Measuring the Reaction Rate of Lactic Acid With Calcite and Dolomite by Use of the Rotating-Disk Apparatus

Abstract

Summary Lactic acid has been examined in various laboratories and applied in the oil field for acid fracturing and drilling-fluid-filter-cake removal, and as an iron-control agent during acid treatments. However, the reaction of lactic acid with calcite has not been addressed before. Determination of the reaction rate and the acid-diffusion properties is a critical step for successful treatments in matrix acidizing and acid fracturing. Therefore, the objective of this work is to conduct a detailed study on the reaction of lactic acid with calcite. Mass transfer and reaction kinetics are reported for the lactic acid/calcite system by use of the rotating-disk apparatus. Disk samples were cut from Indiana limestone or Silurian dolomite and were used in the reaction-rate experiments. The effect of lactic acid concentration (1, 5, and 10 wt%), temperature (80–250°F), disk rotational speed (100–1,800 rev/min), and different inorganic salts on the reaction rate was investigated. The diffusion coefficient of 5 wt% lactic acid was determined at low disk rotational speeds and reported at 80, 200, and 250°F. A model that accounts for the effect of the kinetics of the surface reactions and the transport of reactants and products was developed. The activation energy and the rate constant at 80, 150, and 250°F for the reaction of lactic acid with Indiana limestone were reported. Reaction experiments of lactic acid with dolomite at 150°F over disk rotational speeds of 100–1,800 rev/min, and at 1,500 rev/min over a temperature range of 80–250°F, were conducted and the results were compared with those obtained for the calcite reaction. At 80°F, the reaction of lactic acid with calcite was controlled by mass transfer at low disk rotational speeds (up to 500 rev/min) and was surface reaction limited at higher speeds. At higher temperatures (150, 200, and 250°F), both mass transfer and surface reaction influence the overall calcite dissolution. The kinetics of the surface reaction were influenced by both forward and backward reactions. At 80°F, the surface reaction contributes to 28% of the overall resistance. This dependence becomes much less (13 and 10%) at higher temperatures (150 and 250°F, respectively). The reaction of lactic acid with dolomite at 150°F was mainly controlled by mass transfer up to 1,000 rev/min and by the kinetics of the surface reaction after 1,000 rev/min. At 80 and 150°F, the rate of reaction of lactic acid with calcite was an order of magnitude higher than that with dolomite. At temperatures of 200 and 250°F, the rate of reaction of lactic acid with calcite is twice the rate of reaction with dolomite. The presence of Ca2+, Mg2+, and SO42− ions reduced the reaction rate, which is most likely because of the reduction in the concentration gradient of the products. The reduction in the concentration gradient will cause a reduction in the rate of diffusion of the generated calcium away from the surface, and hence a lower rate of dissolution.