Study on evolution characteristics of water ions and isotopes produced by coalbed methane wells

In order to reveal the geochemical characteristics of the produced water from high rank coalbed methane wells and to explore the evolution law of the produced water from coal reservoirs in time and space, water chemistry tests were carried out for the drainage from the 17 production wells in Shanxi Formation in the study area with recovery time of more than 5 years.After having tested the concentrations of anion and cation, hydrogen and oxygen isotopes, and dissolved inorganic carbon isotopes in the water sample, we found significant difference between this study and the water chemistry data for the same study area from 2011 to 2014. The results showed that the width=27,height=16,dpi=110 concentration and TDS content in the water sample were lower than those in previous reports, and the δD test value was higher than older data. By comparing the test data from different seasons, it is concluded that the concentration of width=36,height=17,dpi=110 and Cl- is the most susceptible to seasonal change; the concentration of Ca2+ and Mg2+ can undergo abrupt spike; and the impact on concentration of width=27,height=16,dpi=110 from seasonal changes is the least significant. Further study also reveals that the concentration of width=36,height=15,dpi=110 and Cl- is also affected by seasonal precipitation to larger degree, the concentration of Ca2+ and Mg2+ is mostly affected by geological factors, and the concentration of width=27,height=16,dpi=110 is mainly controlled by desulfurization. The isotope analysis of water samples reveals a positive correlation between δ13CDIC and δ18O, and a negative correlation between δ13CDIC and δD, It is also found that δ13 CDIC is disproportional to the concentration of width=32,height=16,dpi=110 The δD tends to intensify as the discharge year increases, and gradually shifts to the upper level of the atmospheric precipitation line, indicating that as the discharge year increases, the drainage range from coal reservoirs would expand from cracks to distant places, resulting in the continuous discharge of original pore water from the coal seam which causes the gradual intensification of δD along with time. Analysis also indicate that δ13 CDIC can be useful for assessing the local sealing of coal reservoirs. Finally, this work led to the establishment of a geochemical reaction model illustrating how the produced water in the coalbed methane well evolves along with time and space.