The No. 8 Block of Xinjiang represents a typical ultra-low-permeability fractured oilfield in which conventional water-quality evaluation techniques fail to reproduce the water-quality evolution and associated reservoir responses that occur within fractured reservoirs. To address this limitation, a graded water-quality evaluation method and an associated apparatus tailored for water injection in ultra-low-permeability fractured reservoirs were independently developed. Using this method and apparatus, fracture–matrix core water-injection displacement experiments were conducted to quantify water–reservoir compatibility and to characterize permeability evolution in fracture–matrix cores during displacement. An economic evaluation of fracture–matrix core water-injection schemes was subsequently performed. The results indicate that, for fracture–matrix cores with a permeability of 8 mD, suspended particles with a median diameter ≤ 1.5 μm and a concentration ≤ 8 mg/L cause a total permeability impairment of < 20%, demonstrating favorable compatibility and unobstructed migration, with particle retention concentrated primarily within fracture zones. For fractures with a permeability of 20 mD, suspended particles with a median diameter ≤ 3 μm and a concentration ≤ 6 mg/L similarly result in permeability impairment < 20%, indicating good compatibility and successful passage. By integrating reservoir-permeability variations and historical water-injection data, an economic assessment model that accounts for reservoir evolution, oil prices, and injected-water quality was established. The model enables the identification of water-quality standards that both ensure effective reservoir development and maintain economic viability.