Geochemical processes during hydraulic fracturing: a water-rock interaction experiment and field test study

Abstract

Various environmental problems related to flowback fluid have recently become a concern for hydraulic fracturing during shale gas production. Detailed studies on the geochemical reactions during hydraulic fracturing are performed for the Lower Silurian Longmaxi shale formation, Fuling block, Sichuan basin, SW China, through water-rock interaction experiment conducted under reservoir conditions, and comparisons with field test data are made. Results show that the total dissolved solids (TDS) of the flowback fluid increases and Ca2+, Na+, Cl– and SO4 2– make dominant contributions. The increases of Na+ and Cl– are assumed to be caused by halite dissolution and mixing with in situ formation water in the marine facies reservoir. The main geochemical reactions are inferred to be pyrite oxidation and the dissolution of calcite, dolomite and plagioclase, resulting in increases of major ions in the flowback fluid. Heavy metals, including Mn, Ni, Mo and Zn, exceed the drinking water criteria of China, and the potential threat from leakage should be a concern. Besides of CH4, as the specific pollutant to groundwater system, inorganic geochemical monitoring of shallow groundwater system should track TDS, major ions of Cl–, SO4 2–, Na+, and Ca2+, trace elements of Li, Ni, Pb, Cu, Zn, Mn, Mo and Rb, and stable isotopes, such as 18OH2O, 2HH2O and 13CDIC.