Properties and Interfacial Stabilization of a Crosslinked Hydroxypropyl Guar Aqueous Gel in a CO2-Rich Mixed-Phase Fracturing Fluid

A crosslinked hydroxypropyl guar (HPG) aqueous gel was investigated as the continuous, structure-forming phase of a CO2-rich mixed-phase fracturing fluid for unconventional gas reservoirs. The CO2 fraction was first screened by fluid-loss tests, after which the 30%, 40%, and 50% formulations were compared through proppant-settling tests. Although the 30% formulation exhibited the lowest cumulative fluid loss, the 40% formulation showed the lowest proppant-settling velocity and was selected for subsequent evaluation of phase behavior, gel-phase rheology, flow resistance, interfacial stabilization, and true-triaxial fracturing performance. Stable mixed-phase structures were maintained at 20–80 °C and 10–20 MPa, while bubble characteristics, rheology, and pipe-flow resistance showed a distinct response near 40 °C. Comparison with previously reported AM/AA–CO2 visualization showed that different water-soluble polymers generated different structures after CO2 mixing. Newly measured surface-tension and viscoelastic data, together with molecular dynamics simulations, indicated that bulk gel rheology alone could not account for this difference. HPG exhibited pronounced enrichment near the CO2–water interfaces, whereas AM/AA remained predominantly within the aqueous phase. Combined with the component-removal and viscoelasticity results, these findings suggest that HPG-specific interfacial behavior contributes to the formation and organization of dispersed structures, while the crosslinked gel network provides additional bulk viscoelastic support.