Summary Xanthan polysaccharides are shown to contain some large but deformable multimolecular translucid aggregates called microgels, causing partial plugging at some distance from injection wells. A new laboratory procedure has been developed for testing the filterability of available products, among which fermentation broths are preferred to powder polymers. Introduction Xanthangums, which are biologically produced polysaccharides, are considered viable alternatives to hydrolyzed polyacrylamides fot use in enhanced oil recovery operations. Contrary to polyacrylamides, xanthan gums are less sensitive to salinity changes and mechanical degradation. Nevertheless, industrially available products are known to have plugging tendencies. High cost and poor processing efficiency for. improving their injectability also explain why only a few pilot tests with xanthan gums have been extended to a field scale. The origin of this formation plugging has been attributed either to residual bacterial cell debris coming from the fermentation process1–5 or to clumps of poorly dissolved polymer aggregates, called microgels,3–5 created by interactions with multivalent ions existing in field brines. So far, no publication has dealt with microgels in polysaccharide powder and with their plugging behavior. This, paper aims to contribute to a better understanding of microgel plugging mechanisms. A previously developed testing method6 was used to compare the flow properties of various industrial products or laboratory samples through porous media. Special attention also has been paid to the advantages of the. direct use of clarified and diluted fermentation broths rather than polysaccharides isolated in powder form. Polysaccharide Plugging Behavior Plugging Process With any mixing conditions and salt content, polysaccharide solutions exhibit a turbid appearance which has been related to cellular debris or undissolved polymer particles. Many methods have been proposed for removing these insoluble particles from biopolymer solutions - i.e., flocculation processes, enzyme clarification,8 chemical treatments,1 diatomaceous earth, or Millipore™ filter filtration.2,3,5,9 A clarification procedure is recommended before injecting polysaccharide solutions into wells. Filtration tests of unclarified solutions through Millipore filters,2,4,5,10 and sometimes through Berea sandstone cores,4,5 were carried out at extremely high pressure gradients, probably to simulate hydrodynamic conditions in the vicinity of injection wells. Plugging Process With any mixing conditions and salt content, polysaccharide solutions exhibit a turbid appearance which has been related to cellular debris or undissolved polymer particles. Many methods have been proposed for removing these insoluble particles from biopolymer solutions - i.e., flocculation processes, enzyme clarification,8 chemical treatments,1 diatomaceous earth, or Millipore™ filter filtration.2,3,5,9 A clarification procedure is recommended before injecting polysaccharide solutions into wells. Filtration tests of unclarified solutions through Millipore filters,2,4,5,10 and sometimes through Berea sandstone cores,4,5 were carried out at extremely high pressure gradients, probably to simulate hydrodynamic conditions in the vicinity of injection wells.