Drilling fluid types - PetroWiki

Petrowiki 

There are several different types of drilling fluids, based on both their composition and use. Selecting the correct type of fluid for the specific conditions is an important part of successful drilling operations. Water-based fluids (WBFs) are the most widely used systems, and are considered less expensive than oil-based fluids (OBFs) or synthetic-based fluids (SBFs). The OBFs and SBFs--also known as invert-emulsion systems--have an oil or synthetic base fluid as the continuous(or external) phase, and brine as the internal phase. Invert-emulsion systems have a higher cost per unit than most water-based fluids, so they often are selected when well conditions call for reliable shale inhibition and/or excellent lubricity.