Well treatment compositions comprise water, a polysaccharide and cellulose fibers. The cellulose fibers may have lengths between 100 nm and 10 mm, and diameters between 4 nm and 40 μm. The compositions may be used as spacer fluids during well cementing operations. The spacer fluids may prevent commingling of a cement slurry with a drilling fluid. The drilling fluid may be water-base, oil-base, synthetic-base or an emulsion.