An Integrated Experimental Method for Investigating Sequential Injection Acidizing in Carbonate Formation

Sequential injection acidizing is usually applied for acid diversion and, in recent years, for achieving deep acid penetration in high-temperature reservoirs. However, there is limited research on the impact of this technique on wormholing. Building on a successful application of this technique in a 180 °C carbonate formation, this study investigates its underlying mechanisms, particularly its impact on wormhole development. The current study employs an integrated experimental approach, combining fluid performance evaluation and selection, core flooding experiments, and computerized tomography (CT) imaging to select the acid combinations and to investigate the performance of this technique in a carbonate reservoir. The results indicate that sequential injection acidizing requires less acid and a lower pressure drop to penetrate the same length of core compared with conventional single-acid-stage stimulation. This suggests that this technique leads to deeper wormhole penetration, thereby improving acidizing efficiency. CT imaging reveals the mechanism behind this phenomenon: the differences in reaction kinetics and rheological properties further enhance the randomness of wormholing and generate a new type of wormhole composed of dispersed and interconnected large dissolution pores. This synergistic effect of sequentially injecting different acids in carbonate reservoirs is particularly important for high-temperature reservoirs, where wormhole penetration depth is usually shallow. This study provides insight into the design of sequential injection acidizing and offers guidelines for selecting and arranging the acid stages. Further experiments using more stages of acids could be conducted based on this integrated experimental method.