Cellulose Nanocrystal-Stabilized Dispersions of CO₂, Heptane, and Perfluorooctane at Elevated Temperatures and Pressures for Underground CO₂Sequestration

Stable liquid carbon dioxide (CO2) emulsions and supercritical CO2 foams, stabilized by cellulose nanocrystal (CNC), were prepared at ambient temperature and underground reservoir conditions (T = 343 K; P = 20.7 MPa) in American Petroleum Institute (API) brine. Emulsions and foams prepared with CNC only were resistant to creaming and foam drainage and macroscopic coalescence over 24 h. On the contrary, emulsions prepared using a combination of CNC and dodecyltrimethylammonium bromide (DTAB) creamed at 298 K, and macroscopic bubble coarsening was observed at 343 K. The interfacial tension (γow) between CO2 and CNC suspensions decreased with increased CO2 pressure at both temperatures. Foams and emulsions containing DTAB had larger initial viscosities yet lower stability over a period of 24 h. Thus, reduced CO2 mobility during injection may not be indicative of longer-term foam stability for geological carbon sequestration projects. Ambient condition emulsions with heptane and perfluorooctane (PFO), as model compounds, showed that the less polar PFO could not be emulsified, while heptane formed stable emulsions at 298 and 343 K. The results suggest that polarity is a highly significant factor contributing to emulsion stability. This paper reports the first CNC-stabilized CO2 foam in reservoir conditions, with the potential for geological greenhouse gas sequestration.