In this study, the effect of the rheology of viscoelastic fluids on the formation of internal filter cakes has been investigated. A series of core flow experiments have been conducted to investigate the possible relationship among the polymer concentration, fluid extensional viscosity, shear viscosity, filtration loss characteristics, and pressure drop across the core samples, and hence, any change in the original rock permeability. Results of extensional and shear viscosity measurements, API filtration loss tests, and formation damage tests conducted by using three different partially hydrolyzed polyacrylamide (PHPA) solutions (0.5, 1.5, and 3.0 lb/bbl) are shown. Comparison of the PHPA test results with the previously published xanthan gum (XG) test results is also provided.