Simulation study on evolution of coalbed methane reservoir in Qinshui basin, China

A dynamic equilibrium model has been proposed to describe the coalbed methane concentration–dissipation evolution process in coal, including its generation, storage and migration. The model, developed with a computational program, has been applied to the Qinshui basin, China, which allows to evaluate features of coal seam of the basin including coal quality and burial, palaeo-geothermal field, and coal organic material maturation evolution history. The simulation study was focused on coalbed methane concentration–dissipation evolution history of the upper and lower main coal seams in the basin. The results indicate that the whole evolution process can be divided into three stages. At the first stage, i.e. the primary concentration stage, gas generation quantity and gas content were lower and only weak diffusion took place. The second stage can be called the active concentration–dissipation stage. During this period, coal organic material was matured at a higher rate, generating a large amount of gas because of abnormal high palaeo-geothermal field in the Middle and Late Yanshan Epoch. As a result, gas content in coal reservoir was higher and gas could be lost in different ways including diffusion, cap outburst and permeation. The third stage can be defined as the absolute dissipation stage at which coal organic material no longer generates gas. Meanwhile much gas was lost mainly by diffusion and partly by permeation. Simulation results further show that, although the high gas generation led to active dissipation in the coal reservoir, there still remains comparatively high gas content on the south and north ends of the Qinshui basin due to the preserving geological conditions. This suggests that these regions are favorable for coalbed methane exploitation which has been confirmed by recent geological exploitations in Qinshui basin.