沁水盆地郑庄区块煤层气井产出水化学成分演变及其高产响应

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

煤层气井产出水的化学成分变化对煤层气高产具有重要的指示作用。以沁水盆地郑庄区块进入稳定排采阶段的煤层气直井产出水为研究对象,长期跟踪采样与测试,开展矿化度和主要离子特征分析,探讨产出水的化学特征及其对煤层气井高产的响应机制,取得了一些对生产实践具有参考价值的认识:(1)在煤层气井进入稳产阶段后,产出水的矿化度为2000~3000 mg/L,化学类型主要为Na-HCO3型,反映出具有较好的水文地质封闭环境;(2)在排采过程中,由于郑庄区块的矿化度中心迁移,引起各采区产出水的矿化度以及Na^(+)^(+)K^(+)和HCO_(3)^(-)等主要组成离子含量随排采时间的推进总体呈不同幅度的“W”型,其中,郑1采区、郑3采区、郑4采区和东采区矿化度的波动幅度相比于区块的平均矿化度分别表现出增大、减小、先小后大和先大后小的特征;(3)随着排采的推进,在相对高产的郑3采区和郑4采区,Ca^(2+)、Cl^(-)含量分别与气水比呈正、负线性相关关系,且郑3采区的变化率明显高于郑4采区,而在其他采区,相关性则不明显;(4)构建了两种煤层气高产的产出水地球化学特征响应模式,即随产气效率增加,煤层中水溶CO_(2)的增加促进了方解石脉的溶解,致使Ca^(2+)富集,而水化的Cl^(-)通过煤中的极性含氮官能团与显微组分反应可增加形成有机配合物的机率,致使Cl^(-)消耗。

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 2000 mg/L and 3000 mg/L,and is mainly of Na-HCO3 type,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 HCO_(3)^(-),across different production areas.Specifically,the salinity fluctuation amplitude in the Zheng1,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 Ca^(2+)and Cl^(-)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 CO_(2)dissolved in water promotes the dissolution of calcite veins,leading to a Ca^(2+)enrichment;hydrated Cl^(-)reacts with maceral components in the coal via polar nitrogenous functional groups,which can increase the likelihood of forming organic complexes,leading to a Cl^(-)consumption.