A New Method for the Quantitative
Determination of Soluble Carbonates in
Water-Based Drilling Fluids
R. L. Garrett, SPE-AIME, Exxon Production Research Co.
Introduction
The mechanism by which bicarbonate (HC03-) and carbonate (C03=) ions adversely affect a deflocculated,
clay-based drilling mud's performance (and even these
ions' true concentrations) has been a controversial point
in the industry for many years. l Under field analysis
conditions, only an estimate of carbonate concentration
in fIltrate samples is obtained by API alkalinity titrations.
Even under ideal laboratory conditions, alkalinity titrations only approximate the true carbonate content of
complex muds.
However, a reliable method that allows direct carbonate analysis of muds recently was developed. The Garrett
Gas Train/Carbonate (GGT/C03 =) method determines
total soluble carbonates as gaseous CO2 freed from the
sample upon acidification in a small, transparent, plastic
gas train. The evolved CO 2 is measured quantitatively
using a commercial gas detector tube. The GGT/C03 =
method was developed with the same concept as the
GGT/S= method for measuring sulfides, which recently
was adopted as an API permanent procedure. 2 The similar chemical behavior of the CO 2 -carbonate and
H2 S-sulfide systems in aqueous solutions allows analogous approaches to quantitative analyses and chemical treatment. Although alike in many ways, the
gas-trainldetector-tube analysis method for carbonates is
more restrictive and sensitive to detector-tube flow conditions than is sulfide analysis. These conditions are
specified later when the procedure is described.
0149-2136/78/0006-6904$00.25
© 1978 Society of Petroleum Engineers of AIME
Uses for GGT/C03 = analyses go beyond the main
application for measuring and treating accumulated carbonates in field muds and include (1) studying carbonate
generation rates caused by thermal degradation of organic additives that produce CO2 in muds (such as lignites, lignosulfonates, carboxymethyl cellulose, tannins,
and starches), (2) studying the rate of CO2 absorption
from air into a circulating mud when influenced by various equipment used to process muds, (3) analyzing soluble carbonates in commercial barite to determine if soda
ash or sodium bicarbonate are present, (4) studying the
chemical reaction efficiency between carbonates and a
given treatment material such as lime - especially useful
for field and laboratory pilot tests of problem muds, and
(5) monitoring accumulation of bacterially and chemically produced CO 2 in packer muds, ballast muds for
ships, and muds stored in tanks.
Carbonate Chemistry in Water-Based Muds
In alkaline, aqueous drilling or workover fluids, soluble
carbonates are likely to accumulate from a wide variety of
CO 2 sources. Regardless of the source, if CO2 is generated internally or enters the mud externally, it reacts
immediately with alkaline hydroxyl ions (OH-) to form
HC03- and C03= ions. A chemical eqUilibrium is established at a given temperature, which is controlled by the
H+ and OH- ion concentration or mud pH. These equilibria involve the components shown in Eqs. 1 and 2.
CO 2
+ OH- ~HC03-
.................... (1)
This paper presents a new, direct methodfor measuring soluble carbonates in a drilling fluid.
The method is designed primarily for field use in guiding mud treatments and controlling
adverse mud rheology. Laboratory data are given to verify the method's accuracy andfield
experience illustrates its usefulness in mud control.
860
JOURNAL OF PETROLEUM TECHNOLOGY
TABLE 1-S0LUBLE CARBONATE ACCUMULATION FROM THERMAL DEGRADATION OF ORGANIC MUD
ADDITIVES IN ALKALINE WATER
(Measured by GGTIC0 3 ~ Analysis)
Lignosulfonate Additive,
10lb/bbl
._--_._---Hot roll- time, hours
- temperature, of
Solution pH - before hot roll, pH
- after hot roll, pH
Carbonates ----: before hot roll, mg/liter
- after hot roll, mg/liter
Carbonate accumulation, mg/liter
Test 1
24
150
11.3
10.6
200
310
110
Test 2
24
300
11.4
8.8
150
560
410
Test 3
24
400
10.9
6.9
150
1,375
1,225
Lignite Additive,
10lb/bbl
------Test 4
24
150
11.0
10.3
440
750
310
Test 5
24
300
11.0
9.0
440
2,375
1,935
Test 6
24
400
11.9
8.9
440
2,250
1,810
All samples filtered through 0.45 ,umillipore filter before analyses to avoid solid carbonates.
+ OH- ~ C03 = + H 20 ............. (2)
HC03 -
Carbonate equilibria with pH is seen more clearly in
Fig. I for the aqueous system of 10-2 •5 moIlliter of total
carbonates in fresh water at 25°C. 3.4 Total carbonates are
the sum of (C0 2 ) + (HC03 -) + (C03 =), the "soluble
carbonates" measured by the GGT/C03 = method. The
three species coexist in various proportions, depending
on solution pH. For example, Fig. 1 shows that at low
pH, gaseous CO2 dissolved in water predominates while
HC03- and C03= concentrations are essentially zero.
