Abstract
Hydraulic fracturing flow-back fluid (FFF) poses a significant challenge to environmental security, and it also can provide valuable information for geological and engineering applications. In order to reveal the source and composition of excess FFF in Jimsar tight oil reservoir, Junngar Basin, Northwest of China, and determine the stage of the hydraulic fracturing flow-back process, this study systematically collected the samples of surface water (SW), hydraulic fracturing fluid (HFF), and FFF from three horizontal wells in the study area. After measuring and analyzing the thousand deviation values of diplogen (δD) and oxygen (δ18O) isotopes from the collected samples, we obtained the geochemical information and the temporal variation characteristics. By analyzing the influence degree of isotope exchange of Water Rock Interaction (WRI) and nonlinear fitting the field measured data of the FFF, the convergent model of δ18O with flow-back time of the FFF was constructed. Based on the law of mass isotope conservation, we proposed a method to calculate the content of formation water in the FFF. The results showed that the SW (δDSW = −73.35‰, δ18OSW = −11.30‰) and HFF (δDHFF = −73.03‰, δ18OHFF = −11.20‰) samples conform to the isotopic characteristics of atmospheric precipitation in the study area. In contrast, the FFF samples are characterized by a positive drift in δ18O and a negative drift in δD, clearly deviating from the atmospheric waterline distribution. The water composition of FFF has changed. The temporal variation characteristics of the δ18O in FFF indicated that the flow-back process can be divided into three stages: The first stage was about 15 days after the flow-back process. As the formation water began to mix into the flow-back fluid, the δ18O of FFF increased rapidly. The second stage was 15–40 days after the flow-back process, and the increase rate of δ18O was smaller than that in the first stage. The isotopic composition of FFF gradually shifted to formation water dominated. The third stage was 40 days after the flow-back process, and the δ18O tended to be stable. To be specific, as formation water was continuously mixed into the fracturing fluid, the composition of FFF gradually changed from HFF dominated to formation water. It was calculated from the field measured data that the original δ18O of formation water was −6.902‰; on the 60th day of the flow-back process, the content of formation water in FFF was about 84% and 81% in well JHW43 and JHW44, respectively. Conclusively, the FFF is mainly composed of formation water.
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