Why are we mixing Bentonite with fresh water for water-based mud? The reason is, Bentonite dispersed in freshwater will give you the following qualities –
  1. Good cuttings lifting capacity
  2. Suitable filtration control
  3. Good drilling rate
  4. Good filter cake
It is important to note how to mix Bentonite to maximize its properties.
  • Prehydrating bentonite in fresh water is a more effective viscosifier than adding directly to salt water
  • So freshwater/drill water should always be used.
Mud tank capacity of 550bbls / 87.4m3
soda ash comes in 50kg sacks
Calcium test result is 700mg/l
  1. Instruct the Derrick man to clean the mud tank to remove all thinners, brine, and chemicals before adding makeup water.
The mud tank needs to be properly cleaned
2. Add drill/fresh water to your mud tank – 500bbls/ 79.5m3 (keep space for volume increment)
3. Stir the water with agitators for at most 10mins before taking the sample for test. (this is to get actual value in case mud tanks are not properly cleaned)
4. Take a sample of the drill water from the mud tank to the mud lab and Test for Hardness (Calcium Ca++)
Note that from the graph above, for pre-hydration of Bentonite to be effective, the system must be a freshwater system, Calcium should be <200mg/L
  • Add 1 ml of the drill water from the mud tank to your beaker
  • Add 20 ml of Distilled water
  • Add 8-N  NaOH OR KOH –  { 1 ml or 20 drops,}
  • Add CALCON or CALVER ll.-{1/4-porcelain/brass  spoonful or 0.2g}
  • Mix with a stirring rod
Color after adding Calcon
  • Using a Pipette, Titrate with Versenate Hardness Titrating Solution (EDTA) (20 EPM, 8 oz, 1 ml = 400 mg/l  Ca2+)
  • The color will change from Wine Red to Blue (Take the Redness out)
Color change from wine red to blue
  • Multiply the ml of  Versenate Hardness Titrating Solution (“B”) by 400.               
 (“B”) x (400)  =  mg/l  Ca2+  Soluble Calcium
  • If your result is more than 200mg/l. –  use the conversion factor to treat the water below.
Conversion factor – mg/l (contaminant) X factor = lb/bbl Soda Ash to add
mg/l (contaminant) X .000925 lb/bbl = lb/bbl Soda Ash to add
mg/l (contaminant) X .00263903kg/m3 = kg/m3 Soda Ash to add
Note this is for 1 mg/L of Calcium
For example, if your Calcium result is 700mg/l
Use (700 – 200) X .000925 lb/bbl = 0.465ppb
= 0.465ppb X volume of water in mud tank
= 0.465 X 500bbls
= 233 lb = 2 sxs.
(700 – 200) X .00263903kg/m3 = 1.32kg/m3
= 1.32kg/m3 X 79.5m3 (vol of water in mud tank)
= 105kg = 2 sxs
So Instruct the derrick man to add 2sxs soda ash to the mud tank
Note do not treat Calcium to zero to avoid carbonate contamination
  • If the Ca test results are less than 200mg/l, proceed to the next step, as no soda ash treatment is needed.
Note that for it to be effective, the system must be fresh water system, and Calcium should be <200mg/L
5. Test for chloride
From the graph above, see the range for chloride content to achieve effective viscosity for the prehydration of Bentonite.
Salt contamination cannot be removed from a drilling fluid or water by chemical treatment; instead, change the water source to drill water/ fresh water or add fresh water to reduce the chloride concentration.
  • Add 1 ml sample from the mud tank to your beaker
  • Add 25ml distilled water
  • Add Potassium Chromate {2 – 3 drops}
  • Using a Pipette, Titrate with 0.0282 N AgNO3 or 0.282 N AgNO3 Silver Nitrate
  • The color will change from Yellow to First permanent Red  or Reddish color
  • If you used 0.0282 AgNO3 –
multiply mls of 0.0282 AgNO3 x (1000) =  Mg/l Cl-
  • If you used 0.282 AgNO3 –
multiply mls of 0.282 AgNO3 X (10,000) =  Mg/l Cl-
Bentonite of less than 5000mg/l can still prehydrate, but with more than 20,000mg/l Bentonite, hydration is completely prevented
6. Add Bentonite to mud tank – Add 30 to 35 ppb bentonite. (85.6kg/m3 to 99.9kg/m3)
7. Add 0.25 (0.71kg/m3) to 0.5 ppb (1.43kg/m3) caustic soda to the mud tank.
Picture of Caustic soda
Caustic is added to the mud tank, as we want our ph to be between 9 and 10
  • Magnesium hardness is treated out with sodium hydroxide (caustic soda)
  • At a pH of 9.7, magnesium ions would all react with sodium hydroxide to precipitate as magnesium hydroxide Mg(OH)2
Caustic concentration of 0.25 to 0.5ppb is suitable (0.71kg/m3 to 1.43kg/m3)
8. Switch off agitators and allow sit for 6 hrs minimum
Check funnel viscosity of the prehydrated mud –
  1. if the Bentonite is too thick, use it for HI VIS sweep while drilling to keep the hole clean.
  2. If the Bentonite is too thick, dilute further with water to the required concentration (seawater if offshore) and add necessary polymers such as fluid loss agents and viscosifiers as per the mud program to improve rheology further
  3. If the Bentonite is too thin, add more drilling fluids additives to it
  1. Continued additions of prehydrated Bentonite to mud system while drilling to maintain desired viscosity as indicated by MBT analysis.
2. As density is increased while drilling, the bentonite content of the mud should be decreased to minimize downhole gelation problems, especially if running a saltwater mud system.

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