Simulation study on acidizing of self-diverting acid and conventional acid in fractured carbonate rocks

Abstract

Matrix acidizing is a key technology for alleviating near-wellbore damage in carbonate reservoirs. However, conventional acidizing often exacerbate formation heterogeneity due to preferential acid flow into high-permeability zones. Although self-diverting acid based on viscoelastic surfactant (SDVA) demonstrate potential for improving acid distribution, the diversion in fractured media remain unclear. This study integrates the two-scale continuum model (TSC), the 3D embedded discrete fracture model (3D-EDFM), heat transfer model, and SDVA viscosity control model to develop a numerical model for simulating SDVA acidizing in fractured carbonate reservoirs. Using this model, the SDVA acidizing process was simulated, and results aligned with existing studies, confirming high accuracy. Comparative analysis with simulation of conventional HCl acidizing revealed the following: SDVA acidizing tends to form slender, less-branched wormholes across various injection rates, with smaller the pore volume of acid required to reach the breakthrough (${PV}_{BT}$) and lower optimum injection rates. High injection rates favor uniform dissolution. SDVA effectively reduces acid leakage along natural fractures, demonstrating stronger diversion suppression in low-permeability fractures. The heat from SDVA acid-rock reactions concentrates near wormhole regions, and elevated initial rock temperatures weaken self-diverting effects. In fractured cores, SDVA and HCl produce similar wormhole morphology under low porosity; As porosity increases, diversion advantages of SDVA emerge, enhancing effective acid penetration distance. As injection rates increase, the propagation mechanism shifts from pore-structure-dominated wormholes to fracture-dominated paths, particularly in reservoirs with well-developed, interconnected fracture networks.