Carbon Storage in Depleted Oil Reservoirs: Experimental Investigation of CO2 Impurities Impact

Addressing global climate change necessitates innovative solutions for carbon capture and storage (CCS). Depleted oil reservoirs, with their vast capacities, emerge as promising repositories for CO2 sequestration. While solubility and diffusion are the main trapping mechanisms for CO2 in oil, a significant knowledge gap exists regarding the role of CO2 impurities and oil composition on CO2 solubility and diffusion in oil. This study provides a qualitative analysis of the impact of the Nitrogen and Methane presence on the solubility and diffusion of the CO2 in dead oil sample from the Eagle Ford formation. The impact of the impurities is evaluated based on analyzing the phase behavior changes of two cases with impurities compared to a pure CO2 injection case. Furthermore, the impact of the impurities on the depleted oil composition was analyzed through a complete compositional analysis. The findings of this study enhance our understanding of the fundamental interactions between CO2 and oil under reservoir conditions, which is critical for optimizing CCS strategies. A total of 9 experiments were conducted to observe CO2 diffusion and solubility when 3 different CO2 impurity mixtures were injected in black oil at reservoir conditions. The experiments were meticulously designed to measure the equilibrium phase behavior of the CO2-oil system, providing important data on CO2 dissolution rates and phase transitions under a range of pressure and temperature settings. The results from this study revealed that the CH4 and N2 presence and their concentration in the injected CO2 stream affect the gas solubility and diffusion in dead oil. The results showed that higher CO2 purity leads to more efficient dissolution into the oil, indicated by a more rapid pressure decay, while lower CO2 purity reduces its dissolution effectiveness and alters the phase behavior of black oil. Furthermore, the saturation pressure and volume of the oil increase with rising temperature. CO2 dissolution into oil alters the composition of the residual oil by breaking higher-chain hydrocarbons into smaller chains. Overall, the study concludes that the presence of impurities like methane and nitrogen reduces the effectiveness of CO2 dissolution and alters phase behavior in black oil. These findings conclude the essence of considering the purity of injected CO2 to evaluate the depleted oil reservoirs’ capacity, storing mechanisms and long-term integrity of the geological formation.