Reasons of low yield and stimulation measures for vertical CBM wells in high-rank coal

The CBM production is usually low and development effect is poor in China, which are mainly due to the poor adaptability of the stimulation measures to the geological conditions. This article, based on the low-yield reasons for vertical CBM wells in Zhengzhuang block in southern Qinshui basin, puts forth corresponding stimulation measures, analyzes the mechanism and the geological adaptability, optimizes the parameters of specific stimulation measures and carries out pilot test. The research and practices show that in deeply buried area, fractures are difficult to form, and re-fracturing technology can make the fracture turn and increase the crack length, which has better stimulation. In order to fully release the stress and increase the deflection degree of the fractures, the extraction time before re-fracturing should be at least 1 000 d, and the amount of proppants should be reduced in the fracturing operation. It is important for the fine sand to be added in sections, in order to plug the fractures made by former fracturing. When the cataclastic coal or broken coal develops overall in coal seams, hydraulic-fractures cannot extend far to have high productions, and the implement of indirect roof-fracturing improves the development performance. And the optimized parameters are as follows:the distance between the coal seam and the new perforation interval in the roof formation is between 0.5 m and 1.5 m. The injection rate of fracturing fluid is better between 5 m³/min and 5.5 m³/min and the better perforation interval is between 1.5 m and 2 m. While the fracturing fluid volume for per-perforation-interval is better between 200 m³/m and 300 m³/m. In natural greatly developed fractures zones, the implementation of ball-off fracturing can dramatically improve the production, which is suitable for the wells having falling fracturing pressure curves and the values below15 MPa. And the optimized water production rate per day is between 2 m³ and 5 m³. The stimulation measure is that before ball-off fracturing, a small pre-fracturing with fine sand is necessary in order to block the original fractures and to improve injection pressure to create new fractures. This paper has significant introduction meaning on the reason analysis and the stimulation measures for high rank coalbed methane reservoir.