Pressure-relief gas flow behavior in the hydraulic flushing coal mass considering creep effect: Theoretical modeling and numerical simulation

Abstract

The hydraulic flushing coal mass is often under high in-situ stress and generally possesses low long-term strength, which endows it with significant creep properties. Under the effect of creep, the stress-strain of hydraulic flushing coal mass changes constantly over time, further affecting gas diffusion-seepage properties and gas flow within the coal. If the influence of creep is ignored, it is not only difficult to evaluate the gas migration ability, but also leads to a decrease in the stability of the extraction borehole, ultimately affecting the gas extraction efficiency. In response to this, a fractional visco-elastoplastic stress-strain analytical solution is first derived from the fractional Maxwell model for the hydraulic flushing coal mass. Volumetric strain is then taken as the bridge to quantitatively characterize the gas diffusion-seepage responses during gas drainage, thereby building a new pressure-relief gas flow model that considers the creep effect. On this basis, the rationality of the model is verified through numerical simulation, and the pressure-relief gas flow behavior and its influencing factors are analyzed. Based on the study findings, it is found that when the gas drainage time reaches 90 days, the pressure-relief range of the borehole extends to 2.2m, the volumetric strain of coal mass in the borehole wall increases by 1.43 times, the diffusion time ratio decreases to 0.01, and the permeability ratio increases to 1146.59. This model effectively describes the pressure-relief gas flow behavior, and ignoring the creep effect will lead to underestimating the gas flow in coal. The research provides a favorable support for further understanding of gas flow behavior in the hydraulic flushing coal mass.