Evolution path of water produced from coalbed methane wells and its significance in judging productivity:a case study of Zhijin block in western Guizhou Province
The water produced from coalbed methane(CBM)wells contains rich information on geology and productivity,which can reveal the geochemical characteristics and main controlling factors of produced water and make clear the evolution path of produced water and its inherent link with production capacity,thus helping understand the heterogeneity of production capacity and provide a basis for the optimization of development programs.In recent years,significant breakthroughs have been made in the CBM explora-tion and development in Zhijin block of western Guizhou Province,providing a technical demonstration for the CBM development multiple thin coal seams in South China.However,there are still a number of problems,including great variability in produced water quality,high uncertainty in production capacity,and unclear control factors for gas production.In this study,hydrogeochemical tests were performed on the water produced from CBM in Zhijin block of western Guizhou Province,which aims to explore the hydrochemical characteristics,evolution path and its significance in judging productivity.The quality of the produced water can be divided into Na-HCO3 and Na-Cl types;the total dissolved solid(TDS)of the former ranges from 944mg/L to 2681 mg/L,and that of the latter ranges from 3 603 mg/L to 8 800 mg/L.According to the scale of TDS,the five clusters of samples show the successively increasing degree of groundwater retention.Desulphidation,cation exchange adsorption,and concentrating action are the main fac-tors controlling the hydrochemical characteristics and evolution of produced water.The produced water is of Ca-SO4 type with low TDS(＜200mg/L)under oxidation conditions,Na-HCO3 type with medium TDS(900-3000mg/L)dominated by desulphidation and cation exchange adsorption under reduction conditions,and Na-Cl type with high TDS(3500-9000mg/L)dominated by concentrating action under high retention rate.The evolution process of produced water can be divided into reduction stage and retention stage,and the produced water(TDS＞3 000 mg/L)in the retention stage corresponds to high-yield wells.High-yield wells can be effectively identified by extracting key hydrochemical indicators and their critical values for identification of CBM production capacity,and ana-lyzing the concentrations of characteristic ions such as Cl-,Na+and Sr2+;the desulphidation coefficient γSO42-/γCl-can be used to effectively identify low-yield wells.The CBM wells in Zhijin block of western Guizhou Province should be located in groundwater retention areas with high TDS,high Cl- and low SO42-,instead of strong groundwater runoff zones.The axial part of Zhucang sub-syncline is a favorable area for deployment of CBM wells.Hydrodynamic interference and its resulted low pressure reduction efficien-cy are still the key factors restricting the multi-seam co-production capacity of vertical and directional wells.In conclusion,these findings can serve as a foundation for the efficient CBM development and produced water treatment in Zhijin block.
produced waterwater chemical evolutionwestern Guizhoucoalbed methane co-productionfluid interference
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