Gas Seepage Under Continuous and Step Loading Based on True Triaxial Seepage Test Apparatus

The conventional triaxial loading in gas seepage studies does not reflect the actual stressing state in the coal seam. Thus, a true triaxial seepage experimental apparatus is developed before seepage tests are conducted with it. The first group of tests is carried out with continuous axial loading under uniform lateral constrain on moulded coal specimen: one test under constant-velocity axial loading, two tests under monotonic variable-velocity axial loading, and two tests under single-cycle variable-velocity axial loading. The tests reflect the multi-step, discontinuous, stressing condition with variable-loading rate and strength in a coal seam. The law of coal deformation and gas permeability variation under constant-velocity and variable-velocity axial loading is obtained. Then, the multi-step seepage tests under different non-uniform lateral pressures are conducted, focusing on permeability variation under different triaxial stresses versus gas pressure and intermediate principal stress. The permeability increase with gas pressure decrease is confirmed and interpreted. Moreover, the turning point between Klinkenberg effect and gas wedging effect in the coal seam is determined. A simplified concept of average effective stress is put forward, and the dominant influence of this average effective stress on coal seam permeability is identified based on the concept.

Automated summary

A true triaxial seepage experimental apparatus was developed to conduct a series of coal seam gas seepage experiments. The results showed that the gas permeability of coal seams varies under different stress states, with an increase in permeability as gas pressure decreases, and the turning point between the Klinkenberg effect and gas wedging effect was determined. The concept of average effective stress was proposed, identifying its dominant influence on coal seam permeability.