Comprehensive Analysis of Conductivity for Complex Acid-Etched Fractures: Findings from Ultra-Large-Scale Acid Fracturing Experiments

In carbonate reservoir stimulation, acid fracture conductivity is a paramount parameter for acid fracturing performance evaluation. Contemporary modeling paradigms exhibit limitations by concentrating primarily on simplified, geometrically ideal fractures at laboratory scales – a stark contrast to the intricate, heterogeneous fracture architectures encountered in actual field operations. More critically, the industry lacks methodologies for evaluating interconnected fracture systems’ conductive properties. In light of this, a conductivity calculation model for complex acid-etched fractures through large-scale acid fracturing experiments was developed. Utilizing 2 m × 2 m × 1 m specimens, the research first obtains acid-etched fracture geometry and zero-closure-stress conductivity data. The methodology combines 3D laser scanning for fracture morphology and triaxial compression tests for post-etching mechanical properties, providing key inputs for model development and validation. The developed model combines an acid fracture closure mechanism based on linear elastic theory with continuity equations, enabling comprehensive conductivity calculations for complex fracture systems. Based on experimental results, the ultra-large-scale acid fracture network exhibits a fracture conductivity exceeding 103 D·cm under zero closure stress, which is close to that calculated from the established model, verifying the accuracy of the model. The numerical simulation results indicate that the conductivity of complex fracture networks is significantly influenced by the characteristics of branch fractures. When the density of branch fractures is high, the connectivity between fractures improves, forming continuous flow channels that enhance overall fracture conductivity. The orientation of branch fractures, in conjunction with the dominant flow direction, also affects the conductivity of complex fracture systems. When the orientation of branch fractures aligns with the dominant flow direction, the conductivity of the complex fracture network improves significantly. Increasing the length of branch fractures not only enhances connectivity between fractures, but longer fractures also improve overall reservoir permeability by connecting a larger portion of the reservoir, thereby further enhancing the conductivity of the complex fracture network. As closure stress increases, the conductivity of branch fractures decreases rapidly, and the overall conductivity of the system becomes increasingly dominated by the primary fracture, with the contribution of branch fractures diminishing. This newly developed acid-etched conductivity model for complex fracture systems is expected to provide fundamental tools for field treatment design.