Wellbore-cleaning compositions comprising microemulsions are useful during cementing operations. The microemulsion solvent may be selected from the group comprising methyl esters and ethoxylated alcohols. The microemulsion surfactants comprise a water-wetting surfactant, a cleaning surfactant, an emulsifying surfactant, a non-ionic surfactant and a non-ionic co-surfactant. The wellbore-cleaning compositions may be pumped alone, ahead of a conventional spacer fluid or scavenger slurry and behind a conventional spacer fluid or scavenger slurry. In addition, the wellbore-cleaning composition may be incorporated into a carrier fluid such as (but not limited to) a conventional spacer fluid or scavenger slurry. The resulting fluid mixture may be pumped as a spacer fluid.