It refers to the drilling fluid with oil as the continuous phase. As early as the 1920s, people used crude oil as drilling fluid to avoid and reduce various complications in drilling. However, in practice, it is found that the use of crude oil has the following disadvantages: small shearing force, difficulty in suspending barite, large filtration loss, and volatile components in crude oil are easy to cause fires. So it gradually developed into two kinds of drilling fluids with diesel oil as the continuous phase—full oil-based drilling fluid and water-in-oil emulsion drilling fluid. In full oil-based drilling fluid, water is a useless component, and its water content should not exceed 7%; while in water-in-oil drilling fluid, water is evenly dispersed in diesel oil as an essential component, and its water content is generally 10%~60%.
