Main Geological Controlling Factors Of CBM Production Differentiation In Fanzhuang Block, Qinshui Basin

Based on the large amount of the data and information from the exploration and development of vertical CBM wells in Fanzhuang block, Qinshui basin, the geology of No.3 CBM reservoir was discussed, a comprehensive optimization system of CBM sweet spots was set up taking the CBM well drainage effect as an objective function, and the reginal distribution of CBM sweet spots was predicted. Further, the contorling factors and mechanisms of CBM productivity differentiation were discussed. The results show that the geology of No.3 coal reservoir is characteried by the ―four-high, two-low and one-complex‖. The ―four-high‖ means the high coal rank, high CBM content, strong adsorptivity and high ratio of critical desorption pressure to reservoir pressure, the ―two-low‖ is relatively low porosity and permeability and low pressure gradient, and the ―one-complex‖ refers to the complex coal-body structure. These characteristics, on the one hand, show that the geological conditions of CBM development in the block is relatively favorable in China, and, on the other hand, the more targeted measures for coal reservoir reconstruction and CBM production stimulation. In view of the three aspects of geological conditions such as the CBM accumulation, coal reservoir conductivity and CBM drainage geology, a mathematical model of CBM sweet spot optimization within a block based on the fitting relationship between evaluation parameters and CBM production is established using the multilevel fuzzy model, which is consisting of 9 key elements and 10 evaluation parameters. Based on the model, the 4 primary CBM productivity areas including 6 sub-areas were distinguished within the block. Then, the 4 drainage dynamic index of CBM wells were proposed, and the contoling factors and mechanism for the productivity differentiation of CBM wells were interpreted. It was considered that the productivity differentiation was controlled jointly by the revervoir factors such as geo-stress, structure and groundwater level with the drainage dynamic ones such as gas-water separation and production pressure difference. It was also revealed that the productivity differentiation was resulted in a unified "constant pressure" drainage working system, especially in Fanzhuang block.