This is the basis for the GGT/C03= analysis. By acidification in the train, all C03= and HC03- ions in the sample
are converted to CO 2 gas that is freed from solution and
analyzed by a CO2-detector tube. Fig. 1 also shows that
for the approximate pH range of 6.3 to 10.3 the HC03ion predominates and above pH 10.3 the C03= ion predominates. This last pH is desirable for chemical removal
of soluble carbonates as insoluble CaC03:
Ca++ + C03= ~ CaC03~' ................. (3)
At mud pH's near 10.3, the reaction proceeds more
rapidly because the C03= ion predominates over the
HC0 3- ion. If lime is selected as the treatment chemical
for C03= removal, the mud's pH need not exceed 10.3
because OH- ions freed from lime become available to
raise pH and convert HCoa- to C03= for subsequent
removal as:
Ca (OH)2
+ C0 3=
~ CaC03~
+ 20H-,
....... (4)
-1
.z
COz+HzO
HCOi
i~
1-5
§-&
Sources of Carbonates in Muds
A widely accepted source of soluble carbonate accumulation in muds is CO 2 intrusion from formations being
drilled. As CO 2 enters an alkaline mud, it reacts with
OH- ions and soluble carbonates accumulate. Another
common (although not widely recognized) carbonate
source is thermal degrac;lation of organic additives such as
lignite and lignosulfonate, found in wells where the mud
encounters circulating or static temperatures of 300°F or
higher. Organic chemistry textbooks5 teach that CO2 is
liberated at this temperature by alkaline decarboxylation
of organic acid groups, especially from aromatic acid
molecules. Such structural groups are found in lignite's
humic acid, for example. 6 Table 1 shows carbonate levels
attained during laboratory hot-roll aging of a commercial
lignite and lignosulfonate in alkaline solutions. Lignite is
a more potent source of carbonates than lignosulfonate
because humic acid is high in aromatic carboxylic acid
groups.
Another source of CO 2 comes from air when mixed
into alkaline mud by mechanical devices such as jet
happers, mud guns, hydrocyclones, and screens. However, no quantitative data currently are available on this
source. Similarly, if rig-engine exhaust gas is bubbled
through mud to displace air as a way to reduce oxygen
corrosion, soluble carbonates can be expected to accumulate from the absorption of CO 2 •
Another source of carbonates is bacterial action on
organic matter. Fig. 2 shows carbonates generated in a
field mud stored at 70 to 75°F for more than 92 days.
Some of the mud was treated on the first day with biocide
and some was not. The biocide-treated sample still increased in carbonates, but not nearly as rapidly as did the
untreated sample.
Alkalinity Methods for Carbonate
Estimation
-7
.
-9
HC03- + OH- ~ C03 = + H20 .............. (5)
Gypsum would not produce these OH- ions. Guidelines
for lime/gypsum treatments based on GGT/C03 = results
are given later with experimental evidence of the low
stoichiometric efficiency attained for the reaction of
Eq.4.
3
4
6
7
pH
Fig. 1-Distribution of carbonate species in fresh water
containing 10-2 . 5 mol/liter total carbonates.
JUNE,1978
Alkalinity is defined as the combining power of a base as
measured by the maximum number of equivalents of an
acid with which it can react to form a salt. 7 In water and
waste-water analysis, "alkalinity" is routinely interpreted to represent C03=, HC03-, and OH- ions, although other alkaline materials are shown to contribute to
861
the alkalinity of waste waters. 7,8 In complex mud filtrates, appreciable amounts of other alkaline materials
can be present that react with the standard acid used to
measure API alkalinity. These interferences and procedurallimitations make API alkalinity methods for carbonate analysis only approximations.
API RP 13B2 offers two filtrate alkalinity tests: (1) the
conventional method of P, and M" and (2) the alternate
method of P" PI' and P 2 • For all "P-type" (phenolphthalein) alkalinities, the acid-titration end-point is
pH = 8.3; thus, P, and alternateP" PI' and P 2 titrations
encounter fewer spurious alkaline materials than does the
M, (methyl orange) titration that ends at pH = 4.3.
Consequently, the alternate method of P" PI' and P 2 is
potentially more accurate as an estimate of carbonates
than is the P, and M, method for a complex mud filtrate.
In fact, theP, and M, method long has been recognized as
error prone and was not given a quantitative interpretation in API RP 13B. RP 13B allows use of the P" PI'
and P 2 method to estimate OH-, HC03 -, and C03 =,
but precautions clearly state that interference errors may
exist.
Several technical and operational problems also occur
with the API alternate P" PI' and P 2 alkalinity as used for
carbonate measurement. The first problem arises from
the 25: 1 initial dilution of the sample. This lowers the
sample pH and makes the ionic distribution of carbonates
in the sample different from the distribution in the mud.
Total carbonates are not changed by this dilution unless
CO 2 escapes as gas or ions react with impurities in the
water. The P, used with M, in the conventional nondilution titration is not the sameP, used as part of the P" PI'
andP 2 in the alternate dilute titration. This is a confusing
point that causes interpretive errors if a conventional P,
value is used in carbonate calculations using the alternate
method. A second problem involves the precise experimental technique and equipment required to measure two
volumes ofO.IN NaOH and get exactly the same volume.
