A method of cleaning a wellbore prior to the production of oil or gas, wherein the wellbore has been drilled with an invert emulsion drilling mud that forms an invert emulsion filter cake, is disclosed. The method may include circulating a breaker fluid into the wellbore, where the breaker fluid includes a non-oleaginous internal phase and an oleaginous external phase, where the non-oleaginous phase includes a water soluble polar organic solvent, a hydrolysable ester of a carboxylic acid, and a weighting agent, and the oleaginous external phase includes an oleaginous fluid and an emulsifier, and where the hydrolysable ester is selected so that upon hydrolysis an organic acid is released and the invert emulsion of the filter cake breaks.