A Coupled Model of Acid Transport, Gel Cleanup, and Fracture Propagation in Prepad Acid Fracturing

In conventional hydraulic fracturing of low-permeability sandstone reservoirs, polymer-gel leak-off creates low-permeability filter cakes that impair productivity. This study proposes a prepad acid fracturing technique using a fluoroboric acid (HBF₄) pre-flush to dissolve gel residues and mineral fines. A fully coupled mathematical model integrates HBF₄ hydrolysis kinetics, multi-mineral surface reactions, porosity-permeability evolution via the Panda-Lake model, and dynamic leak-off coefficient feedback. Simulations show HBF₄ decreases monotonically along the fracture while HF peaks at 40-60 m from wellbore. Acid concentration in the leak-off zone decays exponentially, defining a gel-dissolution zone within 0.5 m of the fracture wall. Acid dissolution increases near-wall porosity to 12-15% and permeability to 2.5-3.5 mD (3- to 4-fold). The leak-off coefficient varies dynamically: high in the acid-dominated zone (1.5-2.2 × 10^-3 m/√min) favoring gel dissolution, and low in the gel-dominated zone (≈0.8 × 10^-3 m/√min) promoting fracture extension. Compared with conventional polymer gel fracturing, the proposed method achieves a 15.9% higher stimulation ratio and 22.5% higher productivity after 100 days, despite slightly shorter fractures. The core advantage is restoring leak-off zone permeability from 0.45 mD to 0.85 mD and increasing gel filter cake permeability from 8 × 10^-4 mD to 0.1 mD, with an average relative error of 8.2% against experimental data. These findings provide theoretical guidance for optimizing prepad acid fracturing in gel-damaged low-permeability sandstones.