The first volume is added to the solution used for PI'
containing the sample. The second volume is added to P 2
solution with water as a blank. Any volumetric difference
between the two O.IN NaOH additions is magnified
fivefold in the volume of weak (0.02N) acid used in the
PI and P 2 titrations. A third problem involves stirring
speed during PI and P 2 titrations. In this method, C03 =
precipitates as solid BaC03 before PI (sample) titration.
MOO,-----------------------------------,
.. <
//
--
...--
_A
.,., .... / NON-IIIOCIDE TREATED MUD
/"/
i:ii 4I11III
i
"
3II1II
~
I-
",,//
.....
BIDClDE TREATED MUD
•
I
-
~RD MUD FROM WELL C . UGNITHIGNIlSULFDNATE TYPE
RRST LAB ANALYSIS
I~ 11I1IlJClDE TREATMENT·
~
0
rn
D.54 ppb SODIUM TRICIILDRDPHENATE
~
~
•
m m M
ElAPSED DAYS SINCE BIOCIDE TREATMENT
~
~
Fig. 2-Carbonate accumulation in a stored field mudcomparing biocide treated with nonbiocide treated samples.
862
However, 10caJized areas oflow pH in the vessel caused
by inadequate stirring and/or too rapid acid addition
causes BaC03 to dissolve and react with the acid titrant.
Experience in laboratory and field use of PI and P 2
titrations has shown the API alternate alkalinity is a
tedious test. Comparison of GGT/C03 = with API alkalinity data using field mud samples is discussed later.
Another approach to carbonate measurement using
alkalinity, which avoids the error-prone M, titration, is a
correlation between P, and pH. The pH-P, correlation is
obtained mathematically from equilibria information, but
is accurate only for dilute water systems. A set of correlation plots would be needed to compensate for ionic
strength effects on dissociation of carbonates in highly
treated or saline muds. Proper use of the pH-P, correlation also requires reliable field pH measurements.
In general, alkalinities are not considered a sufficiently
accurate field method for quantitative carbonate analyses
of highly treated muds, where carbonate problems are
most likely to occur. A field-worthy analysis method is
needed that can measure carbonates directly - as the
GGT/C03 = analysis does. A GGT/C0 3 = test can detect
carbonates and their concentrations, thereby indicating
the level oflime or gypsum treatment needed. Often, it is
just as important to know when carbonates are not present
and when a lime/gypsum treatment is not needed. If
carbonates are absent, unnecessarily adding lime or gypsum can create a serious rheological or filtration-control
problem or worsen an existing problem.
Garrett Gas Train Carbonate Analysis
Equipment
A portable, three-chamber, transparent plastic Garrett
Gas Train is used for GGT/C03 = analyses. This is the
same gas train used for GGT/S= analyses. Other than the
gas train itself, with its attached gas-cartridge holder and
regulator, equipment needed for GGT/C03 = tests are (1)
a I-liter gas bag with an 8-mm stopcock, (2) Drager
hand-operated vacuum pump, (3) Drager No. CH30801 9 CO 2-Detector Tubes, and (4) nitrous oxide (N20)
in small pressurized cartridges as the carrier gas. (If an N2
or He gas supply is available, either can be used in place
ofN20.) Chemicals needed are 5N sulfuric acid, octanol
defoamer, and distilled water. Equipment for GGT/C0 3 =
is available through most major mud service companies
and replacement Drager tubes and N20 cartridges (the
major expendable items) are available worldwide.
Procedure
A GGT/C03 = analysis is a simple, three-stage operation.
Stage 1 is shown in Fig. 3A. The first step in Stage 1 is to
place 20 ml of deionized water in Chamber 1 with a few
drops of octanol defoamer. Chambers' 2 and 3 remain
empty as foam traps. The top is sealed on the gas train.
Carrier gas then is bubbled gently for about 30 seconds
through the water in Chamber 1 and vented to the atmosphere to purge most of the dissolved CO 2 from the water
and air from the train. Next, a fully collapsed I-liter gas
bag is attached by the hose to the outlet of Chamber 3. A
measured volume of filtrate sample (discussed below) is
injected with a hypodermic syringe and needle through
the rubber sep1tUm in the top of Chamber 1. Then, 10 ml
of 5N sulfuric acid is injected by syringe through the
septum and the gas train is rocked to mix the liquids
JOURNAL OF PETROLEUM TECHNOLOGY
in Chamber 1. Gentle, carrier-gas flow is restarted and
continued until the gas bag is full and firm to the touch,
which requires 3 to 4 minutes. The stopcock then is
closed to seal the bag.
Stage 2 (Fig. 3B) involves disconnecting the gas bag
from the train outlet and inserting a Drager No .
CH-30801 CO 2-Detector Tube between the hand pump
and gas bag. The stopcock is opened and all gas is drawn
from the bag through the tube, using 10 strokes on the
hand pump.
