Geochemical characteristics and significance of water produced by coalbed methane wells in Lu’an Mining Area

Hydro-geochemistry analysis is an important method for CBM exploration and development research. In this paper, the distribution characteristics of salinity, H/O stable isotope and ion concentration in sample water was investigated, and the optimal CBM development area was analyzed by the characteristics of regional structural distribution and the CBM development history. The testing water was from the No.3 CBM well of the Shanxi Formation Permian system in the Lu’an Mining area in the south-central part of Qinshui Basin. The results show that there are three high salinity areas in the mining area, with an average groundwater salinity of 2 000-3200 mg/L, including Wuyang Minefield, southwestern Yuwu, and northern Gaohe-Gucheng Minefield. And there is a low salinity area in Changcun minefield and east-central Yuwu Minefield, with an average groundwater salinity of about 1 500 mg/L. The \delta \rmD and \delta ^18\rmO of testing water are close to the atmospheric precipitation line in the research area, indicating that all aquifers are supplied by atmospheric precipitation, and the water in the coal seam mainly comes from atmospheric precipitation. It also shows that the surface water in Lu’an Mining Area moves downward from the exposed rock stratum of Taihang Mountain in the east to the coal-bearing stratum and the lower Ordovician aquifer, and then exposes eastward in the limestone series to supply the Xin’an spring area. The Wenwangshan Fault and Ergangshan Fault are open water-conducting faults, providing channels for groundwater migration. Their distribution characteristics determine the basic flow law of groundwater, with lower gas content in the area. There are three groundwater retention areas with higher salinity, including Zhonghua-Anchang fault, Tiancang syncline and Xucun syncline. They are not only gas enrichment areas, but also preferred areas for CBM development.