Maximising CO2 sequestration efficiency in deep saline aquifers through in-situ generation of CO2-in-brine foam incorporating novel CO2-soluble non-ionic surfactants

Abstract

The geological sequestration of carbon dioxide (CO2) within deep saline aquifers remains a critical pathway for achieving long-term climate stabilisation and net-zero targets. This study presents a novel strategy to augment both the efficiency and security of CO2 storage through the in-situ formation of CO2-in-brine foams facilitated by non-ionic CO2-soluble propoxylated-ethoxylated alcohols surfactants, specifically 2EH-PO5-EO9 and 2EH-PO5-EO15. A comprehensive suite of laboratory investigations was conducted under representative reservoir conditions (70 °C and 3200 psi), including cloud point pressure analysis, foam stability monitoring, interfacial tension (IFT) reduction, wettability alteration studies, CO2 solubility measurements, and core flooding experiments. The results reveal that both surfactants exhibit strong solubility in supercritical CO2 and brine, enabling stable foam generation without degradation. Among them, 2EH-PO5-EO15 demonstrated superior performance, achieving a marked reduction in CO2–brine IFT, from 32 to 6.7 dyne/cm, and enhancing CO2 solubility in brine by over 100 % (from 1.25 to 2.51 mol%) compared to pure CO2 scenario; thus strengthening solubility trapping. In parallel, wettability characterisation showed a transformation of the sandstone substrate from strongly water-wet (θ ≈ 18°) to an intermediate gas-wet condition (θ ≈ 71°), significantly amplifying the potential for residual trapping. Core flooding assessments corroborated these synergistic effects, demonstrating reduced CO2 mobility and improved sweep efficiency through foam stabilisation and rock–fluid interfacial modification. Altogether, this work underscores the efficacy of CO2-soluble non-ionic surfactants in enhancing storage security and injectivity, offering a technically and economically attractive approach to large-scale CO2 sequestration in saline aquifers. Also, the current results establish a strong foundation for advancing toward field-scale implementation and represent a significant step forward in creating sustainable solutions for carbon management.