Noble isotopic gas fractionation characteristics of China's first batch commercial shale wells in the early production stage

In this study, He and Ar isotope fractionation characteristics in production gases of six Jiaoye shale wells were analysed to identify the fugitive gases occurring in overlying rocks. It was found that both radiogenic Ar-40 and He-4 concentrations decreased first and then increased due to the release of blocked Ar-4 and He-40 in the small pores as formation pressure dropped. However, the He-4/ Ar-40 ratio slightly decreased since He-4 mitigated fast in the shale matrix. In addition, the ratio of radiogenic Ar-40 to primordial Ar-36 exhibited a monotonic increasing trend due to the degassing of Ar-40 from rocks. It is reasonable to assume that there was not convention through the pathway in overlying rocks created by hydraulic fracturing as the concentration difference of He-3/ He-4 and Ar-40/ Ar-36 between formation and atmosphere did not decrease. Based on noble gases information, the hydraulic fracturing did not destroy the integrity of shale formation overlying rocks. [Received: May 13, 2021; Accepted: January 27, 2022]