Stage 3 is reading the detector tube. If CO 2 was generated from C03 = and HC03 - ions in the sample, a purple
stain appears in the tube (Fig. 3C). The stain length (read
in tube units) is used to calculate the carbonates present in
the filtrate sample:
GGT/C03 = = stain length x 25,000 , . . . . (6)
sample volume
where
GGT/C0 3 =
Strain length
= tube units on Drager No. CH-3080 1
tube,
and
Sample volume
= mi .
Samples
For testing a drilling fluid, the recommended sample
for GGT/C0 3 = analysis is filtrate from the API roomtemperature filtration test. From 1 to 10 ml of filtrate is
generally enough for field muds . The filtrate should be
free of solids; therefore, the fIrst spurt of filter-press
liquid should be discarded because it could contain
CaC0 3 particles that would cause erroneously high
GGT/C03 = values. In the laboratory tests discussed here,
API filtrates also were fIltered through 0.45JL millipore
membrane filters as a precaution against solid carbonates
in the samples.
"To obtain the GGT/CO,· concentration in units of equivalents per million (epm) or
milliequivalents/liter of CO,-, subsmute 833 for 25,000 in the numerator of Eq. 6.
= mg/liter* , as COa=,
,/
f'
Fig. 3A-Stage 1 - freeing CO. from acidified filtrate in Chamber 1 into gas bag.
--
Fig. 3B-Stage 2 - drawing contents of gas bag through CO2 -detector tube using hand pump.
JUNE, 1978
863
CO 2-Analysis Tube
Recommended Tube
Although Drager CO 2-detector tubes are available for
many analytical ranges, only Tube CH-30801 is recommended for GGT/C03 = tests . Other tubes should not be
substituted. Unlike H2 S-detector tubes , the CO 2 tubes are
very sensitive to incorrect use. Tube CH-30801 requires
exactly 10 pump strokes (1 liter) of gas throughput.
Furthermore, the CO 2 within this specific volume of gas
must be distributed uniformly and the tube flow rate held
in the proper range. To satisfy these tube-chemistry requirements , the I-liter gas bag is used for capturing CO 2
in the effluent gas from the GGT to assure that CO 2 is
distributed uniformly. The gas bag then is emptied as gas
is drawn through the detector tube by the hand vacuum
pump. Ten pump strokes (exactly 1,000 ml) empties the
bag. Therefore, by using the gas bag and hand pump
together, the tube's strict conditions for accuracy are
satisfied.
The manufacturer10 states that the CH-30801 Detector
Tube has a relative standard deviation (RSD) ranging
from 15 to 10 percent, which depends on the "tube
loading" by CO2 • When the stain length is short and near
the inlet of the tube, the CO2 measurement is less accurate
(I5-percent RSD) but improves to 10-percent RSD at the
exit end of the tube's measurement range. In our tests,
where the CH-30801 was compared with known carbonates and with the gas chromatograph, the over-all
accuracy was much better than the manufacturer's stated
accuracy.
Detector Tube Chemistry/Interference
No other gases or vapors cause an interference with the
CO 2-detectortube. 9 This was confirmed in conversations
with Drager representatives . In tests of field muds and of
individual mud components in the laboratory, no evidence of any interference was found . Potential positive
errors can result from spurious CO 2 in the train. For
example, CO2 in air remaining in the train before a test or
any CO 2 dissolved in the dilution water will be recorded
as a stain by the Drager tube to give an erroneously high
C03 = analysis. In the worst case, this error could represent about 500 mg/liter C03 =. The 500-mglliter error
assumes a small (1.0 ml) sample size in Eq. 6. Purging
the train and water by flowing N2 0 for 30 seconds before
connecting the gas bag or adding the sample minimizes
this error in field practice. In the laboratory, using de-
Fig. 3C-Stage 3 -
864
gassed (boiled) dilution water is recommended.
The color developed in the CH-30801 tube is a purple
stain on a white reagent matrix . This color change occurs
at the reaction front as crystal violet (redox indicator)
detects CO2 undergoing a reaction with the hydrazine
reagent 9
CO 2
+ N2H4
crystal
NH2 • NH· COOH. ....... (7)
violet
The stain may fade slightly at the inlet end as the reaction
front proceeds through the tube. Ignore any fading at the
inlet end, but the faintest purple at the leading edge
should be included in the stain length reading used
in Eq. 6.
Notice in Fig. 3C that the tube calibration units are not
spaced evenly. This is because the CH-30801 tubes are
calibrated experimentally at the factory. The mud engineer cannot adjust the tube reading accurately in the
field for any alteration in the procedure, such as incorrect
gas volume, flow rate, or excess sample volumes, that
exceeds tube capacity.
Laboratory Verification ofGGT/C03=
Method
Two laboratory studies verified the accuracy of
GGT/C03 = analysis for different types and sizes of
samples: (1) 11 replicate GGT/C03 = determinations of a
1,000-mg/liter standard sodium carbonate solution
showed good agreement, and (2) results of GGT/C03 =
agreed well with gas chromatographic CO 2 analysis using
split-sample testing offield-mud filtrate samples.
