Fracturing flowback fluids from shale gas wells in western chongqing: Geochemical analyses and relevance for exploration & development

Abstract Understanding the behavior and composition of fracturing flowback water (FFW) can provide insight into in situ water–rock interactions, assessment of the success of the fracturing operations. FFW was collected from three wells (Z202-H1, Z203, and Z205) for up to 108 days after fracturing in the same area of western Chongqing, China. The samples were analyzed for the concentrations of various ions (Na+, K+, Ca2+, Mg2+, Ba2+, Sr2+, Cl−, SO42−, Br−, HCO3−, etc.) and for the stable isotope composition (δD and δ18O) of water. With increasing flowback time, the ionic concentration and total salinity increased (e.g., from 315 mg/L to 37117 mg/L after 38 days for well Z203), stable isotopic ratios became heavier (e.g., δD values changed from −23.59‰ to −14.32‰, δ18O values changed from −3.91‰ to −1.92‰). The total salinity of the FFW is shown to be the result of mixing of the highly saline formation water and the low-salinity fracturing water. FFW from Z205 had higher concentrations of Li+ and NO3−, heavier stable isotope compositions, larger Na+/Cl− ratio, smaller (Cl−-Na+)/Mg2+ ratio, and larger SO42− × 100/Cl− ratio compared to the other two wells. All these phenomena revealed that Z205 is more likely to contact with active aquifers which is not conducive to natural gas preservation, because Z205 is close to (less 300 m from) a grade II fault. The RITS and RSIH with flowback time in Z203 were higher than Z202-H1, which shows that FFW from Z203 contained a greater fraction of formation water released from pores or fractures due to complex the network fractures formed by fracturing. Therefore, the fracturing operations of Z203 is better than Z202-H1. This result can reveal the reason for the production difference of adjacent wells, which is difficult to explain by similar total SRV.