In oilfild operations, it is important to decrease water production and increase the efficiency of oil sweep. A gel treatment is using preformed particle gels (PPGs) to reduce water flow through high-permeability formation in mature reservoirs. The success of PPGs treatment primarily depends on whether or not PPGs can reduce the high permeability zones to the anticipate level. This work sought to find methods that minimized the damage caused by PPGs on unswept, low-permeable zones/areas, thus improving PPG treatment efficiency. These results can help to identify of the best PPG types, particle sizes, and brine concentrations for specific reservoir condition. Both a static filtration test model and load pressure model were used to evaluate the damage caused by various PPGs on sandstone core samples with various degrees of permeability. Two types of particle gels, a strong Daqing gel (DQ) and a weak LiquiblockTM 40k gel (40K), were used for various experiments, each with a different strength. The particle sizes ranged from 30 to 120 meshes. Sodium chloride (NaCl) was used to prepare all brines. Various brine concentrations at room temperature were selected to prepare the swollen PPGs. The effect of particle size, core permeability, and brine concentration on core damage was investigated. Results of these experiments yielded information useful for promoting the best PPG treatment for conformance control in mature reservoirs. These results can be used to optimize PPG's design and, thus, prevent damage to the unswept, low-permeable zones/areas.