Drilling fluids, also known as drilling muds, are used during drilling operations to maintain borehole stability, remove drilled material, control pressures, and cool and lubricate the bit. They can be liquid or gaseous, with water and oil-based solutions being the most common. Petroleum-based fluids are used for higher lubrication and heat generation, while gaseous fluids can include air and other gases. The primary function of drilling fluids is to maintain borehole integrity and prevent fluids from entering the drill string.
Drilling fluids are liquid or gaseous substances that are often pumped down wells during drilling operations. These fluids are usually called drilling muds when liquids are used, although that does not mean they are made from actual mud. Liquid drilling fluids can be water or oil based, and gaseous fluids often contain air. One of the primary purposes of drilling fluid is to maintain the stability of a borehole, although it can perform many other tasks as well. Some other functions include facilitating the removal of drilled material from the hole, controlling pressures within a formation, cooling and lubricating the bit, and even transferring hydraulic power to the lower hole assembly components.
There are many different types of drilling fluids, each of which has qualities that are uniquely suited to particular situations. Most drilling fluids can be divided into the categories of liquids and gases. Liquid-based fluids are usually called drilling muds and can be further broken down into water- and oil-based solutions. The water is sometimes used on its own, although it is more commonly mixed with bentonite clay and various other chemicals. Clay is commonly referred to as a “gel” in the oil and gas industry, and chemical additives are usually intended to control various characteristics such as viscosity.
Petroleum-based drilling fluid can be synthetic or natural, and diesel fuel is a natural substance that is sometimes used. These fluids are often chosen for applications where higher lubricating qualities are required or a large amount of heat is likely to be generated in the drilling process. Synthetic oils are usually chosen when the toxic fumes from natural oils could be hazardous, which is often a consideration in the enclosed working conditions found on offshore oil platforms.
Gaseous drilling fluids sometimes consist of air being pumped into the drill string, although various other gases may also be included. Air is also sometimes mixed with water, which can increase the viscosity of a drilling fluid, provide additional dust control, or help clean the borehole. Polymers are sometimes included in an air/water mixture when a blowing agent or other additive is required.
The primary purpose of a drilling fluid is to maintain the integrity of a borehole and prevent fluids present in the rock formations from entering the drill string. Another primary function is to assist in passing drill bits down the well to the surface, where they can be disposed of. The fluid is often used for cooling and lubricating properties, both for drill bit and bottom hole assembly machinery such as mud engines. The hydraulic pressure present in the fluid can also be used to power mud engines and other components.
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