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.