Differences in the adsorption/desorption characteristics of coal reservoirs with different coal ranks and their effects on the reservoir productivity

Adsorption/desorption parameters are critical to the evaluation of the reserves and productivity potential of coalbed methane (CBM). Moreover, the law governing the desorption process is closely related to the productivity of CBM wells. This study aims to investigate the adsorption/desorption characteristics of coal reservoirs with different coal ranks and their effects on the productivity of the reservoirs. Based on the data of the coal macerals, coal quality, gas content, and isothermal adsorption experiments of 49 coal core samples from the Fanzhuang and Anze blocks in the southern Qinshui Basin and the Baode and Liulin blocks in the eastern Ordos Basin, as well the data on well tests and production data of these blocks, this study revealed the effects of coal macerals and coal quality on isothermal adsorption characteristics, classified the CBM desorption stages and compared their characteristics, and built the mathematical relationship between coal adsorption/desorption characteristic parameters and the coal rank. Moreover, based on the data on 20 typical CBM wells, this study explored the influence of desorption parameters on the productivity of CBM wells and put forward some specific suggestions on CBM exploration and extraction strategies of coal reservoirs with different coal ranks. The results of this study are as follows: (1) With an increase in the coal rank from the Baode block in Shanxi to the Fanzhuang block in Jincheng, the starting, turning, and sensitive pressures gradually increased during the desorption of coal reservoirs, and the ranges of inefficient, slow, quick, and sensitive desorption stages gradually shifted toward the high-pressure interval. High-rank coals had significantly wider intervals of the effective desorption stages than low-rank coals, creating more favorable conditions for CBM development; (2) Compared with the medium- and low-rank coals, the high-rank coals have higher CBM desorption index, shorter time required for stable production, and higher gas production potential. However, low permeability is a critical factor restricting the productivity of high-rank coals; (3) In combination with resource abundance, desorption capacity, and permeability, high-rank coals have a high desorption index, while medium- and low-rank coals have high permeability. Accordingly, the key to high CBM productivity is to employ the large-scale and efficient fracturing technology for high-rank coals and is the precise selection of the optimal sweet spots of CBM enrichment for medium- and low-rank coals.