页岩气开发中返排液/产出水的水质演化机理

The shale gas revolution, promoted by hydraulic fracturing technology, has changed the energy structure of the United State. However, large-scale application of hydraulic fracturing has also lead to environmental concerns, causing shale gas exploration and production being forbidden in many countries. Shale gas resource reserve is huge in China and it’s already commercially produced in the Sichuan Basin, which also has potential environmental issues. With consideration of the source and formation mechanism of pollutants in groundwater and difficulties in tracing or identifying pollutants effectively, the typical marine shale reservoir of the Lower Silurian Longmaxi Formation in the southern China has been chosen to do the following studies with combination use of water chemistry and various isotopes: firstly, to determine the source of water and solute in the formation water; secondly, to characterize the variations of water chemistry and isotopes (boron, strontium, lithium and chlorine) of the flowback fluid and produced water with time during the whole production process after hydraulic fracturing; then, to discuss the source of water (end-members includes fracturing fluids and formation water) and solutes (including the mixing of formation water, water-rock interaction and etc.) and the evolution mechanisms of flowback fluid and formation water and also quantifying contributions from different end-members based on the fracturing fluid-shale interaction studies; finally, to establish the tracer-based methods and effective monitoring indexes for groundwater pollution based on comparisons of hydrogeochemistry of shallow groundwater, formation water, flowback fluid and produced water. This can provide scientific basis for effective monitoring and identifying groundwater pollution during the shale gas production.

页岩气革命改变了美国能源结构，影响了全球能源格局。但大规模水力压裂也带来了诸多环境风险，特别是水资源消耗和水污染问题，引起了公众的广泛关注与争议。针对页岩气开发过程中地下水污染物来源和形成机制不清楚，难以有效地识别潜在污染等问题，本项目拟选取我国南方下志留龙马溪组海相页岩开采井，利用水化学及同位素手段，（1）确定页岩地层水水分及溶质来源；（2）刻画压裂后返排液/产出水的水化学及同位素（硼、锶、锂等）随时间的变化规律，结合室内页岩-压裂液反应实验与模拟，探讨其水分（端元包括压裂液和地层水）及溶质来源和形成演化机理（包括地层水混入和水岩反应等），定量评价不同阶段各个端元的贡献比例；（3）基于浅层地下水、返排液/产出水的水化学同位素差异，建立页岩气开发的地下水环境监测指标和污染识别方法，为地下水环境效应评价提供一定的技术支撑。

由水力压裂和水平钻井技术发展引发的页岩气革命改变了美国能源结构，影响了全球能源格局。但大规模水力压裂也带来诸多环境风险，特别是水污染和水资源消耗问题，引起了公众的广泛关注。针对页岩气开发过程中地下水污染物来源和形成机制不清楚，难以有效识别潜在污染等问题，本项目以中科院贵州习水钻测平台以及中石化重庆涪陵气田为依托，利用野外自主可控的水力压裂科学试验和相匹配的室内水岩反应实验，在获取迄今分辨率最高的返排液基础上，通过应用多种环境示踪剂，精细刻画了水力压裂中水循环规律及地球化学过程，建立了地下水环境敏感性监测与污染示踪指标的确定方法。主要创新性成果有：①精细刻画了水力压裂中水循环规律及主要地球化学过程，发现研究区海相页岩地层水起源于大气降水或与蒸发海水的混合，这与普遍认为的（蒸发）海水起源不同，丰富了页岩地层水来源理论；指出返排液是由压裂液和地层水混合而成，且后者占主要，而不是前人认为全是由压裂液组成；发现了在野外压裂过程中黄铁矿氧化及相应的储层改造作用有限；②建立了页岩气开发的地下水环境敏感性监测与污染识别指标的确定方法，在涪陵气田发现Cl、Br、Na、Li、Ba、B同位素、Sr同位素等7项指标可识别0.05%地层水混入导致的浅层地下水污染；在气体污染示踪方面，除碳氢同位素外还引入了惰性气体同位素来增加可靠性，这些为页岩气开发的水环境监测与保护提供了重要的科学方法；③揭示了吸附-解吸附和离子交换作用对溶质同位素示踪的影响，利用顺序提取法确定了岩土体水溶性、吸附态和碳酸盐三个端元含量及同位素组成，指出粘土的解吸附是返排液溶质的一个重要来源，对流体同位素影响取决于解吸附量及吸附态的同位素特征，并揭示出流体与吸附态Sr的同位素平衡可导致流体87Sr/86Sr的变化，进而影响Sr同位素的示踪。以上研究提升了对深部水文地质过程和复杂体系水岩相互作用机理的认识，并为水力压裂条件下水环境效应评价提供了重要的科学依据。