In preparing a bentonite slurry using fresh water, the bentonite will hydrate and agitation furnished by a mixer is sufficient to separate the hydrated clay plate lets and result in a viscosity and gel strength increase, if the bentonite is placed in salty water or water containing dissolved hardness (calcium or magnesium) the hydration and subsequent dispersion by agitation is reduced.
The question arises, what occurs when salt or hardness is added to a dispersed bentonite drilling fluid and what is necessary to return the slurry to an acceptable condition for drilling?
In this test we will study the effect of contamination of monovalent chemicals (NaCl and KCl) and divalent chemicals that cause contamination are calcium sulfate (CaSO4), cement (Ca (OH2), and Gypsum (CaSO4 – 2H2O). These soluble salts are commonly encountered during drilling, completion or workover operations.
TEST PROCEDURE FOR MUD CONTAMINATION
- Test base mud for weight ppg, plastic viscosity cp, apparent viscosity
c.p., yield point lb/100 ft².
- Add to 400 ml base mud, 0.75, 1.5, 2.5, 3.5 and 5.0 grams NaCl and
repeat step (1) after each addition (stir every time).
- Add to a new 400 ml base mud 0.75, 1.5, 2.5, 3.5 and 5.0 grams Gypsum
(CaSO4 – 2H2O) and repeat step (1) after each addition (stir every time).
- Add to a new 400 ml base mud 0.75, 1.5, 2.5, 3.5 and 5.0 grams
anhydrite (CaSO4) and repeat step (1) after each addition (stir every
- Report the results in a convenient table for the three contaminants and
plot in three different plots the effect of contamination with salt, gypsum
and anhydrite on mud density and viscosities and yield point.
TREATMENT OF THE CONTAMINATED DRILLING FLUID
A SALT CONTAMINATION:
- Test 525 ml of the base mud sample for weight ppg, viscosity cp, yield
point lb/100 ft² and pH.
- Add 27 ml of 10% by wt salt water (NaCl) to the base mud sample (525
ml) stir 2 minutes after adding salt. Age for about 15 minutes and stir
again for 2 minutes. Determine its viscosity, density and pH.
- Add to the contaminated sample 1/2 cc increments until the original
viscosity is restored.
- Continue to add (SAPP) in 1/2 cc increments until the original viscosity
- Plot the result on a curve showing the viscosity and pH as a function of
B GYPSUM CONTAMINATION:
- Repeat step No. 1 in (A).
- Contaminate the base mud by (525 ml) with 6 gr of Gypsum, stir for 3 minutes. Age the sample for 15 minutes and stir again for 2 – 3 minutes. Determine the viscosity cp and density ppg and pH.
- Add 3 gr soda ash (Na2CO3) to remove the hardness from sample in step # 2. Stir for 10 minutes and test.
- Add 4 gr Carbonox and 1 gr of Caustic Soda to sample in step # 3. Stir 10 minutes and test.
- Add 4 gr Gypsum to a 525 ml sample of base mud and stir 10 minutes and add 5 gr Q-Broxin and 3/4 of Caustic Soda. Stir 10 minutes and test.
C CEMENT CONTAMINATION:
- Repeat Step No. 1 in (A).
- Contaminant the base mud (525 ml) by 1.0 PPB and stir for two minutes.
Age for 15 minutes. Stir and test.
- Add to the contaminated sample 1/2 cc (SAPP).
- Continue to add SAPP in 1/2 cc increments until the original viscosity is
restored measure pH every time.
- Plot the result on a curve showing the viscosity and pH as function of
- Add to the base mud 1.5 cc (SAPP) and 1.0 Sodium Bicarbonate and stir
for 10 minutes.
- Add 1.0 gr of cement while mixing the sample after aging and test.