Reservoir properties and gas production difference between No.15 coal and No.3 coal in Zhengzhuang Block, southern Qinshui Basin

With the in-depth exploration and development of coalbed methane (CBM), the development differences between different strata in the same region are gradually emerging. Deepening the understanding of reservoir is crucial to formulate development strategies. Taking the Zhengzhuang Block in the southern Qinshui Basin as an example, in the past, the focus was on the main No.3 coal resources. With the stimulation demand in this block, the CBM needs to be gradually recovered in multiple layers. In this paper, the CBM resource of No.15 coal is analyzed. Comparative analysis with well developed No.3 CBM resources is carried to probe into the cause of reservoir differences and regional differences. Furthermore, the genesis mechanism is speculated to guide the formulation of targeted development strategies. Based on the description of drilling cores and observation under microscope, combined with the indoor test, seismic exploration, logging and production data, the differences in coal thickness, physical properties and gas-bearing properties between the No.15 and the No.3 coal seams in Zhengzhuang Block are investigated. The results show that compared with the No.3 coal, No.15 coal is thinner and the plane distribution is unstable with local bifurcation phenomenon. In general, the gas content of No.15 coal is equivalent to that of No.3 coal, ranging from 10 m³/t to 29 m³/t, although there are regular regional differences within the block. The adsorption time of No.15 coal is significantly shorter than that of No.3 coal, indicating its favorable development potential. The distinction in sedimentary environment and tectonic influence are the main reasons for the difference in petrophysical properties and gas-bearing properties between the two seams. Specifically, because of the barrier coast sedimentary of Taiyuan Formation, the water turbulence and strong reducing environment lead to a varying coal thickness in plane and a higher vitrinite content of the No.15 coal. Tectonically, compared with the No.3 coal, partial No.15 coal is more obviously affected by geological structures such as basement structure, faults and folds. According to the characteristics of coal reservoir parameters, the favorable area of No.15 coal is divided into three categories, and the “sweet spot” is identified. Pilot production suggests that the daily production of casing fractured horizontal wells in No.15 coal in category I area exceeds 10 000 m³ and the production of vertical wells in No.3 and No.15 coal seams has doubled than that of single-mining No.3 coal seam. In category Ⅱ area the output of casing fractured horizontal well developing No.15 coal achieves high yield. This research provides technical supports for regional production increase and scale expansion.