Methylene Blue Capacity
The methylene blue capacity test indicates the concentration of reactive clays present in a drilling fluid. Reactive clays are commercial bentonite and formation solids such as shales.
Methylene blue dye is added to a sample of whole mud until all reactive sites have adsorbed the dye. A faint blue/turquoise halo will form around the drop of solids when there is an excess of dye. Drilling muds frequently contain other materials that adsorb methylene blue. Treatment with hydrogen peroxide is intended to remove the effects of organic materials such as CMC, polyacrylates, lignosulfonates, and lignites. Commercial clays such as bentonite and formation solids such as shales adsorb methylene blue and contribute to the methylene blue capacity of the mud. A variation of the cation exchange capacity test can be used to determine the reactivity of shales and clays to allow better determination of the true bentonite concentration in the fluid (Figure 18).
Figure 18 Methylene Blue Test Set-Up
Figure 18 Methylene Blue Test Set-Up
- If the mud to be tested contains air, stir slowly for 2-3 minutes to release entrapped air.
- Use a 2.5 or 3.0-cm3 syringe to measure exactly 2.0 cm3 of drilling fluid.
- To the Erlenmeyer flask, add 10 cm3 water and 2 cm3 of mud.
- Next, add 15 cm3 of 3% hydrogen peroxide to the flask. Then, add 0.5 cm3 of 5N sulfuric acid.
- Gently boil the suspension for 10 minutes on the hot plate.
- After boiling, remove flask from hot plate and bring the total suspension volume in the Erlenmeyer flask to 50 cm3 with distilled water. Allow to cool before proceeding to the next step.
- With a 1-cm3 pipette, add methylene blue to the flask in 0.5-cm3 increments. If the approximate amount of methylene blue is known from prior testing, larger increments can be added at the beginning of this titration.) After each addition of methylene blue solution, swirl the contents of flask for 30 seconds. While the solids are still suspended, remove a drop of liquid on the end of a glass rod. Place the drop onto a Whatman #1 filter paper. Observe the liquid that wicks out from around the solids on the paper. The solids will be blue. The liquid will become blue (and form a halo) when the initial endpoint of the methylene blue titration is reached. (This initial endpoint means that the active solids are almost saturated with the blue dye, but perhaps not all.)
- To find the final endpoint, after detecting the initial endpoint, swirl the flask for 2 minutes and place another drop on a clean area of the filter paper. If the blue ring (or halo) is again evident, the final endpoint has been reached. If the blue ring is not evident, continue as before adding 0.5-cm3 increments of methylene blue solution - until the blue ring is present after 2 minutes of swirling the flask.
- The methylene blue test results are reported as methylene blue capacity (MBC) or as lb/bbl bento-nite equivalent as in equations (a) and (b) below:
- MBC = cm3 methylene blue solution/cm3 mud sample b. lb/bbl bentonite equivalent = 5 (MBC)
Drilling fluid pH measurements and pH adjustments are fundamental to drilling fluid control because clay interactions, solubility of additives, and contaminant removal are all pH-dependent.
The recommended method for pH measurement is with a pH meter. A portable pH meter (Figure 19) consists of a pH probe and an electronic voltage meter. The probe contains:
- glass electrode
- standard reference electrode
- temperature sensor (optional)
- Measurement consists of: (1) calibration of meter, (2) pH measurement of sample, and (3) cleaning and storing the probe.
- Remove end-cap and rinse probe with distilled water. Blot dry.
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