TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFormation of internal cake due to the invasion of reservoir rock by drilling, completion, and fracturing fluids is known to be one of the major causes of productivity reduction. Although the causes of internal cake formation are relatively well identified, the actual physical mechanisms involved in internal cake formation are yet to be understood. In particular, a quantitative means of relating physical properties of fluids to the degree of productivity reduction are not well established. A detailed investigation on the role of the rheology of viscoelastic fluids on the formation of "internal filter cakes" is, therefore, needed.The extensional viscosity for pure liquids without any structure is 3 times the shear viscosity. For these liquids, contribution of the extensional viscosity to the pressure loss is, therefore, constant and often neglected. However, for viscoelastic fluids, such as polymer based drilling and completion fluids, the extensional viscosity may be several orders of magnitude larger than the shear viscosity. This will lead to a significant increase in pressure loss, which is very often attributed to the development of internal cake.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 two different Partially Hydrolized Polyacrylamide (PHPA) solutions ( 1.5 and 3.0 lb/bbl) are shown in this paper.Comparison of the PHPA test results with the previously published Xanthan Gum (XG) test results are also provided.