Carbonate Bicarbonate Contamination

The most common source of soluble carbonate accumulation in muds is CO2 intrusion from formations being drilled. As CO2 enters an alkaline mud, it reacts with OH- ions and soluble carbonates accumulate. Another common carbonate source is thermal degradation of organic additives such as lignite and lignosulfonate at temperatures in excess of 300°F. CO2 is liberated at temperature by alkaline decarboxylation of organic acid groups such as found in humic acid. Lignite is a more potent source of carbonates than lignosulfonate because of its high humic acid content. Another minor source of carbonates is bacterial action on organic additives.

The mechanism by which bicarbonate (HCO3-) and carbonate (CO3=) ions affect the performance of a deflocculated, clay-based drilling mud is not well understood. The fact remains, however, that high concentrations of total carbonates (>1000 mg/L) will have a detrimental effect on freshwater-bentonite drilling fluids.

Only an estimate of carbonate concentration in filtrate samples is obtained by API alkalinity titrations. There are, however, some physical and chemical indications that carbonate contamination is taking place. Elevated yield point and 10-minute gel strength, particularly on bottoms up after a trip, may indicate a carbonate problem. In addition, if the MF is greater than 5 cm3 and greater than 4 times the PF, then carbonates may be present. If carbonates are suspected, a Garrett Gas Train carbonate analysis (GGT) should be run at the rigsite prior to initiating any treatment.

Soluble carbonates are formed when CO2 enters the mud and reacts with hydroxyl ions (OH-) to form carbonic acid, which further reacts to form HCO3- and CO3 = ions. A chemical equilibrium controlled by the pH of the fluid is established at any given temperature. These equilibria involve the components shown in the following equations:

CO2 + H2O ^ H2CO3 (conversion of CO2 to carbonic acid)

H2CO3 + OH- ^ HCO3- + H2O (conversion of carbonic acid to bicarbonate)

HCO3- + OH- ^ CO3 = + H2O (conversion of bicarbonate to carbonate)

Carbonate equilibria with pH is shown in Figure 2 for the system of carbonates in freshwater at 25°C. Total soluble carbonates are the sum of CO2, HCO3- and CO3=, and are measured by the GGT. The three species coexist in various proportions, depending on solution pH. For example, Figure2 shows that at low pH, gaseous CO2 dissolved in water (carbonic acid) predominates, and HCO3- and CO3= are essentially zero. This is the basis for the GGT analysis. By acidification in the GGT all CO3= and HCO3- ions in the sample are converted to CO2 gas, which is freed from solution and analyzed by a Drager CO2 detector tube. Figure 2 also shows that for the approximate pH range of 6.3 to 10.3 the HCO3- ion predominates; above pH10.3 the CO3= ion predominates. The higher pH is desirable for chemical removal of soluble carbonates as insoluble CaCO3.

Figure 2

Equilibrium Distribution of Soluble Carbonates in Water

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Ca++ + CO3 = ^ CaCO3 (Removal of CO3= ion by Ca++ ion)

At mud pH above 10.3 the reaction proceeds more rapidly because CO3= ions predominate over HCO3- ions. If lime is selected as the treatment chemical for CO3= ion removal, the pH of the fluid need not exceed 10.3 because OH- ions from the lime are available to raise the pH and convert HCO3- to CO3= for subsequent removal:

HCO3- + OH- ^ CO3= + H2O (Bicarbonate to carbonate conversion)

Ca++ + 2OH- + CO3 = ^ CaCO3 + 2OH- (Removal of CO3= ion with lime)

Treatment - Removing carbonates from a deflocculated, clay-based mud typically involves adding lime or gypsum to precipitate CO3= as CaCO3. Adding lime to a solids-laden mud, especially in a hot well, can create severe rheological and filtration control problems. A GGT analysis will indicate if soluble carbonates are strongly present. Up to 500 mg/L of carbonates is not likely to cause a problem, but levels above 1000 mg/L (depending on mud density and solids present) may cause problems. If appreciable carbonates are measured, a pilot test series with a range of treatments should show whether lime or gypsum is the preferred treatment.

The soluble carbonate concentration may be reduced, but the fluid properties may not improve. Experience has shown that carbonate problems often are coupled with, or even mistaken for, excessive low gravity colloidal solids. These solids may be the more significant factor contributing to poor rheological control. Solids contamination and carbonate contamination exhibit similar characteristics, such as: poor response to deflocculant treatment, high yield point and gel strengths, high fluid loss, and poor filter cake quality. Pilot testing is always recommended to verify that the proposed treatment is sufficient, but not excessive. Pilot testing with gypsum and lime can be deceiving, because the reaction is slow and may be interfered with by the presence of other ions in the fluid. In the laboratory it was found that about 30 to 40% of the expected reduction in carbonates occurs. The main point is that added lime does not necessarily all react to remove only carbonates in a complex mud. This is supported by field experiences which have shown that three to four times more lime may be required to improve fluid characteristics than predicted by GGT analysis.

Treatment level of lime to remove CO3= ions in 0.118 lb/bbl for 100 mg/L CO3= ion and for gypsum, the level is 0.043 lb/bbl for 100 mg/L. As mentioned above, however, three to four times this amount may be required to treat out most of the carbonates.

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  • emiliano trevisano
    How to treat out bicarbonate ions in drilling mud?
    5 years ago
  • Rose
    How to treat bicarbonate contamination?
    4 years ago
  • tuukka sainio
    What is thermal degradation of lignite drilling fluids?
    4 years ago
  • franziska
    How to measure CO3 contamination from mud?
    3 years ago
  • nairn mcintosh
    What product is the first line of defence against carbonate contaminant in a mud system?
    2 years ago
  • zofia
    How to treat carbonate contamination drilling?
    2 years ago
  • primrose
    What is the accepted concentration of CO3 in the mud?
    1 year ago
  • alarico
    How to test WBM fo bi carb contamnation?
    1 year ago
  • Alfrida Fairbairn
    How can carbonates as a contaminant affects drilling fluid?
    1 year ago
  • ciriaco
    How to treat for carbonates and bicarbonates in drilling mud?
    10 months ago
  • alem
    How to check for co2 contamination drilling fluids?
    7 months ago
  • ofelia
    What is the effect of sodium bicarbonate in mud?
    6 months ago
  • louvenia
    How to reduce carbonates in drilling mud?
    2 months ago
  • rudolph
    What is the sum of carbinate and biocarbinate?
    2 months ago

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