A spacer fluid made of a viscosity thinner, a weighting agent, an antifoaming agent, and a non-ionic surfactant in a base aqueous fluid. In some instances, the viscosity thinner is a sulfomethylated tannin, the weighting agent is barium sulfate, the antifoaming agent is a silicone, and the non-ionic surfactant is an ethoxylated alcohol. A method of treating a well bore annulus in preparation of introducing water-based cement slurry into a well bore using the spacer fluid. A method of using the spacer fluid to position a first fluid into a well bore annulus of a well bore containing a second fluid. A method for fluidly isolating at least a portion of a well bore annulus in a well bore containing an oil-based drilling fluid using water-based cement slurry and the spacer fluid.