Analysis of Known Carbonate Solutions
Table 2 shows the sample size used, tube stain length
observed, and the GGT/C03 = concentration calculated
from Eq. 6 for 11 replicate analyses of a 1,OOO-mg/liter
C03 = solution. Sample-size variation of this 1,000mglIiter solution allowed different stain lengths and
thus tested the Drager tube's precision with different
CO2 loadings. The greatest error (±6 percent) occurred
when a hypodermic syringe instead of a pipette was used
to measure the sample into Chamber 1. Otherwise, the
repeat analyses all were within a few percent of each
other. (A disposable syringe and needle are recommended to measure and to inject the sample for field use
for reasons of simplicity, speed, and to avoid contamination, despite the potential error in volume measurement.)
Laboratory tests were performed to determine the CO 2
stain length in Drager CO2 -detector tube compared with unused tube.
JOURNAL OF PETROLEUM TECHNOLOGY
TABLE 2-REPLICATE ANALYSES USING GGT/C03 = METHOD OF A
WATER SOLUTION CONTAINING 1,000 MG/LITER SOLUBLE CARBONATES
Analysis
Stain Length on
Drc:igerTube
(tube units)
Sample Size'
(ml)
Carbonates
(mg/liter)
1
2
3
4
5
6
7
8
9
10
11
0.095
0.100
0.150
0.150
0.170
0.280
0.270
0.040
0.040
0.080
0.080
2.5 s
2.5 s
4.0s
4.0s
4.0s
7.0s
7.0s
1.0p
1.0p
2.0p
2.0p
950
1,000
938
1,000
1,063
1,000
964
1,000
1,000
1,000
1,000
Error
JEercent)
-5
0
-6
0
+6
0
-4
0
0
0
0
'Sample volumes measured by syringe (s) are less accurate than when using a pipette (p).
residual in Chamber 1 after 1 liter of carrier gas bubbled
through the sample. Presumably all carbonates (as CO2 )
are swept out and captured within this I-liter gas flow
volume. To verify this point, a second and third I-liter
gas bag were used consecutively to trap any CO2 remaining in the sample after the firs t bag had been filled. In the
normally expected concentrations of carbonate ions in a
sample, 100 percent of the CO 2 was captured in the first
liter, and none was found in the second gas bag. In more
concentrated test solutions, about 90 percent was captured in the first bag and the remaining 10 percent was
found in the second liter volume, with none in the third
bag. For most field-mud filtrate samples where carbonates are moderate to low, we expect essentially all to be
converted to CO2 and detected by this procedure. In
extreme cases of high carbonates in muds, residual carbonates may be left in Chamber 1 of the train, but this
error should be insignificant to practical mud-control
treatments .
Split-Sample Comparison With Gas
Chromatography
Table 3 shows the results of split-sample analyses of
field-mud filtrates using the GGT/C03 = method and a
separate laboratory's CO 2 analysis using gas chromatography (GC). The general procedure followed in this study
was to obtain a sufficient filtrate volume to perform two
GGT/COa= tests. This volume was split into two portions
processed through the Stage 1 GGT procedure in quick
succession. The effluents from the two tests were cap·
turedin I-liter gas bags. One gas bag was analyzed by GC
in Exxon Production Research Co. 's hydrocarbon
analysis lab. The other gas bag was analyzed (described
in Stages 2 and 3) using Drager CO 2-detector tubes. To
enhance the GC's CO2 -measurement sensitivity, helium
carrier gas was used in both the GC and GGT flow
systems. Using the ideal gas law, the GC CO2 analyses
were converted into equivalent milligrams of C03 = per
liter of original filtrate used in the GGT. This gave
a direct comparison of GC with GGT/C0 3 = results
(Table 3).
Table 3 indicates the excellent agreement obtained
between GC and GGT/C0 3= analyses for a mud sample
from Well C. The average absolute difference between
the two methods was 73 mg/liter, a 3.9-percent deviation, which is less than Drager's guaranteed deviation (10
to 15 percent) for the tube. 9 For Well B mud samples,
agreement between the two methods was generally very
good, except for one case where GC was lower than
GGT/C0 3 = by 25 percent (821 mg/liter). (The GC
laboratory changed operators in the last test of Well B
samples, when the 821-mg/liter difference occurred.) In
the case with a 36-percent deviation, this result is not
considered unsatisfactory because the absolute difference
is only 125 mg/liter, or within the precision of the tube. In
general, agreement between GGT/C03 = and GC was
better than Drager's guaranteed precision and is considered suitable verification of the method for field muds
TABLE 3-COMPARISON OF CARBONATE ANALYSES OF FIELD MUD FILTRATES BY
GGTIC0 3 = WITH GAS CHROMATOGRAPHY
Mud
Density
(Ib/bbl)
WeliC
mud
WellS
mud
11.6
12.2
12.3
10.1
10.4
11.3
11.8
11.7
16.0
18.1
18.6
18.8
Carbo.ll ates DeterminEld as C0 3 GGT/C03 =
Gas Chromatography
(mg/liter)
~!Iiter)
2,500
2,517
2,125
2,208
1,875
2,065
750
759
1,125
1,180
2,500
Not available
1,000
475
344
1,188
4,125
5,000
Average difference
-71
-3.9
911
350
330
1,146
3,304
Not available
89
125
14
42
821
9.8
35.7
4.2
3.7
24.8
Average difference
JUNE, 1978
Difference
(mg/liter)
(percent)
-17
-0.7
-83
-3.8
-190
-9.2
-9
-1.2
-55
-4.7
- - , ..
