Staged Acid Fracturing in Ultra-Deep Carbonate Formations: Application to Horizontal Wells in the Sichuan Basin

The Qi Xia Formation reservoir in the western Sichuan Basin is characterized by ultra-deep burial depth, high temperature (>160 °C), and strong heterogeneity. Early stimulation strategies predominantly relied on vertical wells with blanket acid fracturing, while horizontal well completion technologies faced limitations in fully unlocking reservoir potential. Key challenges in staged acid fracturing for ultra-deep horizontal wells include: (1) difficulties in precise staging due to reservoir heterogeneity; (2) stringent performance requirements for fracturing fluids under high-temperature conditions; (3) rapid acid-rock reaction leading to limited etching penetration; and (4) low fracture conductivity under high closure stress. To address these challenges, an integrated approach was developed. First, a refined staging design methodology was established by incorporating 23 geological and engineering parameters (e.g., porosity, permeability, injection rate, and treatment scale). Reservoir stimulation coefficients were calculated using machine learning-based intelligent methods, with optimal staging schemes determined through gas reservoir numerical simulation. Second, high-temperature-resistant self-generating acid and gelled acid systems were engineered. The self-generating acid exhibited a viscosity >150 mPa·s after 60 min shearing at 170 s−1 and 160 °C, with a maximum acid concentration of 4.05 mol/L. The gelled acid system demonstrated both thermal stability (160 °C) and superior friction reduction (>70%), meeting requirements for retarded reaction rates and corrosion inhibition. Third, a deep-penetration, high-conductivity stimulation technique was implemented via alternating injection of self-generating acid and gelled acid. Compared to conventional acid fracturing, this approach increased acid-etched fracture length by 80% and conductivity by 45%. Field applications in horizontal wells targeting the Qi Xia Formation at Shuang yushi yielded a threefold productivity enhancement over vertical wells, validating the effectiveness of this technology in heterogeneous ultra-deep carbonate reservoirs.