Filtration Control – Additives For Water Based Muds

•Several types of filtration-control additives are used in water-base muds.•Clays –sodium bentonite–Attapulgiteand sepioliteare clays but impart no filtration control •Polymers–Polymers are the filtration control products used most often in water-base muds–They can range from natural starches and modified cellulose to sophisticated synthetic polymers capable of providing filtration control under high temperatures and hostile conditions Polymers reduce fluid loss in several ways:–Plugging of openings of the filter cake by polymer particles.–Encapsulating solids forming a larger deformable coating or film which reduces the permeability of the filter cake.–Viscosificationof the liquid phase. •Starch, a natural carbohydrate polymer, has been used to control filtration in drilling fluids since the 1930s. –It is widely available as yellow (untreated) and white (modified) starch. –Starches can be used in seawater, salt water, hard water and complex brines. –The most economical and widely used starches are made from corn or potatoes, but starches made from other agricultural products are also available. •Sodium Carboxymethylcellulose(CMC) is a modified natural polymer used for filtration control.–CMC is an effective fluid-loss control additive in most water-base muds.–It works particularly well in calcium treated systems, where it acts to stabilize properties.–CMC is not subject to bacterial degradation and performs well at an alkaline pH. –CMC’s effectiveness decreases at salt concentrations greater than 50,000 mg/l.–subject to thermal degradation at temperatures exceeding 250°F.–Available in Low, medium and high viscosity grades PolyanionicCellulose (PAC) is a modified natural polymer used for: freshwater, seawater, salt and low-solids muds. –It is a high-molecular-weight, polyanioniccellulose similar to CMC, but has a higher degree of substitution. –It is the most widely used fluid-loss control additive and is generally a much better product than CMC.–Good to 275°F –Available as Ultra low viscosity and regular viscosity •Chemical thinners reduce filtration rates by deflocculating the clays, by increasing the fluid phase viscosity and by changing the solids distribution. –Desco and Lignite are effective at deflocculating and lowering fluid loss. •Also available for fluid loss control: –Complex Resin/lignite blends •For HPHT fluid loss control –Polyacrylites •Not in common use anymore •The API fluid loss of these systems is normally zero, or too low to be an effective measure. •The filtration rate of oil muds, unless otherwise noted, refers to the HTHP filtration. •Most oil-and synthetic-base fluids are emulsions. –Their fluid phase is an emulsion with oil or synthetic as the continuous phase and brine as the emulsified phase. –These systems contain from 10 to 50 volume percent brine, usually calcium chloride. –The emulsified brine forms colloid-sized droplets, which are immiscible in the oil or synthetic. –These brine droplets become trapped in the filter cake and reduce filter-cake permeability and fluid loss. Emulsifiers. –Although emulsifiers are not true filtration-control additives, they can reduce filtration by increasing the emulsion strength if the emulsion is unstable. –A sufficiently stable emulsion should be established before treating with filtration-control additives. –If an emulsifier requires lime to be activated, excess lime should be maintained in the mud. •Viscosifiers. –The primary viscosifier in invert emulsion muds is organophilic clay. –Although this clay does not hydrate, it will reduce the filtration rate by providing a colloidal solid for forming a basic filter cake. •The primary filtration-control additives for invert emulsion muds are: –asphalt, –gilsonite(natural asphalt), –amine treated lignite –various other resins –specialized polymers •The asphaltic materials usually provide better filtration control than the amine-treated lignite at equal concentrations and temperature.