Experimental study on permeability characteristics in complete evolution process of coal structure and its significance

Coal reservoir's permeability is closely related to its coal structure, their relationship is of significance to the method optimization of permeability increase for coal reservoirs with different structures. Based on the introduction of geological strength index (GSI) to quantitatively characterize the coal structure, the corresponding parameters including coal structure, acoustic emission and permeability, etc were tested and analyzed. Subsequently the variation trends of permeability features with different coal structures in a complete evolution process were discovered. The results show that during the improvement phase of coal structure (GSI), the coal samples transit from compaction to elastic deformation; the acoustic emission strength turns from weak to strong and the permeability decreases due to the gradual closure of micro-fissures. During the brittle failure phase of coal structure (GSI), the coal samples become yielded and failed; the acoustic emission strength increases gradually and the permeability has a hysteresis increase and then increases to its maximum as a result of the full extension of fractures. During the plastic decrease phase of coal structure (GSI), the concave and convex parts of early-formed fractures are cut and corroded; the connectivity becomes weak and coal samples are powdered gradually; at the same time, the acoustic emission count increases significantly but with less strength, and with the increase of strain, the stress curve becomes upturned which indicates the effect of re-compaction, and also the permeability has a notable decrease.