Role of Temperature on the Performance of In-Situ CO2 EOR

Abstract

As the temperature dramatically impacts many chemical reactions, the reservoir temperature is an essential parameter in selecting and designing chemical enhanced oil recovery (cEOR) methods. For the in-situ CO2-enhanced oil recovery (ICE), reservoir temperature directly impacts the hydrolysis rate of a CO2-generating chemical agent. Below a critical temperature, the CO2 releasing rate is too low to be effective for ICE. Furthermore, temperature affects the CO2 solubility in oil and water phases and the CO2 partition coefficient between them. When the reservoir temperature is high enough, optimizing the oil recovery and injection slug size of the CO2-generating agent is a problem to be studied in this paper. In this study, urea was injected as a CO2-generating agent. Three light, medium, and heavy hydrocarbon components were used as oils in a synthetic homogeneous 3D quarter 5-spot sector model. The injection temperature was 80 °F (300K). In the reservoir, urea hydrolysis generates CO2, which partitions into oil and water. The urea reaction kinetics used in the study are based on 1D history-matched laboratory data from previous studies. The Arrhenius model was used to calculate the urea hydrolysis reaction rate. Additionally, the Gibbs free energy of the urea hydrolysis reaction was computed to determine the critical reservoir temperature above which the hydrolysis would be favorable and spontaneous. A sensitivity study was conducted to study the temperature effect on ICE performance with an objective function of maximum recovery with constraints of a limited urea mass. Based on both Arrhenius and Gibbs models, it was observed that the urea hydrolysis reaction was prolonged, became negative, and was non-spontaneous at temperatures below 70°C (~340K). It was concluded from the kinetic analysis that the urea hydrolysis reaction would not produce any CO2, and synergetic mechanisms of oil swelling, viscosity reduction, and wettability alteration would not happen if the reservoir temperature is below 70°C. Also, increasing the reaction rate close to this critical temperature would require a catalyst, such as NaOH (Wang 2018 and Wang et al. 2019). The 3D sector model also showed that optimum oil recovery at ten wt% urea concentration would be at ~260 °F (400K); minimal impact on oil recovery was observed above this temperature. Also, an additional 4-5% recovery was obtained post-cold waterflooding. On top of that, almost 90% CO2 generated CO2 dissolved in oil, resulting in oil swelling, viscosity, and IFT reduction. Hence, to apply urea as a CO2-generating agent, one of the critical design parameters is that the reservoir temperature must be higher than 70 °C (343 K). As a promising and innovative EOR technique, ICE will be first studied for performance optimization. The study results can be used in reservoir screening and economic evaluation for the ICE actual field applications.