Study of Acid-Fracturing Model and Optimization of Volume Stimulation Technology Based on Well Logging Data and Acoustic Remote Detection for Fracture-Vuggy Carbonate Reservoirs

Abstract The deep and ultra-deep carbonate reservoirs in the Sichuan Basin are a strategic priority for future natural gas development in China. These formations exhibit extremely high temperatures, pronounced horizontal stress differentials, and a heterogeneous distribution of fracture–cavity systems. To achieve optimal reservoir stimulation, it is crucial to generate artificial fractures through acid fracturing and to enhance the connectivity of fracture–cavity systems in three dimensions by leveraging both physical and chemical mechanisms. This study presents an integrated approach to optimizing acid fracturing parameters. First, a geological characterization model is developed to capture the heterogeneity of near-wellbore and far-field fracture–vuggy systems. The porosity and permeability distributions of far-field systems are inversely calibrated using acid fracturing history matching from 12 wells. Next, a thermal–hydro–mechanical–chemical (THMC) coupled acid fracturing model based on planar 3D fracture propagation is established. The Embedded Discrete Fracture Model (EDFM) is employed to incorporate natural fractures, simulating fluid flow and acid etching along these features. Equivalent permeability is used to quantify the stimulated reservoir volume (SRV) of acid-etched fracture systems. Numerical simulations yield several key insights. Fracture–vuggy systems significantly alter hydraulic fracture propagation, reducing total fracture length by 53∼60%. Increasing fracturing fluid viscosity (35–50 mPa·s) improves penetration capacity but raises the risk of vertical breakthrough. During the acid etching phase, substantial acid loss reducing effective etched length by 27.3%. Acid volume escalation yields limited improvement in etched length but significantly enhances lateral connectivity. Field application in Well A demonstrates the effectiveness of the optimized approach, increasing SRV from 4,605 m3 to 35,335 m3 and achieving a daily gas flow rate of 1.565 million cubic meters, and pressure decline analysis confirms effective connectivity with far-field fracture–vuggy systems.