-~.
218
15.6
865
and for the correctness of the analytical factor (25,000)
calculated from Drager's calibration for CO 2 in air
mixtures.
Comparison of GG T /C0 3 = With Alkalinities
Errors reasonably can be expected when alkalinity measurements are used for estimating carbonates in muds.
The PrMf combination has potentially more errors than
does the Pr P r P 2 series or Pr pH correlations because M f
requires titration to a low pH and encounters more interferences in a mud filtrate.
Comparison WithMf
Fig. 4 shows laboratory data comparing GGT/C0 3 =
analyses with M f alkalinities for field mud filtrates from
Wells A and B. Note that for the two different field muds
a linear relationship was obtained for GGT/C0 3 = vs M f .
Mf has a positive intercept at about 2.0-ml standard acid
when GGT/C0 3= concentration trends to zero, indicating
that alkaline materials other than hydroxyl and carbonate
ions are titrated down to a pH 4.3 end-point. The upper
range of these data also supports the industry's rule-ofthumb that when Mf exceeds about 5 ml, soluble carbonate levels can be rather high.
Comparison ofP" PH andP 2
Calculated total carbonates based on the API alternate
alkalinity method are shown in Fig. 5 as compared with
GGT/C03 = analyses. Again, the field mud samples were
from Wells A and B and filtrates were tested under
laboratory conditions. Well A had close agreement between GGT/C0 3 = and alkalinity-estimated carbonates.
However, Well B had about 200-percent difference with
considerable data scatter. Both sets of data trend toward
the zero-zero intersection of Fig. 5. Since the P-type
alkalinity titrations to pH 8.3 did not encounter major
interferences from other soluble materials in the two
filtrates, the PrpH concept of carbonate measurement
appears practicable. This is contrasted with M f intercept
of2 ml (Fig. 4) when carbonates were near zero.
Field Experience With GGT/C03 =
The GGT/C03= method is being used on deep wells by
Exxon affiliates in several operating areas. Operators
other than Exxon have also used the GGT/C03 = procedure through various mud service companies in Texas,
Wyoming, and Louisiana.
Fig. 6 shows field results of a 37-day period where a
clay-based, lignite-lignosulfonate, fresh-water mud was
monitored daily for soluble carbonates using GGT/C0 3 =
analyses. Early in this period, casing was set and cement
was drilled, during which time the mud was weighted
from 12.5 to 16.0 parts per gallon using barite. On Day
14 the mud weight was increased again to 17.7 parts per
gallon. Excessive gel strengths caused concern from Day
1 and were attributed to soluble carbonates; however,
lime was not approved as a mud treatment until Day 8,
when an independent GGT/C03 = analysis verified the
presence of excessive carbonates. The mud property that
was most adversely affected by the carbonates and that
also markedly improved with lime treatments was the
Fann lO-minute gel strength. In Fig. 6c, the lime treatment (plotted downward) is seen to follow the downward
trends of gel strengths and GGT/C03 = analyses. From
Day 17 on, the soluble carbonate concentration leveled
out. A few days later, gel strengths stabilized and lime
additions continued on a more routine basis thereafter.
The GGT/C0 3 = concentration in this stable period
trended near 400 mg/liter, which is considered "background" carbonate values for field testing. (Dissolved
CO 2 in ordinary distilled water gives about 200- to
500-mg/liter carbonates.)
Most field experience with GGT/C03 = analyses has
been favorable. In early usage, there were cases of experimental error at the rig resulting from equipment leaks
and mistaken procedure. With familiarity of the test,
most of these problems have been solved. An API task
group now is evaluating the new method in detail with the
aim of adopting GGT/C0 3 = as an API test procedure.
Carbonate Treatment Guidelines
Removing carbonates from a deflocculated, clay-based
12
~
2600
~
iii
2200
~
.....
cc
1800
•
.5 2400
10
S
c:3
CI:
:::;
.;;:
8
~
S
~
z 6
::::I
~
-'
CI:
~
FIELD MUD FILTRATE SAMPLES FROM:
• WELL A
• WELL B
o~
o
1200
ffi
z0
1000
aI
800
5
600
aI
400
........
::::I
0
.....
en
200
I~
'/~
/1/
/ !~
/ I~
1/
-II
1/
il •
;/
FIELD MUD FILTRATE SAMPLES FROM:
• WELL A
• WELL B
__~__~~~~__~~~
1000
2000
3000
4000
5000
SOLUBLE CARBONATES BY GGT/COj (mg/II
Fig. 4-Correlation of API alkalinity, M r, and soluble carbonates
measured by GGT/C03 ~ analyses.
