Patent ID: 8598090

Claim:
A method of cleaning a wellbore, wherein the wellbore has been drilled with an invert emulsion drilling mud that forms an invert emulsion filter cake, the method comprising: circulating a breaker fluid into the wellbore, the breaker fluid comprising: a non-oleaginous internal phase; and an oleaginous external phase, wherein the non-oleaginous phase comprises: a water soluble polar organic solvent; a hydrolysable ester of a carboxylic acid; and a weighting agent; and wherein the oleaginous external phase comprises: an oleaginous fluid; and an emulsifier; breaking at least a portion of the invert emulsion filter cake by an organic acid released by hydrolysis of the hydrolysable ester, wherein the hydrolysable ester is selected so that upon hydrolysis the organic acid is released and the invert emulsion of the filter cake breaks; wherein the hydrolysable ester of the carboxylic acid is present in the non-oleaginous phase in an amount ranging from about 20 to 60 volume percent of the breaker fluid.