TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractXanthan gum has been used extensively in the oil industry as a viscosifier for different applications due to its unique rheological properties. In this paper we explore how the rheological behavior of xanthan-based fluids can be used to control fluid loss. Linear and radial flow tests were performed in 100-1,000 md rocks. The rheological characteristics of xanthan gum were measured in linear core flow tests. This constitutive flow behavior was used in a radial flow simulator to predict the invasion profile of xanthan gum in the formation. Radial flow tests were performed to validate the predictions from the simulator and to observe the effect of fluid loss additives such as starch and ground Berea.A laboratory scale drilling simulator was used to determine the leakoff and formation damage of xanthan-based drilling fluids. The fluid was circulated through tubing and cuttings were removed from the annulus. Thin section analysis and environmental SEM were performed on rock samples taken at different distances from the wellbore to determine the nature and depth of the damage.Results show that fines generated during the drilling process form an external filter cake which in combination with xanthan gum results in considerable fluid loss reduction. Damage due to xanthan gum is small and limited to a narrow thickness around the wellbore, resulting in negligible skin factors. The use of starch can lead to considerable damage and large skin factors if allowed to invade the formation.