Oilfield Glossary
Oilfield Glossary
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carbonate ion

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1. n. [Drilling Fluids]
An anion with formula CO3-2. Carbonate chemistry involves a pH-dependent equilibrium between H2O, H+, OH-, CO2, HCO3- and CO3-2. At low pH, carbon dioxide [CO2] dominates. As pH rises from acidic toward neutral, HCO3- ions dominate. As pH rises above neutral, CO3-2 ions dominate. If no component is lost from the system (such as CO2 gas evolving), changing pH up and down continually reverses the relative proportion of the carbonate species. Carbonates play several important roles in water mud chemistry. One role is the corrosion of metals by acidic CO2. A second is the formation of calcium carbonate [CaCO3] scale on surfaces by carbonate and calcium ion reactions. Another role is in the chemistry of deflocculated mud, where bicarbonate ions prevent attachment of deflocculants such as lignosulfonate, onto clay edge charges.