Well bore fluid

A method for the completion or work-over of a well comprises the step of using a solids-free, non-aqueous well-bore fluid comprising a halogenated organic compound as a completion or work-over fluid. The fluid has a specific gravity in the range 0.9 to 2.3. Preferred halogenated organic compounds are brominated aromatic ethers, diphenyls, aliphatic hydrocarbons, benzene and alkyl benzenes. The halogenated organic compound may be dissolved in an organic solvent. The relative proportions may be chosen to provide a well-bore fluid having a desired specific gravity. The fluids are non-corrosive, thermally stable and non-damaging to formations.