Each drilling fluid vendor provides a wide array of basic and specialty chemicals to meet the needs of the drilling industry. The general classification of drilling fluid additives below is based on the definitions of the International Association of Drilling Contractors (IADC):
- Alkalinity or pH control additives are products designed to control the degree of acidity or alkalinity of a drilling fluid. These additives include lime, caustic soda, and bicarbonate of soda.
- Bactericides reduce the bacteria count of a drilling fluid. Paraformaldehyde, caustic soda, lime, and starch are commonly used as preservatives.
- Calcium removers are chemicals used to prevent and to overcome the contaminating effects of anhydride and gypsum, both forms of calcium sulfate, which can wreck the effectiveness of nearly any chemically treated mud. The most common calcium removers are caustic soda, soda ash, bicarbonate of soda, and certain polyphosphates.
- Corrosion inhibitors such as hydrated lime and amine salts are often added to mud and to air-gas systems. Mud containing an adequate percentage of colloids, certain emulsion muds, and oil muds exhibit, in themselves, excellent corrosion-inhibiting properties.
- Defoamers are products designed to reduce foaming action, particularly that occurring in brackish water and saturated saltwater muds.
- Emulsifiers are used for creating a heterogeneous mixture of two liquids. These include modified lignosulfonates, certain surface-active agents, anionic and nonionic (negatively charged and noncharged) products.
- Filtrate, or fluid loss, reducers such as bentonite clays, sodium carboxymethyl cellulose (CMC), and pregelatinized starch serve to cut filter loss, a measure of the tendency of the liquid phase of a drilling fluid to pass into the formation.
- Flocculants are used sometimes to increase gel strength. Salt (or brine), hydrated lime, gypsum, and sodium tetraphosphates may be used to cause the colloidal particles of a suspension to group into bunches of “flocks,” causing solids to settle out.
- Foaming agents are most often chemicals that also act as surfactants (surface-active agents) to foam in the presence of water. These foamers permit air or gas drilling through water-production formations.
- Lost circulation materials (LCM) include nearly every possible product used to stop or slow the loss of circulating fluids into the formation. This loss must be differentiated from the normal loss of filtration liquid and from the loss of drilling mud solids to the filter cake (which is a continuous process in an open hole).
- Extreme-pressure lubricants are designed to reduce torque by reducing the coefficient of friction and thereby increase horsepower at the bit. Certain oils, graphite powder, and soaps are used for this purpose.
- Shale control inhibitors such as gypsum, sodium silicate, chrome lignosulfonates,
as well as lime and salt are used to control caving by swelling or hydrous disintegration of shales.
- Surface-active agents (surfactants) reduce the interfacial tension between contacting surfaces (e.g., water—oil, water—solid, water— air); these may be emulsifiers, de-emulsifiers, flocculants, or deflocculents, depending upon the surfaces involved.
- Thinners and dispersants modify the relationship between the viscosity and the percentage of solids in a drilling mud and may further be used to vary the gel strength and improve “pumpability.” Tannins (quebracho), various polyphosphates, and lignitic materials are chosen as thinners or as dispersants, because most of these chemicals also remove solids by precipitation or sequestering, and by deflocculation reactions.
- Viscosifiers such as bentonite, CMC, Attapulgite clays, sub-bentonites, and asbestos fibers are employed in drilling fluids to ensure a high viscosity–solids ratio.
- Weighting materials, including barite, lead compounds, iron oxides, and similar products possessing extraordinarily high specific gravities, are used to control formation pressures, check caving, facilitate pulling dry drill pipe on round trips, and aid in combating some types of circulation loss.
The most common commercially available drilling mud additives are published annually by World Oil. The listing includes names and descriptions of more than 2,000 mud additives.
References:
1. Drilling Operations.
2. Drilling Equipment and Operation.
