A New Method to Evaluate Reaction Kinetics of Acids with Carbonate Rocks Using NMR Diffusion Measurements

In this work, a new method to evaluate the reaction kinetics of different stimulation fluids with carbonate rocks was introduced. NMR diffusion measurements were used to determine the acid diffusion coefficient and the acid tortuous path inside carbonate rocks. Reaction kinetics can also be evaluated using rotating disk apparatus (RDA) in which a disc is rotated in the bulk fluid at different rotational speeds. RDA does not represent the actual, restricted acid diffusion that takes place in the porous media because only the surface of the rock is exposed to the reaction and the acid is not confined in the porous media. NMR diffusion measurements can accurately describe and determine the acid restricted diffusion in porous media. The diffusion coefficient of the acid is a crucial term in describing the reaction kinetics of acids with carbonate rocks. It is also used to predict the optimum injection rate required during the acidizing treatment and the soaking time for different fluids required to remove scales and deposits in the wellbore. The restricted diffusion was determined for different fluids such as GLDA chelating agent, HEDTA chelating agent, and EDTA chelating agent. Core flooding experiments for each fluid were conducted to determine the optimum injection rate. NMR restricted diffusion measurements then were conducted to determine the restricted diffusion and in turn to determine the optimum injection rate. The optimum injection rate estimated from the NMR was compared to that from the core flooding experiments. The results were a good match showing that NMR is a suitable, reliable, and robust method to evaluate reaction kinetics of different fluids with carbonate rocks.