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Applied Drilling Engineering

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high and, thereby, form sodium bisulfite (NaHS) and 
sodium sulfide (Na2S): 
H 2S+NaOH-NaHS+H20 
NaHS + NaOH- Na2 S + H 2 0. 
If the pH is allowed to drop, the sulfides revert to 
H 2S, which is foul smelling and deadly. 
A common method to remove the sulfide ion is by 
the addition of zinc ions to the mud in the form of 
basic zinc carbonate: 
S - 2 + Zn +2 -ZnSI. Oxygen. The presence of oxygen in the 
drilling fluid causes an acceleration of corrosion rates 
that can produce pitting of the pipe and loss of ex-
cessive amounts of metal. Detection of such adverse 
corrosion conditions usually is determined by use of 
corrosion rings placed in the tool joints of the 
drillpipe and by galvanic probes placed in the stand 
pipe on the rig floor. 
Most oxygen enters the mud in the surface pits. 
Rate of entrainment can be reduced greatly by proper 
inspection and operation of mud-stirring and other 
surface equipment. In some cases it may be necessary 
to remove the oxygen from the mud as sodium 
sulfate. This is accomplished by the addition of 
sodium sulfite at the suction of the mud pumps: 
0 2 + 2Na2S03 -2Na2S04 1. 
Amount of Treating 
Agent to Add to Remove 
Contaminant Contaminating lon To Remove Add 1 mg/L Contaminant (lbm/bbl) 
soda Ash if pH okay 0.000928 lbm/bbl/mg/L 
Gypsum or anhydrite calcium (Ca2 +) SAPP if pH too high 0.000971 lbm/bbl/mg/L 
sodium bicarbonate if pH too high 0.00147 lbm/bbllmg/L 
SAPP or 0.000971 lbm/bbllmg/L Cement calcium (Ca 2 +) 
hydroxide (OH - ) sodium bicarbonate 0.00147 lbm/bbl/mg/L 
calcium (Ca2 +) sodium bicarbonate 0.00147 lbm/bbl/mg/L 
Lime SAPP or 0.000535 lbm/bbl/mg/L hydroxide (OH - ) sodium bicarbonate 0.000397 lbm/bbl/mg/L 
magnesium (Mg 2 +) caustic soda to pH 10.5 
Hard water then add soda ash 0.00116 lbm/bbl/mg/L 
calcium (Ca 2 +) soda ash 0.000928 lbm/bbl/mg/L 
Hydrogen sulfide sulfide (S 2 -) keep pH above 10 and add 
zinc basic carbonate 0.00123 lbm/bbl/mg/L 
carbonate (C0 3 2 -) gypsum if pH okay 
0.00100 lbm/bbl/mg/L 
Carbon dioxide lime if pH too low 0.000432 lbm/bbl/mg/L 
(HC0 3 -) lime Concentration of Chemicals for Con-
tamination Removal. After it has been determined 
that a certain chemical contaminant has entered the 
mud and the type of chemical to be added (i.e., 
treating agent) has been evaluated, the final step is to 
decide how much agent to add. 
A basic chemical principle is that for complete 
chemical removal of contaminants it takes a con-
centration of treating agent, expressed in equivalent 
weights per unit volume, equal to the concentration of 
contaminant, also expressed in equivalent weights per 
unit volume. 
If [A] denotes the concentration of treating agent 
in mole/L and [C] denotes the concentration of 
contaminant in mole/L, 
[AJVa=ICJVc, ..................... (2.15) 
where Va and Vc are the effective valences exhibited 
by Agent A and Contaminant C, respectively, in the 
chemical reaction involved. Using this concept, Table 
2.12 was prepared. This table lists the amount of 
treating agent required to remove 1 mg/L from 1 bbl 
of mud for various contaminants present. 
Example 2.14. A titration test has shown that a 
drilling mud contains 100 mg/L of calcium. The mud 
engineer plans to add enough soda ash (Na2C03) to 
his 1 ,500-bbl system to reduce the calcium con-
centration to 50 mg/L. Determine the amount of 
soda ash he should add to each barrel of mud for 
each mg/L of calcium present in the mud. Also 
determine the total mass of soda ash needed in the 
desired treatment. 
