A drilling-mud additive used to co
ntrol fluid loss in water muds ranging from freshwater to saturated-salt to high-pH lime muds. Starches have thermal stability to a
bout 250°F [121°C]. They are subject to bacterial attack unless protected by high salinity or bactericide. Drilling-grade natural starch has API/ISO specifications for quality. Starches are carbohydrates of a general formula (C6H10O5)n and are derived from corn, wheat, oats, rice, potatoes, yucca and similar plants and vegetables. They co
nsist of a
bout 27% linear polymer (amylose) and a
bout 73% branched polymer (amylopectin). The two polymers are intertwined within starch granules. Granules are insoluble in cold water, but soaking in hot water or under steam pressure ruptures their covering and the polymers hydrate into a colloidal suspension. This product is a pregelatinized starch and has been used in muds for many years. Amylose and amylopectin are no
nionic polymers that do not interact with electrolytes. Derivatized starches, such as hydroxypropyl and carboxymethyl starches, are used in drill-in fluids, completion fluids and various brine systems as well as in drilling-mud systems. The use of starch typically causes a minimal increase in viscosity while effectively co
ntrolling fluid loss.