Maja Arnaut, mag.ing.petrol.
Enhanced Oil Recovery (EOR) methods are used to produce additional oil after the primary production phase (where production is based on natural reservoir energy) or, most often, after waterflooding (secondary phase). Some EOR methods are related to CO2 injection (CO2-EOR), which is attractive since part of injected CO2 retained in the reservoir, enabling a positive effect on storage capacity and cost-effectiveness of CO2 storage. Therefore, these methods are of particular importance due to the emission reduction obligations under European Union international agreements within the climate change domain (Kyoto protocol from the year 1997 and Paris agreement from the year 2015). Carbon Capture Utilization and Storage (CCUS) comes to focus when possibilities of CO2 storage and reduction of storage cost are assessed. Although there are other utilization types such as utilization through beverage production or in agriculture, only the CO2 enhanced oil recovery (CO2-EOR) is implemented at a commercial level on an industrial scale –.
By injecting CO2 above the miscibility pressure (or minimum miscibility pressure, MMP), microscopic displacement efficiency is improved due to viscosity reduction, oil swelling, lower interfacial tension and change of density of oil and brine . Regardless of the injection conditions, part of the CO2 is always re-produced so injected CO2 includes recycled CO2 and CO2 that should be brought to complete the total required injected volume (Figure 1)
Oil recovery, injection cost and amount of carbon dioxide permanently stored can be optimized by application of methods which include water alternating gas injection (WAG). Simulation of the WAG process can help establish optimal relation between the stated parameters.
CO2 injection into a reservoir can be implemented at miscible and immiscible conditions and the distinction between the two is defined by the minimum miscibility pressure. If the conditions are miscible, CO2 increases the oil mobility as it is dissolved in oil between the injector and the producer, and if the conditions are immiscible, there is no CO2 dissolution in oil, which means that CO2 flows much faster than oil toward the producers, causing lower oil recovery and higher production of previously injected CO2. The value of the minimum miscibility pressure depends mostly on oil
composition and the reservoir temperature, and to determine the exact value of the minimum miscibility pressure a detailed pressure volume temperature (PVT) characterization of oil and the mixture of oil and CO2, is necessary.
As a CO2-EOR process can be classified as immiscible, near-miscible and miscible, the same classification may be applied for the WAG process. The water is used to maintain the reservoir pressure above the MMP, and to prevent the early breakthrough of CO2 to the production wells. Near miscible or immiscible WAG injection implies three-phase flow for which, no matter the longtime application of WAG processes in the world, there is still no complete understanding of changes in fluid composition which happen during such flow .
The amount of CO2 retained in the reservoir as part of the CO2-EOR project will become important in the EU if trading with European emission allowances (EUA) will be possible on the emission market EU ETS. Vulin et al.  investigate development EU ETS, historical EUA price volatility and trends correlated with price movements of natural gas, coal, and oil to forecast long-term EUA price probability using momentum strategy and geometric Brownian motion. Same as oil price, EUA price is influenced by policy but some dependency on natural gas price and consumption was detected.
To prove that CO2-EOR represents a feasible, mature, and clean carbon capture utilization and storage (CCUS) option, it is crucial to single out the most economically favourable option that can be applied considering the parameters crucial for CO2-EOR operations. Earnings of a CO2 EOR storage project comes from oil production and avoided CO2. Avoided CO2 refers to the amount of CO2 retained in the reservoir during the EOR project, and this volume of CO2 can be considered as allowance and therefore used for trading in the EU ETS. Price of EUA on EU ETS can be observed as possible additional income besides oil production and on the other side highest costs are related to transport and injection of CO2 in CCUS projects.
Optimization of the CO2 injection in CO2-EOR projects can be evaluated through CO2 utilization factors (UF) since that factor is the ratio of utilized CO2 and produced oil. Additional recovery and retention are opposed parameters, i.e., maximum retention does not always bring maximum additional recovery (Figure 2). In the first years of CO2-EOR, retention to additional recovery ratio is higher, and with time this ratio decreases.
Figure 3 shows two extreme cases of WAG injection. Left (Model 3) shows the CO2 molar density change under fully miscible conditions (greater depth), while on the right side the same is shown for the case where the CO2 is injected under immiscible conditions (shallow reservoir). As CO2 can appear both in a gaseous and liquid state (supercritical is treated as a liquid in Eclipse), molar density is the best way to visualize the change of composition in space. Favourable cases (like that on left) will result in greater areal sweep efficiency and consequently in higher CO2 retention (i.e., CO2 stored).
Figure 3. CO2 molar density change during WAG injection with WAG ratio 1:1
The aim of my research within ESCOM and STRATEGY CCUS projects is to develop a systematic method for estimating the potential of CO2 use for additional oil recovery and storage.
In order to develop an optimization function for the above, it is necessary to use different numerical simulators characteristic for reservoir engineering ( Petroleum Experts PVTp, Schlumberger PVTi, Schlumberger Eclipse) to develop a statistically representative number of cases or a matrix of input parameters of the model so that later results can be analyzed (using other computer programs, SQL database and Python code for dana processing and charting) in order to get weighting factors of each parameter and correlation matrix.
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 A. Ettehadtavakkol, L. W. Lake, and S. L. Bryant, “CO2-EOR and storage design optimization,” Int. J. Greenh. Gas Control, vol. 25, pp. 79–92, 2014, doi: 10.1016/j.ijggc.2014.04.006.
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 D. Vulin, M. Arnaut, and D. Karasalihović Sedlar, “Forecast of long-term EUA price probability using momentum strategy and GBM simulation,” Greenh. Gases Sci. Technol., vol. 10, no. 1, pp. 230–248, 2020, doi: 10.1002/ghg.1957.
Maja Arnaut, mag. ing. petrol. is an research assistant at Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb. She is currently in final phase of completing her doctoral thesis: Development of optimization criteria for CO2-EOR methods and simultaneous CO2 storage.