Stress Sensitivity Analysis of Seepage Pores in Coal Briquettes Based on Compressed Exhaust and Pressure-Transient Tests

Abstract

Seepage pores are the main space for methane transport in the coal seam. To further investigate variation laws of seepage pores with stress, this study established a new method based on the compressed exhaust and pressure-transient tests to characterize the seepage pore size distribution under different effective stress conditions. The reconstituted coal briquette and nonabsorbent helium were adopted as the experimental samples. The results showed that the deformation of coal mass mainly originates from seepage pores. With the increase of confining stress, the volume reduction of seepage pores presents a logarithmic growth. The initial seepage porosity of the tectonic coal specimen, i.e., the seepage porosity when effective stress is zero, was estimated to be 5.28%, accounting for 36.24% of the total porosity obtained by helium saturation. When effective stress is in the range of 13–48 MPa, the size of seepage pores generally ranges from 0.1 to 1.0 μm. The volume of seepage pores with sizes ranging from 0.1 to 0.5 μm always accounts for more than 80% of the volume of total seepage pores, and this proportion is not affected by effective stress. The inverse proportional coefficient between the threshold pressure gradient of helium and seepage pore diameter was finally determined to be 0.55 ± 0.19 mPa.