Drilling fluids

- Introduction :

Density is a measurement of fluid weight per unit of volume. This measurement is often referred to as mud weight and is reported as pounds per gallon, pounds per cubic foot, grams per cubic foot, grams per cubic centimeter, kilograms per cubic centimeter, or as Specific Gravity (sg) gms/cm³.

The primary function of monitoring density is to control formation pressures and minimize loss of returns.

- Drilling fluids:

Drilling mud, also known as drilling fluid, is a product used in the process of drilling deep boreholes. These holes may be drilled for oil and gas extraction, core sampling, and a wide variety of other reasons. The mud can be an integral part of the drilling process, serving a number of functions.

Drilling fluid is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid is often called drilling mud. The three main categories of drilling fluids are water-based muds (which can be dispersed and non-dispersed), non-aqueous muds, usually called oil-based mud, and gaseous drilling fluid, in which a wide range of gases can be used.

- Types of drilling fluid

Many types of drilling fluids are used on a day-to-day basis. Some wells require that different types be used at different parts in the hole, or that some types be used in combination with others. The various types of fluid generally fall into a few broad categories:

· Water-based mud (WBM): A most basic water-based mud system begins with water, then clays and other chemicals are incorporated into the water to create a homogenous blend resembling something between chocolate milk and a malt (depending on viscosity).

The clay (called "shale" in its rock form) is usually a combination of native clays that are suspended in the fluid while drilling, or specific types of clay that are processed and sold as additives for the WBM system. The most common of these is bentonite, frequently referred to in the oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a "gel" structure that resists flow.

· Oil-based mud (OBM): Oil-based mud can be a mud where the base fluid is a petroleum product such as diesel fuel. Oil-based muds are used for many reasons, some being increased lubricity, enhanced shale inhibition, and greater cleaning abilities with less viscosity. Oil-based muds also withstand greater heat without breaking down. The use of oil-based muds has special considerations. These include cost, environmental considerations such as disposal of cuttings in an appropriate place to isolate possible environmental contamination and the exploratory disadvantages of using oil based mud, especially in wildcat wells due inability to analyze oil shows in cuttings, because the oil based mud has fluorescence confusing with the original oil of formation. Therefore induces contamination of cuttings samples, cores, sidewall cores for geochemical analysis of TOC and masks the real determination of API gravity due to this contamination.

· Air: Compressed air is pumped either down the bore hole's annular space or down the drill string itself.

· Air/water: The same as above, with water added to increase viscosity, flush the hole, provide more cooling, and/or to control dust.