Geochemical Characteristics And Indicator Function Of Drainage Fluid In CBM Wells In Tiefa Mining Area

In this thesis,about 14 coalbed methane(CBM)wells are selected as research objects in the Beier Mining Area of Daxing Mine in Tiefa mining area.Ion chromatograph,inductively coupled plasma mass spectrometer,laser water isotope analyzer,isotope ratio mass spectrometer and other instruments were used to test the conventional ions,trace elements,hydrogen and oxygen isotopes and dissolved inorganic carbon isotopes in the produced water,gas components and carbon and hydrogen isotopes of methane in the produced gas.Based on geological data and the achievement of CBM exploration.The time-varying characteristics,spatial evolution characteristics of geochemical characteristics of CBM well drainage fluid(produced water and produced gas)are analyzed.The evolution mechanism of the geochemical characteristics of the drainage fluid is clarified.The source of the drainage fluid and the closeness of the strata are discussed.The significance of the geochemical characteristics of produced water to the productivity of CBM wells are revealed.The main understandings are as follows:The time-varying characteristics,spatial evolution characteristics of geochemical characteristics of CBM well drainage fluid are analyzed.The conventional ions of produced water increase or decrease with the drainage time and then tend to be stable,which is due to the flowback of fracturing fluid.Water-rock interaction results in different spatial distribution of different ions.The concentration of Li,Cr,and Sr is negatively correlated with the buried depth and is affected by faults.The positive correlation between δD and δ18O in produced water and buried depth is due to isotope exchange and isotope fractionation caused by methane production activity.The negative correlation between δ13CDIC and burial depth is due to the forced use of heavy carbon isotopes by methanogens.The volume fraction of CH4 is positively correlated with the buried depth,and the volume fraction of CO2 and N2 is negatively correlated with the buried depth.The positive correlation between δ13CCH4 and δDCH4 and burial depth is due to the dissolution and fractionation of water and the change of CH4 generation pathway by methanogens.The evolution mechanism of the geochemical characteristics of the drainage fluid of CBM wells are clarified.In the shallow oxidizing environment,dissolution,oxidation,etc.increase the conventional ions and some trace elements in the water,and produce negative δ13CDIC,evaporation makes the water rich in D and 18O.In the process of runoff,the alternate adsorption of cations reduces Ca2+ and Mg2+and increases Na+;isotope exchange causes D and 18O in coal to enter the underground water;methanogens preferentially absorbs the lighter H and 12C,makes CBM lighter,enriches in D and 13CDIC in water.In the deep reducing environment,sulfate reduction,carbonate precipitation and methanogenesis increase the concentration of HCO3-,Na+,Cl-,decrease Ca2+,Mg2+,SO42-enrich D isotopes,and decrease δ13CDIC slightly,13CCH4 and DCH4 are more enriched;The restriction of HCO3-,Ca2+concentration and the influence of the reducing environment cause the concentration of Li,Cr,and Sr to decrease with the increase of buried depth.The source of the drainage fluid of CBM wells was discussed,and the indication significance of the geochemical characteristics of the produced water to the productivity was revealed.The water source identification index based on conventional ions was established to classify coal seam water and fracturing fluid polluted water.The initial source of formation water is atmospheric precipitation,and the produced water comes from the mixture of coal seam water and sandstone water in the upper and lower fracturing sections.The stratum sealing is relatively general,in the vertical direction,the stratum sealing increases as the depth increases.CBM is of mixed origin,and nearly half is biogenic gas.CO2 reduction biogas is more advantageous than acetic acid fermentation biogas.In the plane,high TDS and HCO3-,Na+concentrations,and low Li,Cr,Sr concentrations indicate high productivity of CBM wells.In the vertical direction,high δ13CDIC values and low δD and δ18O values indicate high productivity of CBM wells.