A C14(PO)4SO3Na-Synergized Microsphere–Microgel System for Heterogeneous Reservoirs in the Changqing Oilfield: Evaluation of Its Synthesis and Enhanced Oil Recovery Performance

Long-term water flooding in heterogeneous Changqing Oilfield reservoirs leaves substantial residual oil trapped in low- and ultra-low-permeability pores, where small pore throats, strong capillarity, and high seepage resistance restrict waterflood sweep efficiency and residual oil mobilization. To address the wide carbon-number distribution, strong low-permeability heterogeneity, and fracture-induced channeling in Block X of the Changqing Oilfield, we proposed a synergistic strategy combining interfacial modification and preferential-pathway regulation. We constructed a C14(PO)4SO3Na (TDS)-synergized microsphere–microgel system comprising the interfacial modifier TDS, microspheres, and microgel particles. Core flooding showed that TDS alone outperformed conventional surfactants (Cocamidopropyl betaine, CAB-35). In heterogeneous models, the microsphere/TDS system achieved 54.45% recovery; in fractured models, microgel showed better plugging and flow-diversion performance than microspheres. Using a 0.3 PV microgel pre-slug followed by a 0.7 PV microgel/TDS mixture, Model II reached 45.58% recovery, indicating that plugging preferential pathways before synergistic flooding is an optimal injection strategy. Microvisualization revealed that water flooding readily channeled through large pores, high-permeability pathways, and fractures. In micromodel experiments, the composite system increased areal sweep efficiency to 60.4%, 49.8%, and 66.4% for low-, high-, and ultra-low-permeability models, respectively. In comparison, core flooding gave recovery factors of 54.45% (heterogeneous matrix) and 45.58% (fractured Model II). Produced-fluid and core SEM results indicated that regulating particles migrated, were retained, bridged, and possibly re-migrated in pore throats, increasing flow resistance in preferential pathways and diverting subsequent fluids to unswept zones.