Thermally activated self-adhesive flexible granules for sealing fractures in high-temperature and high-salinity reservoirs: Synthesis, tunability, and performance evaluation

To overcome the limitations of conventional preformed particle gels in ultra-high-temperature and high-salinity fractured reservoirs, this study developed a novel self-adhesive flexible granule (SAFG). SAFG is synthesized through a two-step process involving the hydrophobic modification of polyvinyl alcohol, followed by chemical cross-linking with terephthalaldehyde, resulting in a stable granular structure. Comprehensive characterization confirmed the successful synthesis, tunable granule size controlled by stirring speed, and exceptional long-term thermal stability in high-salinity brine (2.1 × 10⁵ mg/L, 130℃ for 180 d). Response surface methodology (RSM) optimization determined the optimal synthesis parameters, yielding granules with superior comprehensive performance. SAFG exhibits a unique, tunable self-adhesion mechanism activated by elevated temperature in high-salinity brine, transforming into a cohesive bulk gel. Core-flooding experiments demonstrated effective migration and dynamic plugging in simulated fractures (fracture widths of 0.5 and 1.0 mm), with optimal performance observed at injection rates of 1.5–2.5 mL/min and SAFG concentrations of 5.0 wt%. Critically, after thermally assisted adhesion aging, the adhered SAFG structure achieved a breakthrough pressure of 3080 kPa, approximately 6.3 times the initial stabilization pressure, confirming its robust sealing capability. This work demonstrates SAFG as a robust and tunable conformance control agent for extreme reservoir conditions, offering a viable solution to enhance oil recovery in challenging fractured formations.