Xanthan 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 100–1,00 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. Introduction The rate of leakoff is of critical importance during drilling, completion operations (i.e. sand control) and stimulation treatments, such as acid treatments and hydraulic fracturing. In all of these cases, fluid loss control has been achieved by two basic mechanisms: 1Increasing the overall viscosity of the fluid using high polymer concentrations or by crosslinking the polymer.2,3Developing an internal and/or external filter cake using fluid loss additives (starch, sized CaCO3, mica, silica flour, oil soluble resins, etc.) to plug the pore-throats of the formation.4 Both fluid loss control mechanisms may result in a loss of permeability when flow is initiated in the production mode. Furthermore, if fluid loss additives are not used properly, they can cause significant loss of permeability due to their plugging mechanism if they enter the formation.5,6 Xanthan gum has been used extensively as a viscosifier in the oil field for drilling, drill-in and completion fluids due to its unique rheological properties.7 In this paper we explore the rheological properties of xanthan-based fluids in Berea sandstone rocks and how these properties can be used to control fluid loss. Prior attempts to simulate the flow of non-Newtonian fluids in porous media have not been entirely satisfactory because of the lack of an adequate correlation between the deformation rates inside the pore-throats and the velocity of the fluid. 3,8,9 Linear and radial flow tests were performed in 100 to 1,000 md rocks. The rheological behavior of xanthan gum was 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. Simulations of field scale wellbore invasion profiles are presented using both xanthan gum and HEC. The effect of fluid loss additives, such as starch and sized CaCO3 was also studied in radial flow leak-off tests. The damage left over associated with those additives was quantified and compared to pure xanthan-based fluids.
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