Gas-bearing and reservoir heterogeneity characteristics of deep coalbed reservoir in the Dacheng block, Bohai Bay Basin

The resources of deep coalbed methane are abundant in Dacheng block of Bohai Bay Basin. The production of deep coalbed methane Well has reached more than 1.1×10⁴ m³/d in the early stage, which has great exploration and development potential. However, the gas content, occurrence state and reservoir heterogeneity of deep coal seams are unclear, which restricts the process of large-scale and efficient development of deep coalbed methane. To solve this problem, the pressure-holding coring and analysis test of No. 10 coal seam of Taiyuan Formation in Dacheng Block are carried out for the first time, the gas bearing and coal reservoir heterogeneity characteristics of deep coal seam are analyzed, and the corresponding development strategies are put forward. The results show that coal rock, carbonaceous mudstone, sandstone and mudstone alternately develop longitudinally in the deep No. 10 coal reservoir, and the alternate development of lithology greatly changes the longitudinal heterogeneity of coal quality, physical property, pore fissure and rock mechanical characteristics. Overall, the upper coal body structure of the No. 10 coal seam is relatively complete, while the lower part is relatively fragmented. The rock strata with a thickness of about 2 meters in the middle and lower parts of the coal seam are affected by the intrusion of magmatic hydrothermal fluids, and the degree of metamorphism of coal and rock is relatively high. The gas content test of coalbed methane with pressure-holding coring eliminates the influence of lost gas on the gas content test, and the accurate gas content of deep coal seam in Dacheng block is obtained. The gas content of No. 10 coal seam ranges from 0.3 to 17.8 m³/t, and the longitudinal gas content of the coal seam varies greatly. The longitudinal gas content of No. 10 coal seam shows a trend of decreasing first, then increasing and then decreasing. The high temperature and high pressure isothermal adsorption experiment shows that the theoretical adsorption amount is less than the measured gas content at the top of No. 10 coal seam, the coal reaches a supersaturated state, and the methane in deep No. 10 coal seam shows coexistence of adsorbed gas and free gas, the proportion of free gas is 24.2%, and the free gas is mainly distributed in the top of the coal seam. The heterogeneity of lithology, coal rock and coal quality leads to large variations in gas content and occurrence state. The top of deep coal seam with high free gas content, primary structural coal, good physical property and fracture development is selected as the sweet spot section for drilling and fracturing development, and horizontal well drilling and fracturing are carried out in this section. The results provide important guidance for the exploration evaluation and scale development of deep CBM in Dacheng block and similar CBM resources.