Compositional Modeling of Carbonate Acidizing Processes with CO2 Evolution in Aqueous Environments
Authors: Kumar Harish T. , Muhemmed Sajjaat , Nasr-El-Din Hisham A. 
Published: February 2020

Summary
Several modeling studies have been conducted in carbonate acidizing, particularly in the area of aqueous environments. Yet, complete understanding of this complex subsurface process remains elusive. Characterizing the effects of evolved CO2, a product of the chemical reaction between carbonates and HCl (hydrochloric acid), has been ignored to date under the assumption that high operating pore pressures keep CO2 completely dissolved in the surrounding solution. However, the presence of CO2 in the porous media of the formation itself changes fluid-flow dynamics throughout the entire system.
This paper describes a numerical simulation study to accurately model the physics of carbonate acidizing. A validation of the model is conducted by replicating experiments described in the published literature and by performing laboratory coreflood experiments of carbonate acidizing. The acid efficiency curve and initial pore pressure variations for single-phase experimental studies from the literature is matched by including the effects of evolved CO2 in the model. Two Indiana limestone cores of 6 in. length and 1.5 in. diameter were used to conduct (1) a tracer-injection study with 5 wt% KCl (potassium chloride) solution and (2) an acid-injection study with 15 wt% HCl solution. The experiments were conducted at 72°F, and 1,180 psi pore pressure. The Indiana limestone cores were characterized via computed tomography (CT) scans, and a detailed, accurate porosity profile of each core was used as input to the numerical model. The tracer fluid was used to characterize the porous environment and mechanical dispersion coefficients, and for subsequent calibration of the simulation model. From the conducted single-phase acidizing coreflood experiment, pressure drop values across the core were closely monitored with time to assess acid breakthrough, and the core effluent samples were collected at regular intervals and analyzed to determine the concentrations of calcium chloride (CaCl2) and HCl. CT scans of each core conducted post-acidizing describe its wormhole pattern. These parameters are accurately matched using the simulation model.
A high pore pressure of 1,000 psi and above is not sufficient to keep all the evolved CO2 in solution during carbonate acidizing. The presence of CO2 as a separate phase hinders acid efficiency. Up to 24% by volume of pore space is shown to be occupied by the evolved CO2 that exists as a separate phase, and is located ahead of the acid front during the acidizing process, thus competing for flow with the incoming acid. The modeling of CO2 as a component for simulating the acid coreflood played a key role in acquiring a better match with experimental results, with limited dependency on empirical pore-scale parameters. In addition to wormhole propagation, the current model accurately forecasts effluent concentrations collected and quantity of rock dissolved from the acidized porous media.
A new approach to accurately predict carbonate acidizing in porous media for an aqueous environment has been presented via compositional modeling using a reservoir simulator. The presented methodology can be incorporated in large field scale reservoir models.