Drilling fluids water-based

Water-Based Drilling Fluids. Water-based drilling fluids consist essentially of either a freshwater- or seawater-based fluid with a weighting agent (usually barite), bentonite clay and several water-soluble polymers, together with added salts (commonly NaCl, KCl) and sodium or potassium hydroxide for pH control. 

Common salt, or sodium chloride, is also present in dissolved form in drilling fluids. Levels up to 3,000 mg/L chloride and sometimes higher are naturally present in freshwater muds as a consequence of the salinity of subterranean brines in drilled formations. Seawater is the natural source of water for offshore drilling muds. Saturated brine drilling fluids become a necessity when drilling with water-based muds through salt zones to get to oil and gas reservoirs below the salt. 

Drilling fluids are classified as to the nature of the continuous phase gas, water, oil, or synthetic. Within each classification are divisions based on composition or chemistry of the fluid or the dispersed phase. 

Chemical additives for gas-based drilling fluids are limited to surfactants (qv), certain polymers, and occasionally salts such as sodium or potassium chloride. An aqueous solution of the additives is iajected iato the air or gas flow to generate a mist or foam. No additives are used ia dry air or gas drilling operations. Gas-based fluids are not recirculated and materials are added continuously. As the fluid exits the well, air or water vapor escapes to the atmosphere, gas and oil are burned, and water and formation soflds are collected into a pit for later disposal. Stable foams must be destabili2ed to separate the air from the Hquid phase for disposal. 

Water-Based Muds. About 85% of all drilling fluids are water-based systems. The types depend on the composition of the water phase (pH, ionic content, etc), viscosity builders (clays or polymers), and rheological control agents (deflocculants or dispersants (qv)). 

Oil-Based Muds. Oil-based drilling fluids have diesel or mineral oil (11) as a continuous phase with both internal water and soHd phases. 

The specifications for drilling fluid hematite have been set by the API and are Hsted in Table 2 (24). Hematite is used most frequently in high density oil-based muds to minimise the total volume percent soflds (26). The abrasivity of hematite limits its utiUty in water-based muds. 

Although numerous mud additives aid in obtaining the desired drilling fluid properties, water-based muds have three basic components water, reactive soHds, and inert soHds. The water forming the continuous phase may be fresh water, seawater, or salt water. The reactive soHds are composed of commercial clays, incorporated hydratable clays and shales from drilled formations, and polymeric materials, which may be suspended or dissolved in the water phase. SoHds, such as barite and hematite, are chemically inactive in most mud systems. Oil and synthetic muds contain, in addition, an organic Hquid as the continuous phase plus water as the discontinuous phase. 

Table 5. Polymers Used in Water-Base Drilling Fluids...

Alkalinity Control. Water-base drilling fluids are generaHy maintained at an alkaline pH. Most mud additives require a basic environment to function properly and corrosion is reduced at elevated pH. The primary additive for pH control is sodium hydroxide [1310-73-2] in concentrations from 3 to 14kg/m (1—51b/bbl). 

High initial cost and environmental restrictions prevent use of oil and synthetic muds in many cases where shale problems are expected. It is necessary then to treat a water-base mud to minimize the destabilizing effect of the drilling fluid. Salts, polymers, and other organic materials are added to the mud to reduce the water sensitivity of the shale, shale sweUing, and weakening arising from mud contact, or the rate of water uptake by the shale. 

Other regulations apply in different offshore drilling areas in the United States and around the world. AH have had a profound effect on drilling fluid technology (169,170). Very few instances of water-base muds failing the mysid bioassay test exist in the 1990s. Operators and service companies have eliminated use of the mote toxic additives, reformulated old mud systems, and developed new ones to ensure acceptable environmental performance based on pertinent regulations. 

Mixtures containing sulfated castor oil were used to increase the lubricity of water base drilling fluids (123). Sulfated castor oil is also used in dishwashing compounds as a hand softener. A typical cleaning composition contains sodium dodecylben2ene sulfonate, sulfated castor oil, ethanol, and water. A sulfated derivative of castor oil is used as a dispersant for plaster of Paris, reducing the water needed to form a plastic slurry (124). Pesticide emulsions can be stabilized using ethoxylated castor oil (125). 

