页岩气压裂返排液/采出水同位素地球化学特征与鉴别指标

Isotopic geochemical characteristics and identification indexes of shale gas hydraulic fracturing flowback water/produced water

中国页岩气资源丰富,主要分布在人口密集的南方海相碳酸盐岩分布区,地质地表条件复杂,生态环境脆弱,水资源匮乏或分布不均,大规模压裂开采存在很高水资源利用压力和严重水环境污染风险。综合分析页岩气压裂返排液/采出水的氢、氧、硼、锂、锶等传统与非传统稳定同位素地球化学特征显示:四川盆地返排液/采出水的氢氧同位素与寒武系、二叠系和三叠系的须家河组、嘉陵江组、雷口坡组常规井采出水演化趋势相近,与震旦系常规井采出水不同,表明四川盆地返排液/采出水为压裂注入液与滞留在志留系页岩中的地层卤水的混合产物,其高盐度端元化学组成更接近寒武系地层卤水,但其δ^(11)B值要高于后者。四川盆地页岩气压裂返排液/采出水δ^(11)B值与美国Marcellus的相近,都与蒸发相海水有关,与盆地内不同层系常规井采出水有一定的重合,无法进行精确区分,但其δ^(11)B和B/Cl值与河水和非海相的柴达木盆地页岩气井返排液/采出水能够明显区分。四川盆地页岩气压裂返排液/采出水δ^(7)Li值稍高于美国Marcellus的,与长江水的δ^(7)Li值有一定重合。威远页岩气示范区返排液/采出水^(87)Sr/^(86)Sr均值为0.7197,长宁示范区则为0.7193,远高于不同层系常规井采出水的^(87)Sr/^(86)Sr值,这是因为志留系页岩受到陆源硅质沉积物和下伏富Sr流体的影响,导致四川盆地南部志留系地层实测^(87)Sr/^(86)Sr值偏高,这使得^(87)Sr/^(86)Sr值成为鉴别四川盆地页岩气压裂返排液/采出水与常规气井采出水和浅层地下水的有效指标。

China is rich in shale gas resources,which are mainly distributed in densely populated southern marine carbonate areas.The geological and surface conditions are complex,the ecological environment is fragile,and water resources are scarce or unevenly distributed.Therefore,large-scale fracturing mining has high pressure of water resources utilization and serious risk of water environment pollution.In this paper,the geochemical characteristics of traditional and non-traditional stable isotopes such as hydrogen,oxygen,boron,lithium,strontium in shale gas hydraulic fracturing flowback water/produced water were comprehensively analyzed.The results show that,the hydrogen and oxygen isotopic composition of the flowback water/produced water in Sichuan Basin has similar evolution trend with the produced water of conventional wells from Cambrian,Permian and Triassic Xujiahe,Jialingjiang and Leikoupo formations,but different from that of the Sinian conventional produced water.It indicates that the flowback water/produced water in Sichuan Basin is a mixture of fracturing injected fluid and formation brine retained in Silurian shale.The saline end member is close to the formation water of Cambrian,but with higher δ^(11)B values.The flowback water/produced water in Sichuan Basin has δ^(11)B values close to that of the Marcellus flowback water/produced water,and both are derived from the evaporated seawater.The δ^(11)B values of flowback water/produced water in Sichuan Basin have overlap with that of the conventional produced water from different strata,so it can not be precisely distinguished.However,the δ^(11)B and B/Cl values of the flowback water/produced water in Sichuan Basin can clearly distinguished from the river and the flowback water/produced water from non-marine facies shale in Qaidam Basin.The flowback water/produced water in Sichuan Basin has slightly higher δ^(7)Li values than that of the Marcellus flowback water/produced water,but has overlap with that of the Yangtze River.The average value of ^(87)Sr/^(86)Sr of flowback water/produced water is 0.7197 in Weiyuan and 0.7193 in Changning,which is much higher than that of the produced water from conventional wells in different formations.This is because the Silurian shale is affected by terrestrial siliceous sediments and the underlying Sr-rich fluids.As a result,^(87)Sr/^(86)Sr values measured in the Silurian strata in southern Sichuan Basin are high,which makes ^(87)Sr/^(86)Sr values become an effective index to distinguish shale gas fracturing flowback water/produced water from conventional gas well produced water and shallow groundwater in Sichuan Basin.