A study of the optimum injection rate of carbonic acid during matrix acidizing of carbonate reservoirs: Implications for reducing CO2 emissions

This study investigates the process of optimizing the injection rates of carbonic acid during matrix acidizing in carbonate formations, highlighting its impact on wormhole formation, dissolution, and the changes in rock petrophysical properties. Indiana limestone core samples, with porosity ranging from 14 % to 16 % and permeability from 7 to 12 mD, were flooded using diffrent injection rates (0.5, 1, 2, and 5 cm³/min) to determine the optimal injection velocity. The carbonic acid is prepared by mixing 70 % fresh water with 30 % sc-CO₂ at 1500 psi and room temperature. A micro-CT scan was employed to visualize the wormhole patterns, measure its size, and calculate the optimum Damköhler Number. Moreover, inductively coupled plasma (ICP) was used to measure the calcium concentrations from effluents collected at different intervals. Two models were used to ascertain the optimum injection rate; the first model based on PVBT as a function of injection rate, indicated that 2 cm³/min is the optimum injection rate with 22 pore volumes to breakthrough (PVBT). The second model considered the relationship between PVBT and interstitial velocity (Vi), leading to an optimum injection rate of 1.7 cm³/min, corresponding to an interstitial velocity of 1 cm/min. Calculations of the optimum Damkhöler number reveal a value significantly different from the 0.29 proposed by Fredd and Fogler. This study provides valuable insights into the utilization of CO₂ to enhance well productivity as a replacement for conventional acids.