Drilling fluids are classified according to the type of fluids and other primary ingredients
2.Aqueous: Gasified–foam, energized(including aphrons)clay, polymer, emulsion
3.Nonaqueous:Oil or synthetic–all oil, invert emulsion
True foams contain at least 70% gas(usually N2,CO2,or air)at surface of the hole,while energized fluids, including aphrons, contain lesser amounts of gas.Aphrons are specially stabilized bubbles that function as a bridging or lost circulation material(LCM)to reduce mud losses to permeable and microfractured formation.Aqueous drilling fluids are generally dubbed water-based muds (WBMs), while nonaqueous drilling fluids (NAFs) are often referred to as oil-based muds (OBMs) or synthetic-based muds (SBMs). OBMs are based on NAFs that are distilled from crude oil; they include diesel, mineral oils, and refined linear paraffins (LPs). SBMs, which are also known as pseudo– oil-based muds, are based on chemical reaction products of common feedstock materials like ethylene; they include olefins, esters, and synthetic LPs.
Detailed classification schemes for liquid drilling fluids are employed that describe the composition of the fluids more precisely. One such classification scheme is shown in Figures 2.1 and 2.2. An even more precise classification scheme is described in Table 2.1, which includesthe mud systems most commonly used today, along with their principal components and general characteristics.