Oil Muds

•The origin of non-aqueous drilling fluids can be traced to the 1920s when crude oil was used as a drilling fluid.
•The advantages of oil as a drilling and completion fluid were obvious even then:
–Clays do not hydrate and swell.
–Wellbore stability is improved.
–Production is improved from sandstones containing clays.
–Problems are reduced when drilling evaporites(salts, anhydrite, etc).
–Wellbore enlargement is reduced.
–Mud properties are more stable.
–Contamination resistance is increased.
•Oils also have certain characteristics that are undesirable.
–They are flammable and may contain compounds that cause the failure of rubber goods such as hoses, O-rings, gaskets and Blowout Preventer (BOP) elements.
–Oils lack gel structure and are difficult to viscosifyso they can be weighted.
–Many oils contain toxic or hazardous compounds that cause Health, Safety and Environmental (HSE) concerns.
–They have high gas solubility for many of the gases encountered when drilling wells (natural gas, carbon dioxide and hydrogen sulfide).
–This can interfere with kick detection and well-control procedures.
–Oils may not degrade readily under certain conditions.
–Oils also float on water and can migrate a significant distance from their source
•Today, an invert emulsion mud is a fluid with diesel oil, mineral oil or synthetic fluid as the continuous phase and water or brine as an emulsified phase.
–The emulsified water or brine is dispersed within the oil
–This is the internal phase.
–Calcium chloride salt is used to increase the emulsified water phase salinity to a level where it does not influence (soften or swell) water-sensitive formations and cuttings.
•Invert emulsion muds should be used when conditions justify their application.
•Environmental acceptability, disposal, initial makeup cost, daily maintenance cost, anticipated hole problems, formation evaluation and formation damage issues should all be considered.