In-situ Monitoring and Prediction of the Height of the Water-conducting Fractured Zone in a Jurassic Coal Seam in Northwestern China

The height of the water-conducting fractured zone (HWFZ) plays a significant role in environmental protection and engineering safety during underground mining. In this study, to measure the HWFZ in the Cuimu coal mine (CCM), in-situ monitoring, including water leakage tests and video capture through a borehole camera, were conducted. Considering mining height, panel width, and buried depth, 26 sets of samples were collected from mines in the northwest to obtain a multi-factor regression formula. Additionally, we used the 26 sets of samples as training material for machine learning to predict the HWFZ. Then, to assess why the traditional empirical formula proposed to measure the HWFZ is inapplicable for the Jurassic coal mines in the northwest, the sedimentary environment of the Ordos Basin, the RQD of drilled cores, and the overburden structure characteristics were investigated. Finally, a strata subsidence model was built to analyze the mathematical mechanism of the HWFZ induced by mining activity. The results show that the proposed regression formula and machine learning model can accurately predict the HWFZ of mines in the northwest. Due to the more stable sedimentary environment, the RQD of drilled cores from the Jurassic coal seam is higher and the strata tend to have better continuity as compared to the cores from the Carboniferous coal seam. The strata subsidence model indicates that the residual bulging coefficient influences the HWFZ significantly, which can explain why the measured HWFZ in the northwest is much larger than the predicted values based on the traditional empirical formula. This study is useful to accurately predict and understand the HWFZ in Jurassic coal seams in northwestern China.

Gas generation from coal: taking Jurassic coal in the Minhe Basin as an example

The gas generation features of coals at different maturities were studied by the anhydrous pyrolysis of Jurassic coal from the Minhe Basin in sealed gold tubes at 50 MPa. The gas component yields (C1, C2, C3, i-C4, n-C4, i-C5, n-C5, and CO2); the δ13C of C1, C2, C3, and CO2; and the mass of the liquid hydrocarbons (C6+) were measured. On the basis of these data, the stage changes of δ13C1, δ13C2, δ13C3, and δ13CO2 were calculated. The diagrams of δ13C1–δ13C2 vs ln (C1/C2) and δ13C2–δ13C1 vs δ13C3–δ13C2 were used to evaluate the gas generation features of the coal maturity stages. At the high maturity evolution stage (T > 527.6 °C at 2 °C/h), the stage change of δ13C1 and the CH4 yield are much higher than that of CO2, suggesting that high maturity coal could still generate methane. When T < 455 °C, CO2 is generated by breaking bonds between carbons and heteroatoms. The reaction between different sources of coke and water may be the reason for the complicated stage change in $$\delta^{{{13}}} {\text{C}}_{{{\text{CO}}_{{2}} }}$$ δ 13 C CO 2 when the temperature was higher than 455 °C. With increasing pyrolysis temperature, δ13C1–δ13C2 vs ln (C1/C2) has four evolution stages corresponding to the early stage of breaking bonds between carbon and hetero atoms, the later stage of breaking bonds between carbon and hetero atoms, the cracking of C6+ and coal demethylation, and the cracking of C2–5. The δ13C2–δ13C1 vs δ13C3–δ13C2 has three evolution stages corresponding to the breaking bonds between carbon and hetero atoms, demethylation and cracking of C6+, and cracking of C2–5.