Evolution of chemical components in produced water from coalbed methane wells in Zhengzhuang block of Qinshui Basin and its response to high productivity

The chemical characteristics of produced water from coalbed methane (CBM)wells are critical indicators of high productivity. This study targets at the produced water from vertical CBM wells in the Zhengzhuang block of Qinshui Basin, which have entered a stable drainage and production phase. Long-term sampling and testing were conducted to analyze the salinity and major ion characteristics of produced water, and then explore chemical features and response mechanisms of produced water to high CBM productivity, thus providing valuable insights for production practices. (1) After entering the stable production stage, the produced water from CBM wells has the salinity ranging between 2 000 mg/L and 3 000mg/L, and is mainly of Na-HCO3type, reflecting a well-sealed hydrogeological environment. (2)During the drainage and production process, the migration of salinity centers in the Zhengzhuang block led to the W-shaped variation in the salinity and the main ionic components, such as Na++ K+and HCO3-, across different production areas. Specifically, the salinity fluctuation amplitude in the Zheng 1, Zheng3, Zheng4, and Dong production areas shows the characteristics of increasing, decreasing, becoming smaller and then larger, and larger and then smaller, respectively, as compared with the average salinity of the block. (3) In the relatively high-yield Zheng3 and Zheng4 production areas, the contents of Ca2+and CI show positive and negative correlations with the gas-water ratio over time, respectively. The change rate in Zheng 3 production area is significantly higher than that in Zheng 4 production area, while other areas show no significant correlation. (4)Two response models of chemical characteristics to high productivity in CBM wells are proposed for the produced water. As gas production efficiency increases, more CO2dissolved in water promotes the dissolution of calcite veins, leading to a Ca2+enrichment; hydrated CI reacts with maceral components in the coal via polar nitrogenous functional groups, which can increase the likelihood of forming organic complexes, leading to a CI consumption. © 2024 Science Press. All rights reserved.