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chemical

Cation Exchange (MBT)

Purpose

Determine the cation exchange capacity (CEC) of the mud, expressed as pounds of bentonite equivalent per barrel (lb/bbl). The Methylene Blue Test (MBT) measures the amount of reactive clay (bentonite, drill solids, and contaminants) present in the system.

Equipment

  • Methylene blue solution (0.01 N standardised)
  • 3% hydrogen peroxide (H₂O₂)
  • Sulphuric acid, dilute (5 N H₂SO₄)
  • Titration burette or syringe
  • Stirrer or magnetic stir plate
  • Whatman filter paper (or equivalent)
  • Glass rod
  • 2 mL mud sample

Procedure

  1. Add 2 mL of mud sample to a 250 mL Erlenmeyer flask.
  2. Add 15 mL of distilled water, 0.5 mL of 5 N H₂SO₄, and 5 mL of 3% H₂O₂.
  3. Heat the mixture gently and boil for 10 minutes to oxidise organic matter; allow to cool.
  4. Add 40 mL of distilled water and continue stirring.
  5. Using the burette, add 1 mL increments of methylene blue solution to the flask while stirring vigorously.
  6. After each addition, place a drop of the suspension on filter paper and observe for a turquoise “halo” ring around the central blue spot.
  7. Continue adding methylene blue until the halo persists for at least 2 minutes after stirring.
  8. Record the total volume of methylene blue solution added (mL).

Calculations

  • MBT (lb/bbl bentonite equivalent) = mL methylene blue solution × 5

Notes

  • The persistent turquoise halo indicates saturation of all exchangeable cation sites.
  • If the halo appears immediately at the first addition, dilute the mud sample before repeating.
  • Report MBT value in lb/bbl. Elevated MBT may indicate drill-solids build-up or bentonite contamination.