Chemical Analysis Mud Check
Table 6 pH of 10% Solution (approximate)
Soda Ash Sodium Hydroxide Potassium Hydroxide Calcium Hydroxide Barium Carbonate Gypsum
Calcium Lignosulfonate
Lignosulfonate
Chrome Lignite
Quebracho
Lignite
Sapp
- 4 - 4
- 0 5
- 8
Figure 2
pH Scale and Common Chemicals
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-12
10-13
10-14
10-14
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
10 11 12
Strength strong acid neutral strong base
Examples
1M HCl gastric juice lemon juice
Coca Cola, vinegar orange juice black coffee urine coffee with milk milk saliva blood baking soda milk of magnesia household ammonia caustic soda (0.1M)
1 M NaOH
Alkalinity of the whole mud, or PM, is a measure of both the soluble and insoluble materials in a mud. Soluble ions, such as hydroxyls, carbonates and bicarbonates, and insoluble compounds, such as lime and magnesium hydroxide, have an affect on the PM. The PM is expressed as the volume in cm3
of 0.02N sulfuric acid required to lower the pH of 1 cm3 of the mud to 8.3. This is the pH at which phe-nolphthalein, a color indicator, changes color from pink to clear. When titrating whole mud, the endpoint color is the original color of the mud before phenolphthalein was added.
Alkalinity of the mud filtrate, or PF, is a measure of only soluble materials and is reported as the volume
(cm3) of 0.02N sulfuric acid required to lower the pH of 1 cm3 of filtrate to 8.3. Common soluble ions are hydroxyls, carbonates and bicarbonates. PF will usually be lower than PM due to removal of the insoluble alkaline materials in the filter cake.
The sum of the PF plus the additional volume of 0.02N sulfuric acid required to reduce the pH of the filtrate to 4.3 is dubbed MF. In this pH range, bicarbonate ions are commonly titrated. Humic acids from lignites, lignosulfonates, acetate ions, as well as some other organic additives, also have an effect on the amount of acid required to reach the MF end point.
Total Hardness As Calcium
Total Hardness is a measure of the concentration of divalent cations, and is reported as mg/L Ca++ ion. Hardness is primarily calcium and magnesium, although other cations may influence the results. It is possible to differentiate calcium from magnesium through a different test. The titrating solution is standard versenate (EDTA) solution.
Chlorides
Chlorides is a measure of chloride ions dissolved in the filtrate from all soluble sources. The filtrate is titrated with either 0.0282 N or 0.282 N AgNO3 (silver nitrate) and reported in mg/L chlorides. Mg/L chlorides can be converted to mg/L NaCl by multiplying by 1.65, or to mg/L CaCl2 by the factor 1.57. The derivation of the sodium chloride factor is shown below.
Molecular wt of Cl = 35.5
Molecular wt of NaCl = 58.5
The assumption in using any such factor is that all chlorides titrated result from that particular salt dissolved in water and, consequently, this titration is subject to error in interpretation. The color change observed is from yellow to red-orange. The proper endpoint in the titration is the first permanent orange color seen.
Carbonates
The Garrett Gas Train (GGT) is the most consistent test for the presence of soluble carbonates. A sample of filtrate is acidized causing all carbonates and bicarbonates to be converted to CO2 gas, which is then measured with a Drager tube and reported as mg/L total carbonates in the filtrate.
An alternative titration method for carbonates determination is the P1-P2 titration method. This titration is seldom used because it requires the use of barium chloride (BaCl2), which is extremely toxic. Relationships between the various alkalinities (PM, PF, MF) can also indicate a carbonate problem. A rule of thumb is: if the MF is greater than 5, and greater than 4 times the PF, then carbonates may be present. If these conditions exist, a Garrett Gas Train should be used to verify the presence of carbonates.
Methylene Blue Capacity (MBT)
The methylene blue test is a measure of the capacity of anionic clays to adsorb cationic methylene blue dye. The quantity of adsorbed dye is a direct indication of the quantity and activity of the clays in the fluid. The MBT value, in conjunction with the solids analysis, can indicate the amounts of drill solids and commercial clays in the fluid. The methylene blue capacity (MBT) is reported as cm3 of methylene blue per cm3mud and can also be reported as equivalent lb/bbl bentonite as per the following equation:
cm3 mud
A similar test can be run on shale samples such as cuttings or sidewall cores to determine the activity of native clays. In this procedure, a solid sample is used instead of mud. The results are reported as milliequivalents per gram or equivalents per kilogram.
Post a comment