Coupled analysis about multi-factors to the effective influence radius of hydraulic flushing: Application of response surface methodology

• A 2D model of gas seepage was established using the solid mechanics module and darcy module in Comsol Multiphysics.
• The role of the radius of borehole, original gas pressure, and initial permeability to the effective influence radius.
• The coupling effects of all these three factors on gas drainage were studied using the response surface methodology.
• A quadratic polynomial equation was established between effective influence radius and three factors.

Current coal seams in China are characterized by great depth, low permeability, and strong adsorption features, and thus, the extraction of methane is relatively difficult and gas disaster remains serious. To overcome this limitation, the hydraulic flushing and hydraulic slotting are adopted to improve the permeability of coal seams. A 2D model of gas seepage was established by using the solid mechanics and darcy module in Comsol Multiphysics, version 4.3b. The factors influencing pressure relief, such as the radius of borehole after hydraulic flushing, the initial gas pressure, and the permeability of coal seam, were studied. The coupling effects of these three factors on the effective influence radius were studied through the response surface methodology, and a quadratic polynomial equation between effective influence radius and three factors was derived. The results showed that Gas pressure distribution (e.g. gas full emission region, gas pressure transition region and original gas region) near the borehole, corresponded well with stress distribution (stress relief zone, stress concentrating range and original stress zone). The plastic zone formed around the borehole leaded to stress reduction, and the increasing porosity and permeability resulted in pressure decline. The effective influence radius increased with the borehole radius and the initial permeability, but decreased with original gas pressure. Besides, it was found that an increase in one factor would restrict the role of other two factors on the effective influence radius. This study is helpful to choose the most suitable parameters to achieve high extraction efficiency and lower gas content/pressure.