Carbonated Water Injection (CWI) is considered as one of the most sustainable solutions in response to CO2 emissions in oil and gas processes. It has the potential to store CO2 in geological formations, while recovering oil, reducing this way the carbon footprint. This advantage is a key asset of CWI, comparing to other enhanced oil recovery methods (EOR). In this technique, recovering oil is achieved through the dissolution of CO2, which transfers to the oil phase improving the oil mobility and causing oil swelling, both enhancing the sweep efficiency.
In this work, two models are presented for the prediction of CO2 solubility in water and NaCl brines. A third model is, also, presented for the prediction of CO2 solubility, taking into consideration the presence of O2 and N2, in water and NaCl brines. The first model is the model of Duan and Sun, which is based on the theoretical basis that at equilibrium the chemical potential of CO2 in the liquid phase is equal with the chemical potential of CO2 in the vapor phase. The second model is the equilibrium model, which is based on the theoretical basis that in equilibrium the fugacities of CO2 and water in the vapor phase are equal to the fugacities of CO2 and water in the liquid phase. The third model is the model of Li, which is based on the Henry constant, that is valid only when the system is at equilibrium state and interrelates the gas fugacity coefficient of a component with its activity.
The advantage of Duan and Sun’s model is that they developed a non-iterative method to calculate the fugacity coefficient of CO2 in the vapor phase, whereas in the equilibrium model an equation of state is used in order to calculate the fugacity coefficients of CO2 and water in the vapor phase. The advantage of the equilibrium model is that it predicts better the CO2 solubility in higher salinities, higher than 4 molality. The advantage of Li’s model is that, despite the not so accurate prediction of CO2 solubility in the CO2-water-salts system, it includes the effect of impurities, such as O2 and N2, on the CO2 solubility in the CO2-O2-N2-water-salts system.
The effect of pressure, temperature, salinity and impurities’ content on the solubility of CO2 is studied. It is observed that the solubility increases with pressure, decreases with salinity and impurities’ content. The influence of the temperature is more complex. Its effect varies according to the values of the aforementioned factors. In general, in temperatures below 100o C the solubility decreases, whereas over 100oC it increases with it.
The process simulation for the production of carbonated water is developed in Unisim. Owing to the fact that the already existing thermodynamic models don’t describe well the equilibrium of the CO2-O2-N2-water-salts system, the model of Li et al. is implemented via CAPE-OPEN. This model, though, is not able to calculate the thermodynamic properties of the fluid. Subsequently, a comparison is made between the existing complete thermodynamic models, so as to decide which one simulates in a better way the behavior of Li’s model and is going to be used for the calculation of the rest of the properties.
The process simulation is studied based on a case study provided by Equinor. More specifically, it is referring to an offshore process, that produces carbonated water, by mixing a water and a carbon dioxide stream, in the pressure of 180 bar. Since, the CWI is not a mature process, the modelling/simulation studies are not extensive. In this master thesis, a multistage compression of the CO2 stream and the pumping of water in the same pressure, before their mixing, is proposed.
Two cases are studied in order to reach this pressure. The first one is by immediately pumping the water and compressing the carbon dioxide in the pressure of 180 bar. The second one is by pumping the water and compressing the carbon dioxide in an intermediate pressure and then pumping the carbonated water in the pressure of 180 bar. The scheme of the intermediate pressure is conceived since it is noticed that the given amount of CO2 can be dissolved in the given amount of water at a pressure lower that this of 180 bar. The criterion, on which the selection is based, is the comparison of the cooling, compression and water pumping duties.
The simulation’s results indicate that the water pumping duties have similar values. Thus, the compression and the cooling duties are of greater importance. The results indicate that in terms of required compressed and cooling duties, the intermediate pressure case is more profitable.
A sensitivity analysis is conducted in order to examine the effect of some of the most important operational parameters of the CWI on the aforementioned duties. Therefore, the effect of the type of compression (single vs multi-stage), the thermodynamic approach to equilibrium, the pressure drops of each heat exchanger and the mixer and the pressure ratio is studied.
The compression requirements are lower when the pressure ratio of the compression stages is the same and the multistage compression is chosen. The required pressure for the complete dissolution of the given amount of CO2 decreases with the thermodynamic approach to equilibrium and increases with the pressure drops of each heat exchanger and the mixer. The higher the compression pressure, the higher the cooling and the compression duties.
As a result, it is concluded that the most profitable way, in terms of required duties, of producing the carbonated water is by pumping the water and compressing the carbon dioxide stream in an intermediate pressure and then pumping the mixture in the desired pressure.
KEY WORDS: Carbonated Water Injection, Carbon dioxide, Thermodynamic Modelling of carbon dioxide-water-brine system
