Reservoir controlling mechanism and production controlling patterns of microstructure to coalbed methane

There are strong differences in production among different wells of coalbed methane (CBM),and the actual development outcome is far from one expected. The authors analyze the impact of micro structures on the CBM’s geological conditions,development and production,investigate the fracturing effects of different microstructures,pressure drop funnel shape,the output of pulverized coal and the response of gas water dynamics to geological and engineering factors,discuss the formation of “dynamic gas reservoir” and the reservoir controlling mechanism of microstructure,and define its production controlling characteristics. The research shows that the coupling effect of microstructure,the CBM geological conditions and development dynamics leads to some obvious differences in the CBM production at different microstructure locations. In the long term development process,the continuously desorbed CBM migrates along the dominant depressurization direction and converges to the high part of microstructure to form a “dynamic gas reservoir”. Being different from the conventional trapping and controlling mechanism of natural gas,the “dynamic gas reservoir” is a new type of gas reservoir formed by gas water difference,migration and accumulation in the process of development and production. The mechanism has been verified by the production practice. Controlled by this mechanism,even if the CBM content in shallow buried pressure relief area or anticline axis is not high,a high production level may be obtained if the wells are located at the high part of microstructure. Therefore,compared with other geological factors,the influence of microstructure is more important on the CBM development process. The variation of flow field (gas and water fields) in coal reservoir is related to microstructure at different desorption stages. The water production, in which negative microstructure with positive structural background, independent negative structure, groundwater runoff area, near fault and collapse column, open surrounding rock or wells connected with high aquifer by fracturing, is large. In the negative microstructural part of the positive tectonic background,the CBM wells contribute to the surrounding wells and get high gas production in a late stage,forming high production water and gas wells. In the independent syncline,the wells can also contribute to the surrounding wells to form high water and low gas production wells. The other four types of wells with different geological conditions are “ineffective drainage” wells with high water and low gas production. On the basis of above principles,the CBM wells are divided into six categories,of which the five types of wells are closely related to microstructures. In the positive microstructure and slope,there are mainly Type A wells with high stable yield and slow decline. Type B wells with sudden production stop result from the pump stuck by pulverized coal powder,and are mainly distributed in the positions with steep structural changes. Type D wells with continuous high water and low gas production are mainly located in the negative structural position. Type E wells with mainly intermittent gas production are located near the goaf and fault. The above results change the traditional understanding of in situ lean gas in anticline axis,help to improve the accuracy of the CBM sweet spot evaluation and well location layout,and provide a new idea and basis for predicting the productivity and development rules of similar geological conditions in undeveloped areas.