Experimental evaluation of the impact of CO2/N2 mixture on residual water saturation during CO2 storage in saline aquifer

Yi Li; Wendong Dan; Jiaqi Zhao; Zhikai Hu; Ruiting Suo; Liang Xue; Li He; Qingchun Yu

doi:10.1016/j.jconhyd.2025.104674

Experimental evaluation of the impact of CO₂/N₂ mixture on residual water saturation during CO₂ storage in saline aquifer

J Contam Hydrol. 2025 Jul 7:274:104674. doi: 10.1016/j.jconhyd.2025.104674. Online ahead of print.

Authors

Yi Li¹, Wendong Dan², Jiaqi Zhao², Zhikai Hu², Ruiting Suo³, Liang Xue⁴, Li He², Qingchun Yu⁵

Affiliations

¹ Key Laboratory of Coast Civil Structure Safety of the Ministry of Education, School of Civil Engineering, Tianjin University, Tianjin 300350, China; State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China. Electronic address: yi_li@tju.edu.cn.
² Key Laboratory of Coast Civil Structure Safety of the Ministry of Education, School of Civil Engineering, Tianjin University, Tianjin 300350, China.
³ School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China.
⁴ State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China.
⁵ MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China.

PMID: 40645065
DOI: 10.1016/j.jconhyd.2025.104674

Abstract

Residual water critically constrains CO₂ geological storage efficiency, yet we discover that elevated CO₂ concentration in gas mixtures systematically reduces its saturation. By revealing that N₂ co-injection paradoxically enhances storage safety despite increasing residual water, this work provides transformative strategies for optimizing carbon sequestration with impurities. Specifically, in this study, nine sets of core-flooding experiments were conducted using three different ratios of a CO₂/N₂ gas mixture (50 % CO₂ + 50 % N₂, 75 % CO₂ + 25 % N₂, and 99.99 % CO₂). The experimental results indicate that an increase in the ratio of CO₂ within the gas mixture leads to a progressive reduction in residual water saturation, with the order of saturation being 50 % CO₂ + 50 % N₂ > 75 % CO₂ + 25 % N₂ > 99.99 % CO₂. Furthermore, a detailed mathematical relationship that delineates the connection between residual water saturation and drainage duration is presented, with coefficients a and b examined thoroughly. The incorporation of N₂ into the CO₂ mixture raises residual water saturation, as lower concentrations of CO₂ correlate with increased residual water saturation. However, the presence of N₂ effectively extends the CO₂ breakthrough time, making it more difficult for CO₂ to penetrate the rocks and thereby enhancing storage safety. As the concentration of CO₂ in the gas mixture increases, a reduction in contact angle values and interfacial tension (IFT) occurs. This variation results in a decrease in capillary pressure, which facilitates the displacement or migration of fluids within the core pore space. Concurrently, the viscosity ratio of the gas phase to the liquid phase reduces the viscous resistance in the pores and improves displacement efficiency. Furthermore, the experimental results are primarily influenced by capillary forces. This study enhances the theoretical understanding of CO₂ storage, and provides valuable insights for evaluating the feasibility of CO₂ storage projects that include impurities.

Keywords: CO(2) concentration; CO(2) geological sequestration; CO(2)/N(2) gas mixture; Residual water saturation.