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At the front of a working coalface, the stress state can differ significantly across different zones. This seriously affects the permeability of the coal seam. In this study, we adopt a discrete element method to study the coal permeability under different stress states. The results indicate that an increase in stress causes the permeability to decrease significantly according to an exponential attenuation model. As the stress ratio increases, the change in x-direction permeability is initially small. However, when the stress ratio reaches a certain value, the permeability starts to increase significantly. This is because of cracks that open under shear dilation. The change in coal permeability is examined in terms of the stress state at the front of the workface in three typical mining layouts. Our results show that the x direction permeability to the left of the peak stress increases slightly, but the y-direction permeability increases significantly. The unloading of x- and y-direction stress to the right of the peak stress causes cleats to open, resulting in a significant increase in both x and y-direction permeability.

During the development of oil field, well operation fluids will inevitably invade the formation, and insoluble is produced when contaminant ions contained in operation fluids react with formation fluids, which will easily plugs the pores, consequently affecting the flow of formation fluids. It is of remarkable significance for formation damage evaluation to study the laws of contaminant ions transportation and distribution near wellbore. In this study, the characteristics of high-speed percolation and contaminant ions diffusion near wellbore are reasonably considered, and then we established the non-Darcy flow reaction-diffusion equations, from which the contaminant ions distribution and transportation will be illustrated, thereby, the effects of pollutants migration and reaction on porosity and permeability are further studied. Thus, the newly developed model can provide theoretical basis and practical reference for plug removal. [ABSTRACT FROM AUTHOR]

We developed a new model for fluid movement around the wellbore by introducing a Dirac function, which simplifies a water-injection well as a point source. The effect of porosity on the reaction and diffusion of contaminant ions is also taken into account in the model, making it more reliable in theoretical prediction. [ABSTRACT FROM AUTHOR]