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Water soluble organic polymers such as hydroxyethyl cellulose have been used to slow the leak-off rate of clear brines into permeable formations. Fluid loss or leak -off, however, can only be effectively controlled by bridging the pore openings with rigid or semi-rigid particles of sufficient size and number. Production engineers have been reluctant to use particle bridging because of the possibility of particle transport into the formation resulting in formation damage and/or costly and often ineffective stimulation treatments. A particle bridging fluid has been developed that quickly and effectively controls fluid loss in a wide range of permeabilities and pore diameters. The filter cake formed in this process, however, is highly dispersible to the produced fluid and thus effectively removed by placing the well on production. No acid treatment or other removal techniques are required.

The primary bridging agent in this fluid is a sized calcium carbonate with particle sizes (Fig. 1) capable of initiating bridging in pore diameters in excess of 100 micro meters. The particle size distribution also compliments the smaller openings created during the filter cake formation process, thus assuring that fine particle transport into the formation is minimized. This external bridging has been documented by Scanning Electron Microscopy. Cross sections of a dynamic filter cake obtained at elevated temperatures and pressure were examined under the SEM, shows sharp delineation between the cake and filter media. That is there were no significant calcium carbonate particles transported into the filter media pores. This property greatly enhances filter cake removal on back flow and thus provides for good return permeability.

Water soluble organic polymers are added to provide rheological and filtration control for the fluid system. The filtration control polymers are sufficiently flexible and deformable to fill the small irregular pore openings of the final particle bridge and thus provide positive leak-off control for the fluid. Since the calcium carbonate particles will settle in water or even high to decrease adhesion of filter cake particles thus greatly enhancing the dispersability of the cake. By utilizing this technique, the filter cake can be more effectively removed from the sand face by the produced fluid.

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