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Acid stimulation is a widely employed technique in the oil and gas industry to enhance the permeability of carbonate reservoirs by creating preferential flow channels, known as wormholes. These highly efficient flow pathways are crucial for improving fluid transport in porous media, enabling the bypass of damaged zones near the wellbore. Understanding wormhole formation, structure, and efficiency is essential for optimizing their impact on flow dynamics and the mechanical behavior of the rock matrix. This study investigates the geometrical characteristics of wormholes formed during the acid dissolution of a carbonate rock under varying flow conditions. The wormhole efficiency curve was determined through hydrochloric acid (HCl = 15%) injection at different flow rates (0.6 - 8.0 cm³/min). X-ray microtomography (µCT) scanning provided a detailed, non-destructive visualization of internal structural changes, enabling a comparative analysis of the dissolution process. Based on the experimental results, different wormhole types were distinguished, ranging from simple, straight channels (dominant) to highly ramified structures. The study quantified parameters such as the number of branches, porosity profiles, diameter distribution, channel connectivity/size, fractal dimension, surface area, volume and tortuosity providing insights into the efficiency of fluid transport across the rock samples. As expected, the results revealed a strong dependency of wormhole geometry on flow rate, with lower rates favoring dominant, straight pathways and higher rates resulting in more branched, complex structures. This behavior is consistent with the balance between reaction kinetics and fluid transport, indicating the need for precise control of operational parameters during acid stimulation. The results reveal an optimal flow rate around 0.9–1.1 cm³/min, where the number of branches drops (~200) and the main wormhole channel diameter reaches its minimum (130 μm), confirming high efficiency. Tortuosity stays stable (1–1.5), while the fractal dimension remains high (~2.5–2.8), indicating complex structures. Surface area and volume rise moderately at intermediate rates and reach maximum values (~47 cm² and 650 mm³) at 8.0 cm³/min, where acid penetration and pore enlargement are greatest but cause excessive branching. The porosity profiles confirmed the dependency of wormhole geometry on the flow rate, with an increase in the number of branches for higher flow rates. This work also developed a second approach for analyzing acidification. The images before and after acidification were registered, that is, they were digitally aligned so that the wormhole could be segmented and projected over the pre-acidified sample. That allows for the extraction of the volume that originates the wormhole, called pre-acidified wormhole. This volume can be analyzed and compared to the whole plug in an attempt to understand how it differs and why it was the acid’s preferential path. A pore network can be modeled in both the plug and the pre-acidifed wormhole in order to compare their pore populations and how they differ statistically.This study demonstrates the versatility of X-ray microtomography in capturing details of wormhole development, providing a robust framework for designing acid stimulation treatments. The findings can contribute to optimizing matrix acidizing strategies, ensuring enhanced productivity while minimizing risks to reservoir integrity.
| Country | Brazil |
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