Variation of methane adsorption property of coal after the treatment of hydraulic slotting and methane pre-drainage: A case study

• Six coal samples at different distances from the slotted borehole are obtained.
• Investigation of pore size distribution via organic combination of MICP and N2GA.
• Adsorption pore volume proportion decreases with decrease in borehole distances.
• There exists a significant sectionalized feature around the slotted borehole.
• Effective stress increases after the treatment, controlling the adsorption property of coal.

The variation of methane adsorption property of coal after the treatment of hydraulic slotting and methane pre-drainage was investigated by testing the seven coal samples obtained from the coal seam #10 of Yangliu Mine. The methane isothermal adsorption experiments were conducted to measure the adsorption constants and the Mercury Injection Capillary Pressure (MICP) and Nitrogen Gas Adsorption (N2GA) were organically combined to depict the pore size distribution (PSD). It is revealed that non-significant variation occurs in the various indexes of proximate analysis. With the increase of borehole distance, adsorption constant a presents a tendency toward enlargement and adsorption constant b has an opposite variation tendency, which is consistent with the change tendency of the curvatures of the curves. Remarkable variation occurs in pore size distribution of coal samples and adsorption pore proportion decreases from 55.38% to 33.27% with the decrease in borehole distance. The curves of coal parameters (a, b, Xabs) present a characteristic of boundness and nonlinearity and the variation amplitude is “gentle-drastic-gentle”, which obeys the Boltzmann equation. There exists a significant sectionalized feature after the treatment of hydraulic slotting and methane pre-drainage. That is, region of significant influencing, region of transition and region of non-significant influencing. The gas pressure decreases and the effective stress increases after the treatment, which controls the adsorption property of coal. Hydraulic slotting and methane pre-drainage are sequential processes and each is a dominate factor that changes the methane adsorption property in relevant process. The results can provide reliable theoretical support for the application of hydraulic slotting.

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