BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
treatments is used to prevent excessive pH increases. The treatment amounts given below are
based on exact chemical reactions. In the field these chemical reactions may not be exact and
may take time. Use these treatment amounts as guidelines and verify with accurate pilot testing.
1. For pH’s less than 10.0 (over 50% of the carbonates are HCO3¯) use lime to treat only
bicarbonates:
0.00043 lbs of lime per mg/L of HCO 3¯
2. For pH’s above 10.0 or to maintain a constant pH while treating out carbonates:
0.00022 lbs of lime per mg/L of HCO 3¯
0.001 lbs of gypsum per mg/L of CO 3 = and ½ of the mg/L of HCO 3¯
3. To use gypsum and caustic:
0.00023 lbs of caustic per mg/L of HCO 3
–
0.001 lbs of gypsum per mg/L of HCO 3 − and CO 3
=
The following guidelines are recommended for treatment of carbonates:
• Determine the amount of carbonates to treat carefully. Use more than one calculation method
if possible;
• pilot test;
• when adding lime and gypsum add slowly to allow the chemical reaction to take place;
• add the calculated treatment over several circulations;
• check the mud on each circulation bottoms up and re-calculate the carbonates;
• lignosulfonate additions will keep pH from rising to undesirable levels and help control
rheological increases.
Carbon Dioxide (CO2)
Natural gases can contain carbon dioxide with concentrations that range from small amounts to
relatively high proportions. Much carbon dioxide is clearly of magmatic origin found in areas
where vulcanism is common, with large proportions resulting from local igneous activity.
Metamorphism of limestone can result in high concentrations also. Carbon dioxide is also
produced by the oxidation of many organic substances and crude oils.
In a water solution the carbon dioxide converts to carbonic acid. This carbonic acid lowers pH
and can prove damaging to many polymers. At high concentrations of carbon dioxide the
formation of excessive amounts of soluble carbonates occurs and often results in high viscosities,
gel strengths, and HT/HP control problems. Often calcium hydroxide [Ca(OH) 2 ] is used to treat
out those soluble carbonates with the potential result of scale formation. Nevertheless, calcium
hydroxide remains the most common approach to neutralizing this acid. The measurement and
treatment of carbonates and bicarbonates was discussed earlier in subsection
Carbonate/Bicarbonate Contamination.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 4-13