A spacer fluid composition for use in a method for introducing and positioning a first fluid in a well bore annulus of a well bore containing a second fluid, the spacer fluid composition comprising: a base aqueous fluid comprising fresh water, a viscosifier comprising a chemically modified tannin, a weighting agent comprising barite, an antifoaming agent comprising a silicone-based liquid, and a non-ionic surfactant comprising an ethoxylated alcohol, wherein: the viscosifier is in a range of from 2.3 kg to 4.5 kg per 159 L (5 pounds to 10 pounds per barrel) of base aqueous fluid in the spacer fluid composition, the weighting agent is in a range of from 45.4 kg to 181 kg per 159 L (100 pounds to 400 pounds per barrel) of base aqueous fluid in the spacer fluid composition, the antifoaming agent is in a range of from 0.05 to 0.91 litres per 159 L (0.01 gallons to 0.2 gallons per barrel) of base aqueous fluid in the spacer fluid composition, and the non-ionic surfactant agent is in a range of from 6.8 to 9.1 litres per 159 L (1.5 gallons to 2.0 gallons per barrel) of base aqueous fluid in the spacer fluid composition, the density of the spacer fluid is in the range of from 1121.3 to 1922.2 kg/m 3 (70 to 120 pounds per cubic foot), the chemically modified tannin is selected from the group consisting of sulfoalkylated tannins, causticized tannins, sulfited tannins, sodium-complexed tannins and sulfomethylated quebracho, and wherein the spacer fluid composition is compatible with a first fluid that is a water-based cement slurry and a second fluid that is an oil-based fluid and wherein the first fluid and the second fluid are incompatible with one another, where both compatibility and non-compatibility are determined using American Petroleum Institute Recommend Practice 10B-2 "Recommended Practice for Testing Well Cements". a base aqueous fluid comprising fresh water, a viscosifier comprising a chemically modified tannin, a weighting agent comprising barite, an antifoaming agent comprising a silicone-based liquid, and a non-ionic surfactant comprising an ethoxylated alcohol, the viscosifier is in a range of from 2.3 kg to 4.5 kg per 159 L (5 pounds to 10 pounds per barrel) of base aqueous fluid in the spacer fluid composition, the weighting agent is in a range of from 45.4 kg to 181 kg per 159 L (100 pounds to 400 pounds per barrel) of base aqueous fluid in the spacer fluid composition, the antifoaming agent is in a range of from 0.05 to 0.91 litres per 159 L (0.01 gallons to 0.2 gallons per barrel) of base aqueous fluid in the spacer fluid composition, and the non-ionic surfactant agent is in a range of from 6.8 to 9.1 litres per 159 L (1.5 gallons to 2.0 gallons per barrel) of base aqueous fluid in the spacer fluid composition, the density of the spacer fluid is in the range of from 1121.3 to 1922.2 kg/m 3 (70 to 120 pounds per cubic foot), the chemically modified tannin is selected from the group consisting of sulfoalkylated tannins, causticized tannins, sulfited tannins, sodium-complexed tannins and sulfomethylated quebracho, and wherein the spacer fluid composition is compatible with a first fluid that is a water-based cement slurry and a second fluid that is an oil-based fluid and wherein the first fluid and the second fluid are incompatible with one another, where both compatibility and non-compatibility are determined using American Petroleum Institute Recommend Practice 10B-2 "Recommended Practice for Testing Well Cements".