Dynamic behaviours of reservoir pressure during coalbed methane production in the southern Qinshui Basin, North China

During the coalbed methane (CBM) reservoir depletion, accurate determination of the dynamic characteristics of reservoir pressure is essential to evaluate the effective desorption intensity, interwell interference, multi-layer production, and productivity. Based on the study on the propagation of pressure perturbation and gas desorption area in the stage above and below the critical desorption pressure (CDP) respectively, an analytical mathematical model is established to characterize the dynamics of reservoir depressurization. Then with a field case study on the reservoir pressure depletion in the southern Qinshui Basin, North China, the variation laws of reservoir pressure during CBM production are analysed and discussed. In the stage above CDP, the average reservoir pressure decreases linearly under the constant water production rate. With the location farther away from the wellbore, pressure gradient progressively decreases and rate of pressure change at a fixed location shows a fast to slow pattern. In the stage below CDP, the gas desorption area expands gradually and similar variations are observed between the average reservoir pressure and bottom-hole pressure, both showing a fast to slow pattern. This shows that the reservoir pressure depletion depends dramatically on the variations of bottom-hole pressure. It verifies the reasonability of this mathematical framework after a comparison of calculation results and changing laws between this study and tank-type model. This study is meant to provide a theoretical guidance for the management and optimization of wellbore dewatering during CBM extraction.