Material and energy dynamic balance mechanism in middle-high rank coalbed methane (CBM) systems

Based on the unsaturated characteristic of medium-high rank coal reservoirs and coalbed methane ( CBM) well production data,the material and energy dynamic balance mechanism in middle-high rank CBM systems was re- vealed by considering the critical desorption pressure as a key parameter. The control action of this mechanism on practical CBM development was also analyzed. Results show that four aspects of the requirements in the field can be realized through using above mechanism,including the real-time monitoring of reservoir pressure and water saturation, the single CBM well recoverable reserves calculation,the reservoir absolute or relative as well as effective permeability dynamic prediction and the productivity dynamic numerical simulation. The higher the relative gas content ( the ab- sorbed gas saturation) of the coal reservoirs is,the faster the reservoir pressure and water saturation decline,the easier the CBM desorption is. After the reservoir pressure is under a critical desorption pressure,the longer production time could bring more accurate reserves computation. In the whole process of CBM production,the coal reservoir permeabili- ty are unified as the function of reservoir pressure and the unsaturated effective permeability curves can better reflect the positive and negative effects as well as the gas slippage effect. Meanwhile,the unsaturated effective permeability model is more accurate and precise than that of the saturated effective permeability model in the CBM well productivity prediction.