866
1400
a:
•J
1600
~
I
,"/
2000
>aI
<
4
/ /
I /
I
SOLUBLE CARBONATES BY GGT/C03' (mg/I)
Fig. 5-Comparison of soluble carbonates estimated from API
alkalinities, Pfi P" and P 2 with those measured by Garrett Gas
Train analysis.
JOURNAL OF PETROLEUM TECHNOLOGY
mud typically involves adding lime or gypsum to precipitate C03 = as CaC03 . Adding lime to a solids-laden mud,
especially in a hot well, is a procedure that can create
severe rheological and filtration-control problems if excessive lime is used. Therefore, two tests are important as
an aid for avoiding incorrect lime/gypsum treatment. (1)
A GGT/C03 = analysis should indicate that soluble carbonates are strongly present. Up to 500 mglliter of carbonates is not likely to cause a mud problem; but levels
above 1,000 mglliter (depending on mud density, solids
present, etc. ,) may create a mud problem. (2) If appreciable carbonates are measured, a pilot test series with a
range of treatments should show whether lime (or gypsum) will reduce the soluble carbonates and also improve
the mud's properties. The former may occur, but not
necessarily the latter. Experience has shown that carbonate problems often are coupled with, or even mistaken
for, excessive amounts of low-gravity colloidal solids.
These solids may be the more significant factor. Solids
and contamination problems are coupled and exhibit
TABLE 4-LlME/GYPSUM PROCEDURE
100 mg/liter C03 ~
- - - - - - - - - - -
Treat With
O.043lb/bbllime
or
{
0.1011b/bbl gypsum
Note: 100 mg/liter COa - ~ 3.3 epm ~ 3.3 milliequivalents/liter.
similar characteristics, such as poor response to deflocculant treatment, high yield points and gel strengths, and
high fluid loss and poor filter cake quality. High M f
values or M f/Pf ratios are traditional carbonate indicators.
Most symptoms also could result from excess calcium
in a mud, especially if Pf and Mf values are incorrect or
are ignored.
Therefore, before adding lime to a mud, the mud
engineer should be sure that carbonates are present.
Furthermore, detecting soluble calcium by API-hardness
titration is no assurance that carbonates really are absent
because calcium (and magnesium, the hardness ions) can
be present and soluble in organic-chelate form but will be
0
en
z
250
CI
~-
Ci fa' 500
Ci~
c:e
101-
:IE
::::i
c.LlME TREATMENTS
750
1000
50
:z:
Ic:J
Z
40
b.l0'GELSTRENGTHS
101
1:11: !O'
1;;;5
30
.... CI
101'"
c:J .....
.!@.
20
!:
:IE
...0
10
0
I
I
I
I
I
I
I
...I
...I.
5000
4000
3000
c.i
z
a. SOLUBLE CARBONATES
2000
CI
u=
~"";;,
CI &:
II
1000
r
~-
Ic:J
c:J
500
" BACKGROUND "
CARBONATE LEVEL - - - -
O~____~~~~~~~~~~-L~~~-L~~~~~~~~~~~-L~~-L~~-J
12
16
20
24
28
32
36
TIME ELAPSED (DAYS)
Fig. 6-A field example -
JUNE,1978
Garrett Gas Train used to measure carbonates in mud filtrate, effect on mud rheology, and removal by lime
treatment.
867
TABLE 5-LlME TREATMENT EFFICIENCY TO REMOVE SOLUBLE CARBONATES
(Measured by GGTICOa= Analysis of Pilot Test Samples of Field Muds)
WellCMud
Slaked lime added -Ib/bbl in mud
-Ib/bbl in liquid
Hot rolled - time, hours
- temperature, OF
Carbonates - before, mg/liter
- after, mg/Hter
Carbonates removed - actual, mg/liter
Theoretical carbonates removed,' mg/liter
Actual/theoretical COa= reduction
(18.1-lb/gal density,
60.0 vol. percent
liquid-phase retort)
Test
Test
Test
1
3
_120040
0.80
1.60
0.67
1.33
2.66
16
16
16
300
300
300
1,750
1,750
1,750
1,125
750
500
1,000
1,250
625
3;100
6,200
1,550
0040
0.32
0.20
Well B Mud
(18.8-lb/gal
density, 55.6
vol. percent
liquid-phase
_~.r:!L.._
Test
Test
4
5
1.20
2.10
2.16
3.78
19
19
180
180
6,500
6,500
4,900
3,700
1,600
2,800
5,020
8,780
0.32
0.32
'Based on stoichiometric efficiency ofO.043lb/bbl Ca(OH), to reduce CO,,' by 100 mg/liter in liquid phase, assuming the reaction Ca++
CO,,· .... CaCO" t .
unavailable to react with carbonate ions in the filtrate.
