Water Quality Evolution-Guided Pilot-Scale Hardness Control of Saline Coalbed Methane Fracturing Flowback Fluids

Hydraulic fracturing of the Benxi No. 8 coalbed methane reservoir produces saline coalbed methane fracturing flowback fluids whose chemistry changes during flowback, creating a moving treatment target for hardness control, scaling prevention and internal oilfield reuse. This study combined time-resolved gas and water monitoring from Well M, CO₂ pressurization experiments with carbonate bearing drill cuttings and pilot scale treatment to link water quality evolution with fit-for-purpose process design. During flowback, total mineralization increased from 34502.17 to 324236.66 mg/L, pH decreased from 6.56 to 4.17, and Cl^- and Ca²⁺ increased from 20121.26 to 199918.01 mg/L and from 3191.37 to 38216.28 mg/L, respectively. Mg²⁺ reached 6059.27 mg/L, while total organic carbon decreased from 5053.62 to about 1200 mg/L but remained persistent in late flowback. The enrichment of Cl^- and ΣNa⁺ + K⁺ indicated an increasing formation derived brine contribution, while pressurized tests confirmed that dissolved CO₂ promoted Ca²⁺ release from carbonate bearing solids. In pilot operation, alkaline oxidation removed iron ions to below the detection limit and shifted most Mg²⁺ removal upstream. CO₂ microbubble softening converted residual Ca²⁺ into crystalline CaCO₃, and crossflow ceramic membrane filtration removed the generated carbonate particles. The final effluents contained no detectable suspended solids or iron ions, with Ca²⁺ of 75.15 and 57.62 mg/L and Mg²⁺ of 12.16 and 31.30 mg/L in two treatment batches. The integrated process reduced hardness and scaling risk while retaining the dissolved salt matrix, providing a selective treatment route for internal oilfield reuse of saline coalbed methane fracturing flowback fluids.