Evaluation of acid retardation based on acid-etched fracture morphology

Acid fracturing is the most effective production enhancement measure for carbonate reservoir stimulation. The retardation performance of acid systems is one of the key parameters influencing the effectiveness of acid fracturing. However, current methods for evaluating acid retardation, based on static dissolution experiments and acid-rock reaction kinetics, have certain limitations. This study introduces a new method for evaluating acid retardation by simulating acid flow in formation fractures. The morphology of acid-etched fractures was obtained, and the depth and width distribution of the etched fractures were quantitatively analyzed to assess the acid retardation performance. In addition, high-temperature dissolution experiments were used to evaluate the acid's dissolution capacity, and acid-rock reaction kinetics experiments were conducted to determine the acid-rock reaction rate and activation energy of the acid systems. The results show that at 130 °C, the reaction rate between hydrochloric (HCl) acid and rock (2.12 × 10⁻⁵ mol/(s·cm²)) was the fastest, followed by Diverting acid, while G acid had the slowest reaction rate (1.51 × 10⁻⁶ mol/(s·cm²)). The activation energy of weak acid systems was much higher than that of strong acid systems. The new evaluation method revealed that the average fracture width and depth etched by HCl acid were the largest (7.57 mm and 6.39 mm, respectively), while the fracture depth etched by G acid was the smallest (0.87 mm), and the average fracture width etched by Acetic acid was the smallest (2.30 mm). This indicates that acetic acid has a stronger etching ability along the fracture length compared to G acid. Additionally, the fracture width and depth curves of strong acids (e.g., HCl acid and Diverting acid) showed a downward trend, whereas those of weak acids (Acetic acid and G acid) showed an upward trend. This suggests that Diverting acid has poor retardation performance, while G acid has the best retardation performance. Furthermore, the roughness of fractures etched by strong acids was much greater than that of fractures etched by weak acids, indicating that strong acids have a stronger non-uniform etching ability. To balance the non-uniform fracture etching morphology and the effective reach of the acid, a combination of strong and weak acids can be used.