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is reported in 
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72 
TABLE 2.14-SPECIFICATIONS FOR BARITES 
Specified OCMA 
Property API Values 
Minimum specific gravity 4.2 4.2 
Wet screen analysis, wt% 
Maximum residue 
on No. 200 sieve 3 3 
Minimum residue 
on No. 350 sieve 5 5 
Maximum residue 
on No. 350 sieve 15 
Maximum soluble metals 
or calcium, ppm 250 
Maximum soluble solids, wt% 0.1 
Performance of 2.5-g/ml 
barite, cp (apparent) 125 
Water susl)ension after viscosity at 
contamination with gypsum 600 rpm 
milliequivalent weights (meq) of methylene blue per 
100 mL of mud and that the equivalent montmorillonite 
content of the mud in pounds mass per barrel is ap-
proximately five times the cation exchange capacity. 
While application of this simple rule of thumb is often 
sufficient when performing a mud solids analysis, it 
is sometimes desirable to refine the use of the cation 
exchange capacity data. This is accomplished by per-
forming methylene blue tests on a sample of the ben-
tonite being used and a sample of the drilled solids being 
encountered. These cation exchange capacity (Z v) re-
sults are reported in meq of methylene blue per 100 
g of solids. Typical results for several common solids 
are shown in Table 2.6. 
The cation exchange capacity of the mud, Z vm, 
reported in meq/100 mL of mud sample, can be pre-
dicted from a knowledge of the cation exchange ca-
pacity of the bentonite clay Z vc, the bentonite clay 
fraction.2..L , the cation exchange capacity of the drilled 
solids, Z Vd.n and the drill solids fraction,Jd.,, using the 
following equation. 
_ ( Zvc Zvds) Zvm=lOO mL fcp,.--+fd,Pd,-- · 100 . . 100 
Since both Pc and Pds are approximately 2.6 g/mL 
this equation reduces to 
Zvm =2.6(/,.Zvc +fttsZVds) 
Substituting the identity 
fds=ftg-fc 
and solving for fc yields 
Zvm -2.6/tgZVds fc = ... · .............. (2.37) 
2.6(Zvc -Zvds) 
Eq. 2.37 allows the direct determination of the ben-
tonite clay fraction from the methylene blue titration 
results on samples of mud, bentonite clay, and drilled 
solids and a knowledge of the total fraction of low-
gravity solids. 
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Example 2.22. A mud retort analysis of a 15-lbm/gal 
APPLIED DRILLING ENGINEERING 
TABLE 2.15-THEORETICAL SOLUBILITY 
OF Ca 2 + AS A FUNCTION OF pH 
[OW] [Ca 2 + I 
....e_l2_ (moi/L) [OW] 2 (moi/L) 
11.0 0.0010 1.0 X 10-6 1.3000 
11.5 0.0032 1.0x 10-5 0.1300 
12.0 0.0100 1.0x1o-4 0.0130 
12.5 0.0316 1.0 X 10-3 0.0013 
13.0 0.1000 1.0 X 10-2 0.0001 
freshwater mud indicates a solids content of 29% and 
an oil content of zero. Methylene blue titrations of sam-
ples of mud, bentonite clay, and drilled solids indicates 
a Zvm of 7.8 meq/100 mL, a Zvc of 91 meq/100 g, 
and a Zvtts of 10 meq/100 g. The mud has a plastic 
viscosity of 32 cp and yield point of 20 Ibm/ 100 sq 
ft when measured at l20°F. Determine (I) the total 
fraction of low-gravity solids, (2) the volume fraction 
of bentonite, (3) the volume fraction of drilled solids, 
and (4) whether there is sufficient bentonite in the mud. 
Solution. 
1. From Fig. 2.29,J1g = 0.08. 
2. Using this value in Eq. 2.37 yields 
7.8- 2.6(0.08)(10) 
fc = 2.6(91- 10) 
=0.027. 
3. fds = /tg- fc = 0.08-0.027 = 0.053. 
4. For a volume fraction of 0.027, the bentonite 
content in lbm/bbl is 
910(0.027) = 24.6lbm/bbl, 
which is a little high for a 15-lbm/gal mud. Figure 
2.31 suggests such a claim since the yield point is 
slightly above the maximum recommended value. 
