Geochemical Characteristics Of Gas And Water In Coalbed Methane Production Wells In East Yunnan And Identification Of Gas Sources

In this paper,eight CBM drainage wells in Enhong block and Laochang Yuwang block in eastern Yunnan are taken as the research objects.Based on the geological background,main coal seam characteristics and actual drainage data in the study area,combined with the laboratory test results of gas and water produced from the main coal seam and coalbed methane combined production wells,the variation law of the output fluid characteristics of each well in different time periods is analyzed,and the output is revealed Based on the overall change trend of fluid and its productivity response,the identification template of gas water production layer source and its contribution is established,and the effective identification of gas source and its contribution of coalbed methane combined production well is realized.In the study area,the buried depth of the main coal seams in the study area is deep in the periphery and shallow in the middle,and gradually deepened in the layer domain;the thickness of the main coal seams is generally thicker in the central part than in the edge,and the thickness in the northeast is thicker than that in the southwest;the gas content of the main coal seam is higher in the northwest than in the core,and the surrounding is lower,and all of them are in the middle high position in the whole area.The gas produced by CBM wells in the study area is mainly high mature dry gas.The main non hydrocarbon gases are nitrogen(atmospheric origin),followed by carbon dioxide(organic origin).With the increase of drainage and production days,the proportion of methane in six wells in Laochang Yuwang block increases in an inclined S-shape,and there are two inflection points,the first inflection point occurs in about 70days of drainage,and the second inflection point occurs in 170 days;compared with the six wells in Laochang Yuwang block,the methane proportion of two wells in Enhong block is relatively stable,and the change trend with drainage time can be seen Cheng is the lower part of the oblique S-shape.The concentrations of Na⁺,Cl^-,HCO₃^-in the produced water of eight CBM wells in the study area are high,while the concentrations of K⁺,Ca²⁺,Mg²⁺,SO₄^2-and F^-are low.With the increase of drainage time,the produced water of H-1,H-2,L-1 and L-2wells is Na-Cl-HCO₃ type,and that of L-3,L-4,L-5 and L-6 wells is Na-HCO₃ type.There is a positive correlation between HCO₃^-and CBM production in CBM wells.When the HCO₃^-concentration exceeds 2500 mg/L,the gas production will be greatly increased.The highest HCO₃^-concentration in the produced water of L-4 and L-6 wells is 3114 mg/L and 2569 mg/L respectively,and their gas production is also the highest.The isotopic values of produced water from H-1,H-2 and L-1 wells show D-shift characteristics,while those of L-3,L-4,L-5 and L-6 wells show O-shift characteristics,while those of L-2 well fluctuate on both sides of the atmospheric drawdown line.Combined with the actual gas production,it can be inferred that when?D is less than or equal to-72.5‰and?¹⁸O is less than or equal to-10.7‰,it is more favorable for gas production.With the increase of buried depth,the content of trace elements in produced water of gas wells changes in a"wavy"shape.Among them,the trace elements in rocks change with the burial depth,showing the characteristics of"double wave peaks",while the contents of trace elements in coal seams change with the burial depth,showing the characteristics of"single wave peak".The buried depth of 700 m is roughly corresponding to the peak value of trace elements in rock or coal seam.Through the analysis,the quantitative characterization range of trace elements in the produced water of high-yield coalbed methane wells is proposed:(1)300?g/L<?_Y<400?g/L and 150?g/L??_M<180?g/L;(2)500?g/L<?_Y<650?g/L and 100?g/L??_M<180?g/L.When the concentration of HCO₃^-is high,the value of?¹³C_DIC is heavier,and the factors of coal seam itself have great influence on the?¹³C_DIC value of produced water.The ¹³C_DIC value of the produced water is positively correlated with the gas production.When the ¹³C_DIC of the produced water is the dissolved source of carbonate minerals in coal and the?¹³C_DIC value is about-3‰,the gas production is higher.The roof structure of the main coal seam is compact,which can effectively block the gas migration of coal reservoir and increase the reliability of the proportion of layer source gas identification.According to the types and maturity of kerogen in the main coal seams,the six drainage wells are divided into three categories,namely,L-1 belongs to the same type(with different stages of homology),L-3 and L-5 are of the same type(with different stages of homology),and the wells of L-2,L-4 and L-6 are of the same type(multi-source and different stages).According to the above three kinds of drainage wells,the CBM source identification templates are constructed respectively,and combined with the actual gas production data characteristics,the production capacity contribution of the drainage wells is divided according to the main coal seams.The dynamic contribution rate of main coal seam gas production with drainage time is analyzed quantitatively.The distribution area of the gas output data of the main coal seam is in the maturity range,which is basically consistent with the measured maturity value of the main coal seam,which proves that the identification result is credible.Combined with the numerical simulation method,the accuracy of the layer source identification template is further verified.