A method for reducing the loss of fluid through the walls of a well penetrating a permeable subterranean formation during completion, workover and other operations. The method provides for the use of a fluid comprising an aqueous dispersion of finely divided, slowly oil-soluble, water-insoluble solid particles in an aqueous salt solution containing chrome lignite, hydroxyethyl cellulose and xanthan gum. The density of the fluid is controlled by varying the concentration of salt dissolved in the solution. The finely divided solid particles consist of a homogeneous solid solution of (1) wax; (2) an oil-soluble surface active glyceryl or sorbitan partial ester of a saturated higher fatty acid; and (3) a water-dispersible surface active polyethylene glycol monoester of a saturated higher fatty acid. The particles may optionally include an ethylene/vinyl acetate copolymer, and/or a fatty alcohol.