Optimisation of areal CO2 flooding in oil reservoirs

Abstract

<p>One of the most actual environmental problems is the emissions of greenhouse gases into the atmosphere in particular carbon dioxide (CO<sub>2</sub>) emissions. Carbon capture and storage (CCS) in oil reservoirs is often proposed as a solution to this problem. This can not only help in the emissions reduction but can also increase the profit from oil production. Furthermore, CO<sub>2</sub> is also one of the associated petroleum gases to be disposed of. It is unacceptable to release associated gas into the atmosphere and its flaring is also bad for the environment. Therefore, the gas injection is more environmentally and economically profitable.</p><p>In this work we compare the optimal CO<sub>2</sub> flooding strategies into oil reservoirs by means of 1-D and 2-D compositional modelling with the MUFITS simulator. We employ the standard modelling approach based on balance equations for each hydrocarbon component, CO<sub>2</sub> and water. We use the SRK equation of state for predicting phase equilibria in the hydrocarbon mixture with CO<sub>2</sub>. This allows for an accurate modelling of the CO<sub>2</sub> dissolution in the oil phase and oil evaporation into the gas phase. We consider different WAG injection strategies with both alternating and simultaneous water and CO<sub>2</sub> injection. The areal sector model of a five-spot well pattern is coupled with a simple economic model of WAG for the calculations of the injection profitability. The optimisation criterion is the maximisation of economic profit, i.e. the net present value (NPV). We determine the injected volume at which NPV is maximum over various injection strategies. First, a study of the grid independence of the 2-D numerical solution is carried out. Second, we compare the optimal strategies for the 2-D areal study with those for the 1-D study corresponding to a slim-tube experiment [1]. The simulation results demonstrate that the optimal strategies obtained in 1-D simulations qualitatively match those in the 2-D simulations [2], although the sweep efficiency in the 2-D study is smaller. Thus, 1-D modelling is effective for determining optimal strategies for areal injection of water and gas. Also, we investigate the influence of the oil price by plotting the map of optimal WAG strategies against the net profit and the dimensionless injection rate.</p><p>We also consider an alternative optimisation criterion that assumes maximisation of the volume of CO<sub>2</sub> stored in the reservoir after oil production ceases. We show that this criterion results in the optimal WAG strategy different from the strategy with a maximum of NPV. However, the range of parameters exists where both optima are reached on the same strategy.</p><p> </p><p>[1] Afanasyev A., Andreeva A, Chernova A. Influence of oil field production life on optimal CO<sub>2 </sub>flooding strategies: Insight from the microscopic displacement efficiency // 2021 J. Petrol. Sci. Eng. 205. 108803. DOI: 10.1016/j.petrol.2021.108803</p><p>[2] Afanasyev A., Andreeva A., Chernova A. Numerical optimisation of CO<sub>2</sub> flooding using a hierarchy of reservoir models // 2021 Adv. Geosci., 56, 19–31. DOI: 10.5194/adgeo-56-19-2021</p>