Mechanistic analysis of matrix-acid treatment of carbonate formations: An experimental core flooding study

Despite the extensive works carried out on optimal design of the acidizing operations, the detailed mechanisms of the wormholes formation and propagation within the rock structure and their effects on optimum acid injection rate have not been well studied in the available literature. In this work, high pressure-high temperature (HP-HT) acid injection experiments and computed tomography (CT) scan imaging were performed by HCl 15 wt% to discover the mechanisms underlie the creation of wormholes and their extension in the carbonate rocks. The pressure drop profiles and permeability variations before and after acidizing process were employed to identify the optimum acid injection rate. As a final point, core effluent samples were collected and analyzed for justification of the HP-HT experimental results. For performed HP-HT experiments, acid injection rate of 7 cm³/min was obtained as the optimal acid injection rate. The maximum permeability improvement, K_f/K_i = 11.2, was achieved at the optimum acid injection rate corresponded to the minimum acid breakthrough volume, as well as the acid consumption. The results obtained from the CT scan analysis show that the wormhole created in core C.4 is close to the optimal conditions with a single distinctive wormhole to bypass the damage. At acid injection rates lower or higher than the optimum injection rate, the shape of the wormholes changes to conical at very low injection rates and ramified at high injection rates. The highest concentration of calcium at effluent samples was observed for the minimum and maximum injection rates (1 and 15 cm³/min), respectively. The maximum contact time and highest contact area between the acid and rock were attained at minimum and maximum acid injection rates, respectively. As this work was performed at realistic oilfield conditions, it can be used for effective plan, execution and optimization of the acidizing operation in carbonate reservoirs.