Solution. Table 2.2 shows calcium to have a 
0.000424 lbm/bbl/mg/L 
molecular weight of 40 and a valence of two. Thus, 
for a desired change in contaminant concentration of 
I mg/L, 
2+ 1.0 mg/L Ll[Ca ] = ---------
40 g/g mole (I ,000 mg/g) 
=2.5 X 10~ 5 g mole/L 
Since Ca 2 + has a valence of two, the concentration 
expressed as a normality is 
~[Ca2 +] Vc =2.5 X 10~ 5 (2) 
=5.0x 10~ 5 gew/L 
The reaction of interest is 
Ca 2 + + C03 2 ~ -CaC03 1 
and the concentration of treating agent co3 2 ~ is 
given by Eq. 2.15: 
Ll[C0 3 2 ~] Va =~[Ca2 +] Vc 
=5.0x 10~ 5 gew/L 
Solving for ~[C03 2 ~] with Vc equal to 2 yields 
The molecular weights of Na, C, and 0 are 23, 12, 
and 16, respectively. Thus, the concentration of 
Na 2C0 3 needed is 2.5x10~ 5 g mole/L 
. (2(23) + 12 + 3 (16)] g/g mole= 2.65 x 10 ~ 3 g/L. 
Recall that one equivalent barrel for pilot testing is 
350 mL. The Na2 C03 concentration expressed in 
gm/eq. bbl or lbm/bbl is given by 
0.35 L 
2.65 X 10- 3 g/L· ---
eq. bbl 
=0.000928g/eq. bbl. 
Thus, a treatment of 0.000928 Ibm of Na2C03 per 
barrel of mud per mg/L change in Ca2+ con-
centration is indicated. Note that this value is 
already given in Table 2.I2. To reduce the Ca2+ 
concentration of I ,500 bbl of mud from I 00 mg/L to 
50 mg/L requires the addition of 
0.000928 (I ,500)(IOO- 50) 
= 69.6lbm of Na2 C03. 
2.3. 7 Filtration Control. Several types of materials 
are used to reduce filtration rate and improve mud 
cake characteristics. Since filtration problems usually 
are related to flocculation of the active clay particles, 
the deflocculants also aid filtration control. When 
clay cannot be used effectively, water-soluble poly-
mers are substituted. The common water-soluble 
polymers used for filtration control are (I) starch, (2) 
sodium carboxymethylcellulose, and (3) sodium 
polyacrylate. Polymers reduce water loss by m-
creasing the effective water viscosity. 
Starch, unlike clay, is relatively unaffected by 
water salinity or hardness. It is primarily used in 
muds with high salt concentrations. Since thermal 
degradation begins at about 200°F, it cannot be used 
in muds exposed to high temperatures. Also, it is 
subject to bacterial action and must be used with a 
preservative except in saturated saltwater muds or 
muds with a pH above 11.5. 
Sodium carboxymethylcellulose (CMC) can be 
used at temperatures up to approximately 300oF but 
is less effective at salt concentrations above 50,000 
ppm. CMC tends to deflocculate clay at low con-
centrations and, thus, lowers gel strength and yield 
point in addition to water loss. CMC polymers are 
available in a number of grades. In order of in-
creasing effectiveness, these grades are technical, 
regular, and high viscosity. 
Sodium polyacrylate is even more temperature-
stable than CMC but is extremely sensitive to 
calcium. At low concentrations it is a flocculant and 
must be added at concentrations above 0.5 lbm/bbl 
to act as a filtration control agent. Both calcium and 
clay solids must be kept at a minimum when using 
this type polymer. 
All of the above filtration control additives have 
limitations at high temperatures, salinity, or hard-
ness, and also increase mud viscosity. Lignites and 
modified lignites have been used extensively as 
thinners and filtration control agents. Lignite 
products are designed to take advantage of the ex-
cellent temperature stability of lignite molecules. 
Lignite is a low rank of coal between peat and 
subbituminous. The major active component is 
humic acid, which is not well-defined chemically. 
Humic acid is a mixture of high-molecular-weight 
polymers containing aromatic and heterocyclic 
structures with many functional groups such as 
carboxylic acid groups. Humic acid also can chelate 
calcium or magnesium ions in mud. Ground, 
causticized, chromium-treated, sulfonated, or 
polymer-reacted lignites are used to reduce fluid loss 
without increasing viscosity. 
Example 2.15. The unweighted freshwater mud in 
the pit appears too viscous, and