Wellbore fluid

An oil-based wellbore fluid in the form of an emulsion which comprises continuous phase comprising an oil and, dispersed therein, a first dispersed liquid phase and a second separate dispersed liquid phase, the first dispersed liquid phase being a brine phase containing at least 5% by weight of a dissolved salt or a mixture of salts, and the second dispersed liquid phase comprising a polar organic liquid, said polar organic liquid being insoluble or sparingly soluble in the brine comprising said first dispersed liquid phase, but being soluble in pure water.

A method of carrying out a wellbore operation, which includes the steps of:

• (a) introducing into a wellbore an oil-based wellbore fluid comprising a continuous phase comprising an oil and, dispersed therein, a first dispersed liquid phase and a second separate dispersed liquid phase, the first dispersed liquid phase being a brine phase containing at least 5% by weight of a dissolved salt or a mixture of salts, and the second dispersed liquid phase comprising a polar organic liquid, said polar organic liquid being insoluble or sparingly soluble in the brine comprising said first dispersed liquid phase, but being soluble in pure water and additionally comprising at least one additive selected from the group consisting of clays, filtration reducers, weighting agents, bridging particles, and suspended salt particles such that a filter-cake forms on the wellbore wall or in conduits in a rock formation that is penetrated by the wellbore;
• and subsequently (b) allowing a further fluid to come into contact with the filter-cake, said further fluid having a level of salinity which is lower than the level of salinity of said first dispersed liquid phase, and being such that said polar organic liquid is soluble therein.