Geochemical significance of CBM produced water: A case study of developed test wells in Songhe block of Guizhou province

YANG Zhaobiao; WU Congcong; ZHANG Zhenguang; JIN Jun; ZHAO Lingyun; LI Yangyang

Abstract：Based on the produced water tracking test data of CBM wells in Songhe block of Guizhou province,chemical characteristics and geological significance of the produced water in this area were systematically studied.It is found that there is a linkage between the geochemical properties of CBM produced water and production capacity,and the production capacity response index can be used to reflect the CBM production capacity.The results show that the produced water in CBM wells has higher content of Na+ and HCO3 and lower content of Ca2+,Mg2+ and SO42-,which is the performance of CBM reservoir enrichment.The concentrations of K+ and Cl-of characteristic ions are abnormally high and decrease with the increase of drainage time,suggesting that the produced water is contaminated by fracturing fluid.The value of δD is biased showing obvious D drift features,which is the typical characteristic of coal seam water.On the whole,the produced water is characterized by mixed coal seam water with fracturing fluid.As a result of the migration of high position water to the lower part,the spatial distribution of Na+,K+,Cl-shows higher accumulations in the lower part of the well network,which are 3 g/L,160 mg/L,5 g/L respectively,and lower concentrations in the higher part,which are 2 g/L,100 mg/L,3 g/L or so,which is the result of the migration of high position water to lower part.In contrast,the content of HCO4-is high in the higher position at about 900 mg/L,and low in the lower position at about 600 mg/L.It is speculated that this may be caused by the migration of CO2 from the lower to the higher part,which leads to the phenomenon of gas and water differentiation.