The influence of long-time water intrusion on the mineral and pore structure of coal

Abstract

The formation water intruding into the coal seam through cross-layer seriously reduces the gas extraction efficiency. In this work, a series of experiments were carried out to investigate the influence of long-time pressure water on the mineral content and pore structure of coal. Results show that, the water intrusion makes a decrease in the types and contents of mineral in coal. Furthermore, full-scale pore size was analyzed by using low-temperature N2 adsorption and CO2 adsorption experiment, indicating that the macropore shows a different change trend before and after water intrusion. For TY and YZ sample, the macropore volume decreased by 18.56% and 0.98%, respectively. For QC sample, the macropore volume increased by 3.20%, while the micropore volume, mesopore volume and total volume of three samples increase in different extent, which are 16.67%–30.77%, 11.56%–17.59%, and 16.10%–125.00%, respectively. Then, the fractal dimension of coal before and after water intrusion was calculated by using FHH model, showing that the fractal dimension D1 decreases while the fractal dimension D2 increases after water intrusion because the water intrusion is able to create and expand pore of coal but it can reduce the roughness of the pore surface. Finally, the influence mechanism of water intrusion on the pore structure of coal is analyzed based on clay mineral collapse, mineral dissolution and coal swelling. The clay minerals in coal is a key factor impacting the pore structure during water intrusion process due to its poor water stability. In addition, the mineral dissolution and coal swelling also play the significant roles in pore structure. This work is significant for the gas control and prevention of coal mine in rich-water coal strata.