Change types of coal permeability to gas and its identification pattern

Coal shows a variety of permeabilities to gas flow. Coal samples of different ranks were collected from 32 mines in China, and were prepared into 54 cylindrical cores to investigate coal permeability to gas (CO₂) flow. The results were classified based on rebound pressure and permeability damage rate. Furthermore, apattern of change types was established. These included "declining type", "rebounding type" and "increasing type", which could be further sub-divided into 8 types. The permeability change types can be identified from cross-plots of "maximum vitrinite reflectance-initial permeability", "ratio of inertinite content to vitrinite content-initial permeability", "ratio of fixed carbon content to ash yield-initial permeability" and "ratio of inertinite content to vitrinite content-ratio of fixed carbon content to ash yield".