World Oil's Guide to Drilling, Completion and Workover Fluids 2000

Descriptions of fluid-system classifications, product functions and source companies are listed on these pages. System descriptions and product definitions have been kept as simple as possible and, wherever practical, reflect general industry practice and terminology consistent with descriptions adopted by the American Petroleum Institute (API) and the International Association of Drilling Contractors (IADC).

Basic System Classifications

Nine distinct fluid systems are defined, with the first six being water-based. Next are oil- and synthetic-based systems, along with the last, which consists of air, mist, foam or gas as the circulating medium.

All chemicals and fluid systems listed in these tables are designed for use in drilling, completion and workover operations.

Non-dispersed. These systems include spud muds, natural muds and other lightly treated systems generally used for shallow wells or top-hole drilling. Thinners and dispersants are not added to disperse drill solids and clay particles.

Dispersed. At greater depths, where higher densities are required, or where hole conditions may be problematic, muds are often dispersed, typically with lignosulfonates, lignites or tannins. These and similar products are effective deflocculants and filtrate reducers. Potassium-containing chemicals are frequently used to provide greater shale inhibition. Specialized chemicals are also added to adjust or maintain specific mud properties.

Calcium treated. Divalent cations such as calcium and magnesium, when added to a freshwater drilling mud, inhibit formation clay and shale swelling. High levels of soluble calcium are used to control sloughing shale and hole enlargement, and to prevent formation damage. Hydrated lime (calcium hydroxide), gypsum (calcium sulfate) and calcium chloride are principal ingredients of calcium systems.

Gyp systems usually have a pH of 9.5 to 10.5 and an excess gyp concentration of 2 to 4 lb/bbl (600 to 1,200 mg/l calcium); Lime systems typically have either excess lime concentration of 1 to 2 lb/bbl and pH of 11.0 to 12.0 for a low lime system or excess lime concentration of 5 to 15 lb/bbl for a high lime system. Specialized products are added to control individual mud properties. Calcium-treated muds resist salt and anhydrite contamination but are susceptible to gelation and solidification at high temperatures.

Polymer. Muds incorporating generally long-chain, high-molecular-weight polymers are utilized to either encapsulate drill solids to prevent dispersion and coat shales for inhibition, or for increasing viscosity and reducing fluid loss. Various types of polymers are available for these purposes, including acrylamide, cellulose and natural gum-based products. Frequently, inhibiting salts such as KCl or NaCl are used to provide greater shale stability. These systems normally contain a minimum amount of bentonite and may be sensitive to divalent cations such as calcium and magnesium. Most polymers have temperature limits below 300°F, but under certain conditions may be used in wells with appreciably higher BHTs.

Low solids. Included are systems in which the amount (volume) and type of solids are controlled. Total solids should not range higher than about 6% to 10% by volume. Clay solids should be some 3% or less and exhibit a ratio of drilled solids to bentonite of less than 2:1. Low-solids systems typically use polymer additive as a viscosifier or bentonite extender and are non-dispersed. One primary advantage of low-solids systems is that they significantly improve drilling penetration rate.

Saltwater systems. Several mud systems have been included in this classification. Saturated salt systems have a chloride concentration near 190,000 mg/l (saturated) and are used to drill salt formations. Saltwater systems have a chloride content of 10,000 to 190,000 mg/l. The lower levels are usually referred to as brackish or seawater systems. Saltwater muds are usually prepared from brackish, seawater or produced-water sources.

Muds are prepared from fresh or brine water and dry sodium chloride (or other salts such as potassium chloride used for shale inhibition), which are added to achieve desired salinity. Various specialty products such as attapulgite, CMC, starch and others are used to increase viscosity for hole-cleaning properties and to reduce fluid loss.

Oil-based muds. Oil-based systems are used for a variety of applications where fluid stability and inhibition are necessary such as high-temperature wells, deep holes, and where sticking and hole stabilization is a problem. They consist of two types of systems:

1. Invert emulsion muds are water-in-oil emulsions typically with calcium chloride brine as the emulsified phase and oil as the continuous phase. They may contain as much as 50% brine in the liquid phase. Relaxed invert emulsion muds are a "relaxed" emulsion and have lower electrical stabilities and higher fluid-loss values. Concentration of additives and brine content / salinity are varied to control rheological, filtration and emulsion stability.
2. Oil-based muds are formulated with only oil as the liquid phase and are often used as coring fluids. Although these systems pick up water from the formation, no additional water or brine is added. All oil systems require higher additional gelling agents for viscosity. Specialized oil-based mud additives include: emulsifiers and wetting agents (commonly fatty acids and amine derivatives) for viscosity, high-molecular-weight soaps, surfactants, amine treated organic materials, organo clays and lime for alkalinity.

Synthetic muds. Synthetic fluids are designed to mirror oil-based mud performance, without the environmental hazards. Primary types of synthetic fluids are esters, ethers, poly alpha olefins and isomerized alpha olefins. They are environmentally friendly, can be discharged offshore and are non-sheening and biodegradable.