Table 4 shows the pounds per barrel oflime or gypsum
that, if added at liquid phase (filtrate) concentration,
theoretically could remove 100 mg/liter cas = from the
liquid phase.
The theoretically correct lime treatment as predicted
by GGT/COs= analysis usually is an undertreatment in
practice because calcium from the lime is consumed by
components other than carbonates. Calcium ions react
with clays, lignite, and other chemicals present in a mud.
Experience from laboratory tes'ts indicates that only
part of reagent-grade Ca(OH)2 reacts to reduce soluble
carbonates in highly treated lignosulfonate field muds.
Table 5 shows the results obtained by GGT/COs =
analyses. Samples of field mud that had appreciable
soluble carbonates were treated with known amounts of
lime and hot rolled at either 300 or 180°F to allow the
cas = and the Ca++ from the lime to react. The percentages of soluble 'carbonates actually removed in terms of
the theoretically expected reduction in carbonates was
found rather low - 30 to 40 percent. Note, however, that
some additional carbonates probably were generated
during the 300o Phot rolling (conditions of Tests 1,2, and
3 of Table 5). In Tests 4 and 5, carbonate generation at
180°F probably was not significant. The main point of
this study is that added lime does not react only to remove
the carbonates in a complex mud. This is supported by
field experiences reported by field mud engineers, who
say that three to four times more lime may be required to
improve a mud's performance than that predicted by
various carbonate tests, including GGT/COs = tests.
Pilot tests of lime (or gypsum) treatments should be
conducted in sealed cells in a rolling oven at bottom-hole
circulating temperature, allowing 2 hours or more for the
chemical reactions to occur. Lime (or gypsum) test additions should be multiples of the stoichiometric treatment,
based on GGT/COs= analysis (calculated from Table 5).
A four-test series is recommended to include a possible
treatment range. Assume, for example, that GGT/COa=
analysis of a filtrate was 1,160 mg/liter COa= that called
for a 0.5-lb treatmentoflime per barrel of liquid phase in
the mud. A suggested four-cell test series with lime
would be (1) 0 lblbbl as a blank, (2) 0.5 lblbbl, (3) 1.0
lblbbl, and (4) 2.0 lblbbl. In this series, the blank allows a
clear comparison of untreated mud properties with the
868
+
three levels of lime treatments. The stoichiometrically
predicted treatment level of 0.5 lblbbllikely will be an
actual undertreatment. The 1.0 lblbbl of lime may be near
the correct treatment and the 2.0 lblbbllikely is an overtreatment. Results of rheological and filtration tests plus
further GGT/CO a= analyses of the pilot samples will
indicate the correct amount of lime to add to the mud for
removing most of the carbonates.
Acknowledgments
The author thanks the management of Exxon Production
Research Co. for permission to publish this study and
several individuals who provided support and consultation. F. G. Scanlan and A. B. Pearson performed most of
the laboratory studies. G. G. Binder and D. E. O'Brien
gave their support in many ways. L. A. Carlton, P. J.
Trahan, and others at Exxon Co., U.S.A., provided the
drilling wells for mud samples and helpful advice for
early field testing of the GGT/COa= method. L. A. Carlton and M. R. Annis continually contributed their experience of carbonate occurrences in muds throughout this
investigation.
References
1. Green, B. Q.: "CarbonatelBicarbonate Influence in Waterbase
. Drilling Fluids," Pet. Eng. (May 1972).
2. API Recommended Practice
Standard Procedure for Testing
Drilling Fluids. API RP 13B, 7tIred., API, Dallas (1978).
3. Strumm, W. and Morgan, J. J.:AquaticChemistry, John Wiley &
Sons, Inc., New York (1970) Chap. 4.
4. Deffeyes, K. S.: "Carbonate Equilibria, A Graphic and Algebraic
Approach," Limnology and Oceanography (1965) 10, 412.
5. Fuson, R. C.: Advanced Organic Chemistry. John Wiley & Sons,
Inc., London (1954) 210-212.
6. Lignin Structure and Reactions, Advances in Chemistry Series 59,
ACS, Washington, D.C. (1966).
7. Glossary of Drilling Fluid and Associated Terms, Bull. Dll, 1st
ed., API (1965).
8. Standard Methods for the Examination of Water. Sewage and
Industrial Wastes. 10th ed., American Public Health Assn., Inc.,
New York (1955).
9. Detector Tube Handbook. 3rd ed., Dragerwerk, AG, Lubek,
W.Gennany(1976)4,42,182.
10. Garrett, R. L.: "A New Field Method for the Quantitative Determination of Sulfides in Water-Base Drilling Fluids," J Pet. Tech.
(Sept. 1977) 1195-1202; Trans., AIME, 263.
JPT
Original manuscript received in SoCiety of Petroleum Engineers office Sept. 9, 1977.
Paper accepted for publication Nov. 22,1977. Revised manuscript received Jan. 30,
1978. Paper (SPE 69(4) was presented atthe SPE-AIME 52nd Annual Fall Technical
Conference and Exhibition, held in Denver, Oct. 9-12, 1977.
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