Gas content and productivity controlling factors of coal reservoir in Sijiazhuang area, Qinshui Basin

In order to study the characteristics of coalbed methane accumulation and production controlling factors in the northeastern Qinshui Basin, based on the data of coalbed methane drilling and drainage in the Sijiazhuang area, the geological structure, coal seam thickness and buried depth, maceral of the Sijiazhuang coals are analyzed. The geological control mechanism of gas-bearing coal reservoirs is studied, combined with the fracturing and drainage technology of coal-bed methane wells, the production characteristics of coal-bed methane and its influencing factors are discussed. The results show that: (1) The gas-bearing properties of coal reservoirs in the study area are greatly affected by geological structure. In the fold axis and lateral structural compression zone, the gas content is often high, especially in the syncline axis. Under the superimposed action of the collapse column and hydrogeological conditions, the gas content of the No.15 coal seam is lower than that of the No.8 and No.9 coal seams, and the gas saturation of the No.8 and No.9 coal seams is also higher than that of the No.15 coal seam. (2) The gas content of the No.8 , No.9 and No.15 coal seams in the study area all show a trend of increasing with the increase of the coal seam depth, but with the increase of the coal seam depth, the effects of tectonic stress and geothermal field gradually increase, and there is “critical depth” that varies with buried depth(approximately about 700 m). The gas content of coal seams also shows a trend of increasing with the increase of coal seam thickness. The simpler the coal seam structure, the better the coal seam gas content. (3) The area where the NNE-NE trending fold and the EW trending structure are superimposed in the middle of the study area has become a high production area of coalbed methane due to its larger structural curvature and relatively relaxed regional in-situ stress, which has good permeability conditions and gas-bearing properties. (4) Adopting partial pressure combined mining technology in areas with multiple coal seams can effectively increase gas production. Multiple coal seams can provide sufficient gas sources for high productivity of coalbed methane wells, and simultaneous drainage and pressure reduction of multiple layers can produce gas from different coal reservoirs. Production capacity superposition, realize long-term stable production, and long-term sustained high-yield wells can appear in areas where gas-bearing properties are good and free gas may exist.