2.4 Inhibitive Water-Base Muds 
An mhibitive water-base mud is one in which the 
ability of active clays to hydrate has been reduced 
greatly. Inhibitive muds prevent formation solids 
from readily disintegrating into extremely small 
particles and entering the mud. They also partially 
stabilize the portions of the wellbore drilled through 
easily hydrated clays and shales. Inhibitive water-
base muds are formed by the controlled con-
tamination of a clay /water mud by electrolytes 
and/or deflocculants. Thus, they also are highly 
resistant to subsequent contaminants encountered 
during the drilling operation. 
2.4.1 Calcium-Treated Muds. Lime-treated muds 
were among the first inhibitive muds. The beneficial 
effects of calcium-treated muds at a high pH were 
discovered largely by accident as a result of cement 
contamination. 
The properties of a calcium-treated mud depend to 
a great extent on the amount of calcium that enters into 
solution. When Ca 2+ and OH- ions are introduced 
into an aqueous solution, they react to form Ca(OHh. 
• 
DRILLING FLUIDS 
The solubility product, K,P, for this reaction has a 
value of about 1.3 x 10-6 : 
Ca2 + +20H- =Ca(OHh. 
K,p=[Ca 2 +][oH-] 2 =1.3xi0- 6 (at 70°F). 
The solubility principle states that the product 
[Ca 2+] · [OH- ] 2 must remain constant. If we repre-
sent the solubility of the Ca 2 + ion by S, then 
[Ca2 +] 
[OH-J 
s ·(2S) 
S, 
2S,and 
1.3xl0- 6 . 
Solving for the solubility, S, gives 
S=~ 1.3 X 10- 6 = 6.89 X 10- 3 mol/L. 
4 
The concentration of OH- ions in a saturated 
solution of lime is 2S = I. 38 x I 0- 2 . This 
corresponds to a pH given by 
( 
I X 10 -l 4 ) 
pH =-log 2 =12.14. I 1.38 X 10-
The solubility of Ca2+ can be altered greatly by 
changing the pH of the solution. For conditions at 
which the lime solubility reaction is predominant, the 
solubility product principle implies 
2 + 1.3xl0-
6 1.3xl0- 6 
[Ca l = 2 - 2 [OH1 [antilog (pH- 14)) 
Using this relation, the solubility of Ca2 + as a 
function of pH has been tabulated in Table 2.I5. 
The chemistry of a lime-treated mud is more 
complicated than that of an aqueous solution of lime 
because of the presence of clays. However, the 
solubility of Ca2+ in a lime-treated mud still remains 
a function of the pH of the mud. Thus, the solubility 
of Ca2+ in a lime mud can be controlled by the 
addition of caustic (NaOH). 
When Ca2 + enters a clay/water mud, the mud 
begins to flocculate and thicken. At the same time, a 
clay cation exchange reaction begins in which the 
Na + cations of the clay are replaced by Ca 2 + 
cations. Calcium montmorillonite does not hydrate 
as extensively as sodium montmorillonite, and the 
clay platelets begin to stack closer together as the 
cation exchange reaction proceeds. This process is 
called aggregation and is marked by a thinning of the 
mud. The magnitude of the change in viscosity associ-
ated with flocculation and subsequent aggregation 
depends on the temperature and the concentration of 
solids and deflocculants in the mud. 
The conversion to a lime mud generally is made in 
the wellbore during drilling operations. The high 
temperature and pressure environment of the well 
greatly aids the chemical conversion. Lime, caustic, 
and a deflocculant are added to the mud at a rate that 
will accomplish the conversion in about one cir-
culation. A variety of deflocculants can be used. 
Quebracho, chrome lignosulfonate, calcium 
73 
lignosulfonate, and processed lignite are all ap-
plicable. A water-soluble polymer usually is added 
during the second circulation to improve the 
filtration properties and stabilize the mud. 
The amount of lime, caustic, and deflocculant 
added to make the conversion usually is determined 
using pilot tests. The average conversion is made 
using about 8 lbm/bbl lime, 3 lbm/bbl caustic, 3 
lbm/bbl deflocculant, and I lbm/bbl polymer. An 
excess amount of lime present in the mud is desirable 
to prevent the slow depletion of Ca2 + concentration 
in solution by formation solids and mud dilution. A 
frequent practice is to maintain the lime content of 
the mud in pounds per barrel numerically equal to 
theP1. 
Ca1cium-treated muds sometimes are formed using 
gypsum (CaS04 ·2H 2 0) rather than lime 
[Ca(OHh]. Gypsum-treated muds are particularly 
well-suited