Numerical investigation of re-etching behavior and conductivity enhancement during closed fracture acidizing in naturally fractured reservoirs

Closed fracture acidizing (CFA) is a stimulation technique in which acid re-etches the previously closed acid fracture to enhance acid fracture conductivity in carbonate formations. Previous studies typically assumed smooth fracture surfaces, and fracture conductivity models did not account for surface deformation under closure stress well. Consequently, the mechanism of re-etching and conductivity enhancement during CFA, especially in naturally fractured reservoirs, remains inadequately understood. To address this gap, we apply geostatistical methods to reconstruct the fracture surface morphology, while Monte Carlo simulations are employed to generate natural fractures (NFs). Next, a CFA model of acid flow, leak-off, and dissolution of acid fracture with rough surfaces is built. Then, fracture conductivity model is created by considering both acid etching and rock deformation under closure stress. Based on the models, extensive numerical simulations are conducted. The study shows that the live acid penetration distance during CFA is limited (typically 20–45 m), but competitive etching forms channels that retain high conductivity under closure stress. Smaller fracture apertures create high-conductivity channels and promote live acid penetration distance; higher natural fracture (NF) density, longer NF length, and smaller inclination angle enhance acid diversion and localized etching. While retarded acid improves acid penetration distance, conventional acid is beneficial for achieving greater erosion and enhancing conductivity retention under high closure stress. Engineering parameters influence treatment performance by either deepening existing channels or extending penetration distance. This study builds a quantitative relationship among etching pattern, live acid penetration distance, and fracture conductivity, providing valuable insights for optimizing CFA in naturally fractured reservoirs.