Adsorption of Carboxylates on Calcite: The Coupled Effect of Calcite-Brine and Brine-Oil Interactions

The adsorption of polar organic molecules on the surface of brine-saturated carbonate rocks alters the relative mobility of oil and water with important implications for oil production and groundwater remediation. We propose a mathematical model for the adsorption of polar organic acid groups (initially contained in an oil phase) on calcite in the presence of brine. We reduce the problem into smaller subsystems and characterize them by identifying the key interactions. Oil-brine equilibrium is dictated by the partitioning of acidic components between the two phases. The dissolved acids ionize to carboxylate species in the water phase, which may either form complexes with the calcite surface or precipitate as calcium carboxylate salts by binding calcium ions from the solution. All these interactions are implemented into a Phreeqc model as equilibrium and kinetic processes. To obtain the main parameters (e.g., partition, ionization, or adsorption constants) governing the behavior of the different subsystems at different chemical conditions, we tune the sub-models to relevant experimental data (e.g., partitioning, precipitation, adsorption, and electrokinetic measurements). We then assess the performance of the model by coupling the reaction network to the transport equations and simulating the crude oil injection within a chalk core to predict effluent acid concentration history. By defining the total acidity of crude oil as a mixture of several carboxylic acids, our model satisfactorily fits the experimental data. The total acid number that is commonly reported as the sole indicator for the concentration of organic acids in oil does not allow distinguishing between different types of organic acids nor their affinity toward the calcite surface. More sophisticated analytical methods for quantifying the acid species in the crude oil are required for a more accurate description of the adsorption process using our model.