Chemical Interactions Between CO2 Acidified Aqueous Fluids and Carbonate Minerals

Abstract

The chemical interactions between carbon dioxide (CO2) acidified aqueous fluids and carbonate minerals were studied at elevated temperature and pressure conditions for the purposes of modelling fluid flow and reactive transport in carbonate reservoir formations. Experimental measurements were made of the rate of dissolution of carbonate mineral samples in acidified brines, with specific focus on the reaction rate in the surface reaction controlled regime. Another key outcome of this study was the experimental measurement of pH for CO2 acidified aqueous systems within the context of the data requirements of the oil and gas industry. Some studies have also conducted on the equilibrium constants of carbonate dissolution reactions. Three novel pieces of experimental apparatus were used to perform the experimental measurements, each designed to accommodate the corrosive CO2-acidified reservoir fluids at high-temperature, high-pressure reservoir conditions. A newly designed and constructed batch dissolution reactor system, consisting of three vessels, implemented the rotating disc technique to study carbonate dissolution process without the impact of mass transfer effects. A newly designed and constructed high-pressure pH measurement system used an electrometric technique to quantify the impacts of dissolved CO2 on the acidity of the aqueous solution. An in-situ Micro-Raman reactor apparatus used Raman spectroscopy to exam the equilibrium concentration of bicarbonate ions. All three systems were fully calibrated and validated before being applied to conduct systemic measurements with variation of temperature, pressure, salinity and minerals. The pH measurements for CO2-saturated water in the pressure range from (0.28 to 15.3) MPa and temperatures from (308.3 to 423.2) K were conducted first. Commercially-available pH and Ag/AgCl electrodes were used together with a high pressure equilibrium vessel operating under conditions of precisely controlled temperature and pressure. The results of the study indicate that pH decreases along an isotherm in proportion to –log10(x), where x is the mole fraction of dissolved CO2. An empirical equation has been developed to represent the present results with an uncertainty of ±0.06 pH units. We also compare our results with a new chemical equilibrium model and find agreement to within 0.1 pH unit. The pH measurements were further extended to a CO2-saturated aqueous NaCl solution. A new strategy is proposed to calibrate the pH electrodes by using the Pitzer model to quantify the salt effects. Measurements were carried out at temperatures between (308 and 373) K and at pressures up to 15.4 MPa for NaCl solutions with concentrations of (1, 3, 5) mol·kg-1. The pH is found to increase with increase of pressure, decrease of temperature and increase of NaCl concentration. An empirical equation correlating pH with CO2 solubility has been proposed with an uncertainty of ±0.08 pH units. Comparisons of the experimental data with two thermodynamic simulation packages using different aqueous electrolyte models suggest that the Pitzer model provides reasonably accurate predictions, although further improvements at higher NaCl concentrations would be desirable. New experimental data have been measured for carbonate mineral dissolution rates in CO2-saturated aqueous system and the dissolution kinetics have been determined using the pH model derived in this study. Calcite dissolution rates in CO2-saturated water at pressures ranging from (6.0 to 13.8) MPa and temperatures from (323 to 373) K were first measured. The rate of calcite dissolution in HCl(aq) at temperatures from (298 to 353) K was also measured. The impact of mineral sample surface morphology was investigated and the results suggest that at far-from-equilibrium conditions, the measured calcite dissolution rate is independent of the dislocation density due to the development of a dynamic steady-state pattern of etch pits. The results also indicate that the calcite dissolution rates under surface-reaction-controlled conditions increase with increase of temperature and CO2 partial pressure. A kinetic model incorporating both pH and the activity of CO2(aq) has been developed to represent the dissolution rates found in this study. We report correlations for the corresponding reaction rate constants based on the Arrhenius equation and the activation energies so determined are in reasonable agreement with the literature. The dissolution rate studies were then carried out for two other carbonate minerals, dolomite and magnesite, using pure mineral crystals. Dissolution experiments were conducted in CO2-saturated water and HCl(aq) systems at similar temperature, pressure and pH conditions compared to the calcite experiments. The results indicate that the dissolution rates of dolomite and magnesite also increase with increase of temperature and CO2 partial pressure. The dissolution kinetics of both minerals can be modelled as a single first-order heterogeneous reaction for both the HCl system and the (CO2 + H2O) system. It was also noticed that dolomite dissolves in a stoichiometric manner, which greatly reduced the complexity in modelling. Finally, the dissolution rate investigations were extended to two different reservoir analogue samples (Limestone and Chalk) to validate the reaction kinetics models proposed in this project for reservoir samples. Significant efforts were made to estimate the true reactive surface area. Good agreement has been observed between the experimentally-measured dissolution rates of the reservoir analogue samples and the data calculated using the reaction kinetics model established in this project. It is concluded that the kinetic models and the associated parameters derived in this project can be incorporated into reservoir simulators to provide more accurate reactive transport simulations for future large scale CCS projects. Finally, Raman spectroscopy was used to probe the chemical equilibrium constants of carbonate dissolution reactions by measuring the equilibrium bicarbonate concentration (HCO3-). The utilisation of a new calibration procedure has enabled in-situ, non-invasive and non-destructive, online quantitative fluid-rock interaction studies for carbonate-CO2-brine systems. Both calcite and magnesite were studied in CO2 acidified H2O and 1 M NaCl systems at three temperatures from (297 to 373) K and pressures of 7 MPa and 15 MPa. Several geochemical simulators using different aqueous electrolyte models were used and were able to achieve various degrees of success in predicting the equilibrium HCO3- concentration in comparison with the experimental values. Overall, the Pitzer model demonstrates the widest applicability and highest accuracies for CO2 acidified aqueous system. The present work was carried out as part of the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC) program. It provided extensive mineral dissolution rates and pH data that can be used to characterise the chemical interactions between CO2-acidified aqueous fluids and carbonate minerals relevant to oil- and gas-field applications. The results of this study should facilitate more rigorous modelling of reactive transport and fluid flow in the design and optimisation of enhanced oil recovery and carbon storage processes. Areas in which the research might be extended, both through further experimental studies and improved modelling, have been identified.