Stabilization of CO2 aqueous foams at high temperature and high pressure: Small-angle neutron scattering and rheological studies

Understanding the evolution process including foaming, stabilization and ageing of CO2 aqueous foams in high-temperature, high-pressure (HTHP) environments is crucial for their rational use. However, most studies have been conducted through macroscopic observation, leading to inadequate insight into their intrinsic mechanisms. Herein we attempt to gain a comprehensive understanding of the evolution process by a combination of HTHP visualization foam meter, HTHP small-angle neutron scattering (SANS) and HTHP rheometry. A C22-tailed tertiary amine, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM), was first employed to develop robust CO2 foams. It was found the volume and lifetime of UC22AMPM-CO2 foams decrease by 24.7% and 56.9%, respectively, as the temperature rises from 25 °C to 100 °C at 8.5 MPa. The deterioration of the foam properties at elevated temperatures is attributed to the continuous phase viscosity reduced from 70 mPa·s to 25 mPa·s, leading to faster drainage, as well as to the coarsening rate increased from 6.60 × 104 to 9.83 × 105 μm3 min−1, concomitant with the change in bubbles shapes, indicting enhanced coarsening and coalescence. Likewise, the decrease in pressure from 10 MPa to 1 MPa also resulted in a 46.7% and 91.0% reduction in foam volume and lifetime at 100 °C, respectively. The impairment in foam properties with decreasing pressure also comes from the decrement of continuous phase viscosity, accelerating the drainage and thereby lowering the liquid fraction in the foam. The decreasing liquid fraction entails the liquid film to thin and weak, enhancing coarsening of bubbles and their merging. Our findings improve the insights into the foam evolution and pave a new road towards more comprehensive characterization of CO2 foam under HTHP conditions.