Salinity of flowback fracturing fluid in shale reservoir and its reservoir damage: Experimental and field study

In order to study the salinity of fracturing fluid flowback from shale gas wells and its damage to reservoirs, shale gas wells in PS (Peng Shui) and CN (Chang Ning) areas of Sichuan Basin (China) were taken as research objects in the current work. Immersion experiments of shale particles and single mineral particles were carried out using distilled water. The conductivity of distilled water during immersion was monitored using a conductivity meter, and the ionic composition and the content of immersed fluid were analyzed using ion chromatograph and atomic absorption spectrophotometer. Meanwhile, the flowback fracturing fluids from shale gas wells in PS and CN areas of Longmaxi reservoir in Sichuan Basin, China were collected, and their salinity and composition were analyzed. The results of shale particle immersion experiments showed that the mass concentration of salt in shale was 1 mg salt/g of shale. Furthermore, the salinity of flowback fracturing fluid was found to be about 30,000 mg/L. Compositional analyses of shale particle immersion fluid and field flowback fluid show that the contents of Na+ and Cl− in shale reservoir are the highest. The analysis of field flowback fracturing fluid in shale gas wells shows that the salinity of flowback fracturing fluid is much higher than that of fracturing fluid. High salinity flowback fracturing fluid would produce salt crystals in the later stage of flowback and production stage, blocking fractures and pores, and reducing gas seepage capacity.