A New Method for the Quantitative Determination of Soluble Carbonates in Water-Based Drilling Fluids
Authors: Garrett R.L. 
Published: December 2007

This paper presents a new, direct method for measuring soluble carbonates in a drilling fluid. The method is designed primarily for field use in guiding mud treatments and controlling adverse mud rheology. Laboratory data are given to verify the method's accuracy and field experience illustrates its usefulness in mud control.

Introduction
The mechanism by which bicarbonate (HCO3-) and carbonate (CO3=) ions adversely affect a deflocculated, clay-based drilling mud's performance (and even these ions' true concentrations) has been a controversial point in the industry for many years. Under field analysis conditions, only an estimate of carbonate concentration in filtrate samples is obtained by API alkalinity titrations. Even under ideal laboratory conditions, alkalinity titrations only approximate the true carbonate content of complex muds.However, a reliable method that allows direct carbonate analysis of muds recently was developed. The Garrett Gas Train/Carbonate (GGT/CO3=) method determines total soluble carbonates as gaseous CO2, freed from the sample upon acidification in a small, transparent, plastic gas train. The evolved CO2 is measured quantitatively using a commercial gas detector tube. The GGT/CO3 = method was developed with the same concept as the GGT/S= method for measuring sulfides, which recently was adopted as an API permanent procedure. The similar chemical behavior of the CO2 -carbonate and H2S-sulfide systems in aqueous solutions allows analogous approaches to quantitative analyses and chemical treatment. Although alike in many ways, the gas-train/detector-tube analysis method for carbonates is more restrictive and sensitive to detector-tube flow conditions than is sulfide analysis. These conditions are specified later when the procedure is described.Uses for GGT/CO3 = analyses go beyond the main application for measuring and treating accumulated carbonates in field muds and includestudying carbonate generation rates caused by thermal degradation of organic additives that produce CO2 in muds (such as lignites, lignosulfonates, carboxymethyl cellulose, tannins, and starches),studying the rate of CO2 absorption from air into a circulating mud when influenced by various equipment used to process muds,analyzing soluble carbonates in commercial barite to determine if soda ash or sodium bicarbonate are present,studying the chemical reaction efficiency between carbonates and a given treatment material such as lime - especially useful for field and laboratory pilot tests of problem muds, andmonitoring accumulation of bacterially and chemically produced CO2 in packer muds, ballast muds for ships, and muds stored in tanks.

Carbonate Chemistry in Water-Based Muds
In alkaline, aqueous drilling or workover fluids, soluble carbonates are likely to accumulate from a wide variety of CO2 sources. Regardless of the source, if CO2, is generated internally or enters the mud externally, it reacts immediately with alkaline hydroxyl ions (OH) to form HCO3- and CO3 = ions. A chemical equilibrium is established at a given temperature, which is controlled by the H+ and OH- ion concentration or mud pH. These equilibria involve the component shown in Eqs. 1 and 2.
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JPT
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