Abstract: An aqueous spacer composition, compatible with drilling muds and cement slurries including saline slurries, and application there of drilling oil and gas wells.Preferred spacer bases in accordance with the invention comprise a biopolymer of the type produced by Azotobacter indicus, a high molecular weight sulfonated styrene or toluene-styrene polymer, and a dispersing agent. This base fluid has no solid particles. The final spacer composition additionally contains a weighting agent (barite) and water. Total compatibility is provided with muds and/or cement slurries whether or not they are saline, together with excellent fluid loss controlling, rheological, and anti-settling properties. A wetting agent may be added to provide compatibility with oil-based muds.