Applications
•Troublesome shales. •Salt, anhydrite, carnalliteand potash zones. •Deep, hot wells. •Drilling and coring sensitive productive zones. •Extended-reach drilling projects. •Difficult directional wells. •Slim-hole drilling. •Corrosion control. •Hydrogen sulfide (H2S) and carbon dioxide (CO2) bearing formations. •Perforating and completion fluids. •Casing pack or packer fluids. •Workover fluids. •Spotting fluids to free stuck pipe. Emulsion Fundamentals
•Invert emulsion drilling fluids are mixtures of two immiscible liquids: oil (or synthetic) and water. –They may contain 50% or more water. –This water is broken up into small droplets and uniformly dispersed in the external nonaqueous phase. –These droplets are kept suspended in the oil (or synthetic) and prevented from coalescing by surfactants that act between the two phases. •To adequately emulsify the water in oil, there must be sufficient chemical emulsifier to form a film around each water droplet. –The emulsion will be unstable if there is not sufficient emulsifier. –As the water content increases, the required concentration of emulsifier increases. •From the standpoint of stability, the smaller the droplet, the more stable the emulsion since large droplets will coalesce more easily than smaller droplets •Uniform droplet size also makes the emulsion more stable –Shear is required to reduce the droplet size, the fluid through the bit, mud guns and shearing devices will aid in reducing droplet size •The importance of sufficient shear and small droplet size and their relationship to mud stability cannot be overemphasized. –Small, uniform water droplets generate viscosity and gel strengths that help support weight material and aid in the reduction of fluid loss by becoming trapped in the filter cake. •Increasing water content (internal phase) of an invert emulsion: –Increases the size of water droplets. –Increases the chances of water droplets coalescing. –Increases the emulsion plastic viscosity. –Increases the amount of emulsifier required to form a stable emulsion. –Decreases the emulsion stability. •The incorporation of solids into a water-in-oil or synthetic emulsion can have either a positive or negative effect on mud properties, depending upon the manner in which they are wetted. –As long as the solids are maintained in an oil-wet condition and do not coalesce or deplete the required surfactant concentration, they will form a stable emulsion. •Non-aqueous drilling fluids are formulated using additives based on a broad group of chemicals called surface-active agents or surfactants. –These chemicals include emulsifiers, soaps and wetting agents. –They act by reducing the interfacial tension between two liquids or between a liquid and a solid. –Surfactants have a hydrophilic (water-loving) polar head and an organophilic (oil-loving or lipophilic) non-polar tail, •Non-aqueous systems contain wetting agents that coat surfaces and solids to alter the contact angle (wettability) of the solid-liquid interfaces, –These materials allow preferentially wetting of solids by the oil or synthetic. –If a fluid is over treated with wetting agents so that solids are totally wetted, the solids may tend to settle or sag. –Solids must be maintained in the preferentially oil-wet condition to maintain a stable fluid. •The preferential oil-wet condition can be disrupted by contamination with water, increased solids loading and insufficient treatments of wetting agents. –When water-wet solids occur: –Solids tend to adhere to shaker screens. –The appearance of the mud becomes “grainy,”losing its glossy sheen. –The Electrical Stability (ES) will decrease. –The rheology will increase. –Barite settling will be observed in mud cup, heat cup and pits. –The High-Temperature, High-Pressure (HTHP) fluid loss will increase and may contain free water. Additives
•Emulsifiers. –Emulsifiers are surfactants that reduce the surface tension between the water droplets and oil (or synthetic). –They stabilize the mixture by being partially soluble in water and partially soluble in oil –They are usually long-chain alcohols, fatty acids or polymers and can be anionic, cationic or non-ionic. •Soaps. –Some emulsifiers are soaps that are formed by the reaction of a fatty acid ester with an alkali (such as lime) where the hydrogen on the fatty acid is replaced by a metal, such as calcium from lime. –Soaps made with sodium are water-soluble and form oil-in-water emulsions. •Wetting agents. –A wetting agent is a surface-active agent that reduces the interfacial tension and contact angle between a liquid and a solid. –This causes the liquid to spread over the surface of the solid –Wetting agents have one end that is soluble in the continuous-phase liquid and the other that has a strong affinity for solid surfaces, •Viscosifiers. –Although emulsified water increases viscosity, viscosifiers and gelling agents are also required. –Untreated clays cannot be used as viscosifiers because they do not hydrate and yield in oil or synthetic fluid. –If the clays are first coated with an amine, so that they are organophilic, then they will yield and viscosifyin oil and synthetic fluids. •Viscosifiers cont. –Organophilic clay still needs a polar activator (water or alcohol) to produce the maximum yield. –Therefore, their yield decreases as the oil-or synthetic-to-water ratio increases. •Alternative non-clay viscosifiers are available to increase viscosity. –They include asphaltic materials, fatty acid gellants and polymers. •Developing viscosity is a particular problem when mixing new fluids in mud plants where low shear mixing and low temperatures do not allow amine-treated clays to yield. –Freshly prepared muds should not be treated with more organophilic clay than will be required when drilling. •Weight material. –Barite is the most common weight material used in oil and synthetic-base muds. –Calcium carbonate is also used, particularly in lower-density packer fluids, where it is easier to suspend than either barite or hematite. –Hematite may be used in high-density muds where its high specific gravity helps minimize the total solids content of the mud. –Alternative weight materials may require different wetting agents. •Filtration-control additives. –HTHP filtration control of invert emulsion muds is affected by the viscosity of the continuous fluid phase, the oil or synthetic-to-water ratio, the tightness of the emulsion, water-wetting of the solids, the solids content, and the amount of amine-treated clay in the system. –Gilsoniteor asphalt, amine-treated lignite (and polymers are the most common filtration-control additives.
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