Geochemical comparison and its geological significance of CBM produced water in the Songhe and Zhijin blocks

Based on the conventional ion,trace element and hydrogen and oxygen isotope test of 16 CBM wells pro- duced water in Songhe and Zhijin blocks,combined with its geological background and productivity characteristics,the geochemical characteristics of the two blocks’s produced water and significance to production capacity are compared and analyzed. Research shows that,the concentration of Cl- and Na+ in Songhe produced water is abnormally high and the water quality is Na-Cl,indicating that formation water is polluted by fracturing fluid. The water produced in Zhijin comes from coal seams,the Na+ and HCO- are enriched and the water quality is Na-HCO3 ,reflecting the relatively closed groundwater environment. Both the Songhe and Zhijin produced water show obvious D drift characteristics,and the higher content of hydrogen and oxygen in the coal of Songhe area lead to the heavier hydrogen and oxygen isotopic composition in the produced water. Except Li element,the dissolution of most trace elements in Songhe produced water are greater than that of Zhijin block. A more universal closeness index reflecting formation water environment is put forward. The correlation between the closeness index and daily output and accumulated water yield is significant,the medium closeness index is more conducive to achieving high yield,and it can be extracted as the geochemical index to evaluate productivity potential. The identification template of production water source based on HCO- and Cl- concen- tration is preliminarily established,and two zones are divided:low-yield wells producing fracturing fluid zone and high- yield wells producing coal seam water zone.