Implications of the geochemical characteristics of post-fracturing flowback fluids for shale gas exploration and exploitation

Previous studies on post-fracturing flowback fluids focus primarily on their cleaning and discharge, high salinity sources, and damage to gas reservoirs. An analysis of the geochemical characteristics of these fluids helps develop an improved understanding of the preservation condition and fracturing performance of shale gas reservoirs. This study analyzed the ion, total dissolved solids (TDS) concentration, and stable isotope characteristics of post-fracturing flowback fluids from five horizontal shale gas wells in the Luzhou area. Among these wells, two were subjected to hydraulic fracturing using fresh water, and three using reused flowback fluids. The results indicate that with increasing flowback time, the post-fracturing flowback fluids from wells subjected to hydraulic fracturing using fresh water showed increased TDS concentration, heavier stable isotopes, and the presence of new ion components. These results indicate the mixing of a large volume of formation water into the fluids. In contrast, post-fracturing flowback fluids from wells subjected to hydraulic fracturing using reused flowback fluids exhibited a slow increase in the TDS concentration and stable isotopes. As the flowback time increased, the trends in TDS concentration and stable isotope ratios of post-fracturing flowback fluids from shale gas wells subjected to fracturing using fresh water evolved toward those of post-fracturing flowback fluids from shale gas wells undergoing fracturing using reused flowback fluids. Measurements show that post-fracturing flowback fluids from both well types exhibited roughly the same properties after one year of shale gas production. This result suggests that post-fracturing flowback fluids from wells using reused flowback fluids progressively took on the formation water properties. In particular, post-fracturing flowback fluids from well Lu 211—a well subjected to hydraulic fracturing using fresh water—showed a low sodium-chloride coefficient, a low coefficient of variation, high TDS concentration, heavy stable isotopes, and a high nitrate ion concentration. This indicates a formation water source of the fluids and the poor sealing of the formation water, which hinders shale gas enrichment. The quantification of the fracturing fluid and formation water contents in the post-fracturing flowback fluids reveals that higher TDS concentration and heavier stable isotopes in the fluids appear to correspond to higher formation water content and lower fracturing fluid content, as well as higher fracturing performance. A systematic analysis of the geochemical characteristics and flowback pattern of fracturing fluids indirectly provides insights into the flow path of formation water, water body mixing, rock-water interactions, and fluid sources. Besides, the analysis offers a new perspective for understanding the preservation conditions and fracturing performance of shale gas reservoirs.