Experimental study on hydrogen and oxygen isotope exchange in water-rock interaction of Lucaogou formation in Jimsar sag

Hydrogen and oxygen isotopes provide new evidence for estimating the composition of hydraulic fracturing flowback fluid. However, water-rock interaction between the fracturing fluid and the reservoir rock could have a negative effect. Thus, a comprehensive investigation of hydrogen and oxygen isotope exchange during water-rock interaction can greatly help the accurate understanding on the composition of the flowback fluid. In this study, water samples used to configure fracturing fluid and core samples of tight oil reservoirs from Lucaogou formation in Jimsar sag were obtained, and the isotope exchange experiment in water-rock interaction was performed. The variations of the experimental temperature, water-rock reaction time, and particle sizes of core samples on the isotope exchange during water-rock interaction are considered. After water-rock interaction, δ2H and δ18O of two surface water samples and 34 aqueous solution samples were measured. The experimental results indicated that isotope exchange did occur in the water-rock interaction. However, at a reservoir temperature of 90°C and a limited fracturing period, the exchange of isotope during water-rock interaction was not obvious. Reducing the particle size of core samples could increase the isotope exchange rate. In summary, the hydrogen and oxygen isotope exchange caused by water-rock interaction has little influence on the isotope characteristics of hydraulic fracturing flowback fluid in the Lucaogou formation.