The differences on gas-water relative permeability characteristics of low and high rank coal and its controlling factors

A comprehensive comparison was conducted in terms of the gas-water relative permeability characteristics of high rank coals from south Qinshui Basin and low rank coals from Erlian Basin, China. The results show that: High rank coal samples are featured with a larger percentage of micropores. Compared with high rank coal samples, low rank ones display a smaller irreducible water saturation (Swo) and higher methane (Krg) and water (Krw) permeability and wider spans of two-phase flow region. High rank coal samples with a large percent of micropores are featured with a larger Swo and a narrower two-phase flow span. Gas displacement pressure increases Krg, while decreases Krw and Swo, and the changing rule is more pronounced in low rank coal samples. The effective stress is inversely proportional to the Krg and the span of two-phase flow, whereas proportional to the Krw. Influenced by effective stress, the dynamic changes of the cross points are more obvious in the low rank coal samples, however that of two-phase flow spans and Swo are not so pronounced. The conclusion will be helpful for CBM exploration and development.