Drilling muds are a special class of drilling fluid used to drill most deep wells. The term mud refers to the thick consistency of the fluid after the appropriate materials have been added to the water-liquid or oil-liquid base. 

Drilling Fluids Composition and Applications Water-Base Mud Systems Bentonite Mud... 

Salt Concentration. The most cost-effective method of reducing salt concentration levels is to dilute water-base drilling fluid with freshwater. Care must be taken to make sure that the makeup water is compatible with the system. The water must not contain high concentrations of undesirable corrodents. 

Oil Muds. Oil-base muds can be used effectively to minimize the corrosion-related problems in drilling operations. These fluids are composed of a continuous oil phase in which water is emulsified. The performance of oil-base mud is very competitive with that of water-base muds, and is superior under some adverse conditions. However, as pointed out earlier, they are only used in special cases due to their high costs and environmental restrictions. Nevertheless, they are the most effective method to avoid corrosion-related problems. 

Nesbitt, L. E., et al., Properties of hydrogen sulfide scavengers used in water-based drilling fluids, Proceedings of 36th Annual Technical Meeting of the Petroleum Society, Edmonton, Canada, June 2-5, 1985. 

Synthetic-based muds are mineral oil muds in which the oil phase has been replaced with a synthetic fluid, such as ether, ester, PAO, or linear alkylbenzene, and are available from major mud companies. The mud selection process is based on the mud s technical performance, environmental impact, and financial impact. Synthetic muds are expensive. Two factors influence the direct cost unit or per-barrel cost and mud losses. Synthetic muds are the technical equivalent of oil-based muds when drilling intermediate hole sections. They are technically superior to all water-based systems when drilling reactive shales in directional wells. However, with efficient solids-control equipment, optimized drilling, and good housekeeping practices, the cost of the synthetic mud can be brought to a level comparable with oil-based mud [1308]. 

Methylglucosides also could find applications in water-based drilling fluids and have the potential to replace oil-based drilling fluids [801]. The use of such a drilling fluid could reduce the disposal of oil-contaminated drilling cuttings, minimize health and safety concerns, and minimize environmental effects. 

Such a composition reduces friction, permeates drilling mud wall cake, destroys binding wall cake, and reduces differential pressure. Unfortunately, many of such compositions are toxic to marine life. Synthetic PAOs are nontoxic and effective in marine environments when used as lubricants, retum-of-permeabiUty enhancers, or spotting fluid additives for water-based drilling muds. A continuing need exists for other nontoxic additives for water-based drilling muds, which serve as lubricants, retum-of-permeability enhancers, and spotting fluids. 

For many years uintaite and other asphaltic-type products have been used in water-based drilling fluids as additives assisting in borehole stabilization. These additives can minimize hole collapse in formations containing water-sensitive, sloughing shales. 

Neutralized sulfonated asphalt (i.e., salts of sulfonated asphalt and their blends with materials such as Gilsonite, blown asphalt, lignite, and mixtures of the latter compounds) are commonly used as additives in drilling fluids. These additives, however, cause some foaming in water or water-based fluids. Furthermore, these additives are only partially soluble in the fluids. Therefore, liquid additives have been developed to overcome some of the problems associated with the use of dry additives. On the other hand, with liquid compositions containing polyglycols, stability problems can arise. Stable compositions can be obtained by special methods of preparation [1407]. In particular first the viscosifier is mixed with water, then the polyglycol, and finally the sulfonated asphalt is added. 

Polydrill is a sulfonated polymer for filtration control in water-based drilling fluids [1775]. Tests demonstrated the product s thermal stability up to 200° C and its outstanding electrolyte tolerance. Polydrill can be used in NaCl-saturated drilling fluids as well as in muds containing 75,000 ppm of calcium or 100,000 ppm of magnesium. A combination of starch with Poly drill was used successfully in drilling several wells. The deepest hole was drilled with 11 to 22 kg/m of pregelatinized starch and 2.5 to 5.5 kg/m of Polydrill to a depth of 4800 m. Field experience with the calcium-tolerant starch/Polydrill system useful up to 145° C has been discussed in detail [1774]. 

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