Abstract: Fuzzy-ball working fluids (FBWFs) have been successfully applied in different development phases of tight reservoirs. Field reports revealed that FBWFs satisfactorily met all the operational and reservoir damage control requirements during their application. However, the damage-control mechanisms and degree of formation damage caused by fuzzy-ball fluids have not been investigated in lab-scale studies so far. In this study, the degree of fuzzy-ball-induced damage in single-and double-layer reservoirs was evaluated through core flooding experiments that were based on permeability and flow rate indexes. Additionally, its damage mechanisms were observed via scanning electron microscope and energy-dispersive spectroscopy tests. The results show that: (1) For single-layer reservoirs, the FBWF induced weak damage on coals and medium-to-weak damage on sandstones, and the difference of the damage in permeability or flow rate index on coals and sandstones is below 1%. Moreover, the minimum permeability recovery rate was above 66%. (2) For double-layer commingled reservoirs, the flow rate index revealed weak damage and the overall damage in double-layer was lower than the single-layer reservoirs. (3) There is no significant alteration in the microscopic structure of fuzzy-ball saturated cores with no evidence of fines migration. The dissolution of lead and sulfur occurred in coal samples, while tellurium in sandstone, aluminum, and magnesium in carbonate. However, the precipitation of aluminum, magnesium, and sodium occurred in sandstone but no precipitates found in coal and carbonate. The temporal plugging and dispersion characteristics of the FBWFs enable the generation of reservoir protection layers that will minimize formation damage due to solid and fluid invasion.