the drainage of coal bed methane (CBM). Owing to the lower original permeability of coal seams in China, more attention was paid to the study of damage‐induced permeability laws. However, the theory of damage‐induced permeability still needs to be improved. In this paper, the permeability evolution experiments during the complete stress–strain process of coal were carried out firstly. The results are shown that with the increase in axial strain, the coal permeability decreases slowly at first, then increases rapidly, and finally increases slowly or even remains unchanged, and the peak stress point can be regarded as the extreme point of permeability. Then, the equivalent plastic strain was used to describe the damage of coal. Based on the above results, a new damage‐induced permeability model was developed, and the new model can match the experimental data of permeability very well. In addition, solid–gas coupling models of coal were developed based on the new permeability model, and the numerical results show that there is a region of sudden increase in permeability around the borehole where plastic failure occurs, which indicates that the damage‐induced permeability model is reasonable. Finally, the results of numerical simulation of gas predrainage in combination of soft coal and hard coal under different cases were used to optimize the borehole layout of gas predrainage in the field.

Experimental study on strain behavior and permeability evolution of sandstone under constant amplitude cyclic loading‐unloading

“…The author considers that the effective pores and fractures play a decisive role in permeability. 33 Furthermore, high in situ stress (ie, the increase in initial confining pressure in experiment) determines the greater ultimate energystorage capacity of the rock, which increases the possibility of rockburst. This dilatancy capacity indicates that the fracture structure of the sandstone was being dynamically adjusted and was affected by the in situ stress and particle distribution, intergranular fillers, and fracture structure of sandstone.…”

Effect of roof movement on gas flow in an extremely thick coal seam under fully mechanized sublevel caving mining conditions

“…After mining, damaged coals and rocks—residual coals, plus materials from the overlying strata and immediate roof—fill in goafs. Because of its high permeability and porosity, the caving zones contain gases, which may be from the adjacent unmined coal seams and/or the mined coal seams . The impact forces that are caused by the roof collapse have an important effect on the gas seepage field in goafs.…”

“…Because of its high permeability and porosity, the caving zones contain gases, which may be from the adjacent unmined coal seams and/or the mined coal seams. 38,[41][42][43] The impact forces that are caused by the roof collapse have an important effect on the gas seepage field in goafs.…”

Short‐term failure mechanism triggered by hydraulic fracturing

“…The first concentrated seismic activities occurred during the Wenchuan earthquake in 2008, and the second concentrated seismic activities occurred during the comprehensive development of shale gas, that is, 2012. Similarly, it also has been proved that hydraulic fracturing could induce instability in tight sandstone gas and coalbed methane exploitation . It was universally accepted that hydraulic fracturing promoted the initiation and propagation of new fractures …”

“…Similarly, it also has been proved that hydraulic fracturing could induce instability in tight sandstone gas 11 and coalbed methane exploitation. 31 It was universally accepted that hydraulic fracturing promoted the initiation and propagation of new fractures. 27 Published studies mainly focused on purely statistical 8,19 and numerical simulation.…”