Air, mist, foam and gas. Four basic operations are included in this specialized category. These include: 1) Dry air drilling, which involves injecting dry air or gas into the wellbore at rates capable of achieving annular velocities that will remove cuttings; 2) Mist drilling involves injecting a foaming agent into the air stream, which mixes with produced water and coats the cuttings to prevent mud rings, allowing drill solids to be removed; 3) Foam uses surfactants and possibly clays or polymers to form a high carrying-capacity foam; and 4) Aerated fluids rely on mud with injected air (which reduces hydrostatic head) to remove drilled solids from the wellbore.

Function of Additives

The product function classifications for each additive are those generally accepted by the Subcommittee on Drilling Fluids, IADC. Some additives have multiple uses, and for those a primary and two secondary function categories are listed.

Alkalinity, pH control additives. Products used to control the degree of acidity or alkalinity of a fluid include lime, caustic soda, soda ash and bicarbonate of soda, as well as other common acids and bases.

Bactericides. Products are used to prevent bacterial degradation of natural organic additives such as starch and xanthan gum.

Calcium reducers. Soda ash, bicarbonate of soda, caustic soda and certain polyphosphates are used to reduce calcium in seawater, treat cement contamination and overcome contaminating effects of anhydrite and gypsum, both forms of calcium sulfates.

Corrosion inhibitors. pH control along with an appropriate corrosion inhibitor are used to control corrosion, neutralize hazardous acid gases and prevent scale in drilling fluids. Common corrosion inhibitors are amine- or phosphate-based products, as well as other specially formulated chemicals.

Defoamers. Products are designed to reduce foaming action, particularly in brackish and saturated saltwater muds.

Emulsifiers. These products create a heterogeneous mixture (emulsion) of two insoluble liquids. These include fatty acids and amine-based chemicals for oil-based muds and detergents, soaps, organic acids and water-based surfactants for water-based muds. Products may be anionic (negatively charged), non-ionic (neutral) or cationic (positively charged) chemicals depending on the application.

Filtrate reducers. Filtrate, or fluid loss reducers – such as bentonite clays, lignite, CMC (sodium carboxymethylcellulose), polyacrylate and pregelatinized starch – serve to decrease fluid loss, a measure of the tendency of the liquid phase of the drilling fluid to pass through the filter cake into the formation.

Flocculants. These are used to increase viscosity for improved hole cleaning, to increase bentonite yield and to clarify or de-water low-solids fluids. Salt (or brine), hydrated lime, gypsum, soda ash, bicarbonate of soda, sodium tetraphosphate and acrylamide-based polymers may be used to cause colloidal particles in suspension to group into bunches or "flocs," causing solids to settle out.

Foaming agents. These 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-bearing formations.

Lost circulation materials. The primary function of a lost circulation additive is to plug the zone of loss back in the formation away from the borehole face so that subsequent operations will not result in additional drilling fluids loss.

Lubricants. These products are designed to reduce the coefficient of friction of drilling fluid which decreases torque and drag. Various oils, synthetic liquids, graphite, surfactants, glycols and glycerin, as well as other chemicals, are used for this purpose.

Pipe-freeing agents. Consisting of detergents, soaps, oils, surfactants and other chemicals, these agents are intended to be spotted in an area of suspected pipe stickage to reduce friction and increase lubricity, thereby freeing the stuck pipe.

Shale control inhibitors. Sources of soluble calcium and potassium, as well as inorganic salts and organic compounds provide shale control by reducing shale hydration. These products are used to prevent excessive wellbore enlargement and heaving or caving while drilling water-sensitive shales.

Surface active agents. Surfactants, as they are called, reduce interfacial tension between contacting surfaces (water / oil, water / solid, water / air, etc.). These may be emulsifiers, de-emulsifiers, wetting agents, flocculants or deflocculants, depending on the surfaces involved.

Temperature stability agents. These products increase rheological and filtration stability of drilling fluids exposed to high temperatures and those which continue to perform their intended purpose under these conditions. Various chemicals are used, including acrylic polymers, sulfonated polymers and copolymers, as well as lignite, lignosulfonate and tannin-based additives.

Thinners, dispersants. These chemicals modify the relationship between viscosity and percentage of solids in a drilling mud, and may be used, further, to reduce gel strength, increase a fluid’s "pumpability," etc. Tannins (quebracho), various polyphosphates, lignite and lignosulfonate materials function as thinners, or as dispersants. Principal purpose of a thinner is to function as a deflocculant to reduce attraction (flocculation) of clay particles which causes high viscosity and gel strengths.

Viscosifiers. Bentonite, CMC, attapulgite clays and polymers are used to increase viscosity for better hole cleaning and suspension of solids.

Weighting materials. Barite, lead compounds, iron oxides, calcium carbonates and similar products possessing high specific gravity are used to control formation pressures, check formation caving and facilitate pulling dry pipe.