FORM ATE
TE CH N I CA L
M A N UA L
C A B O T
C O M PAT I B I L I T I E S A N D I N T E R A C T I O N S
SECTION B1
COMPATIBILITY WITH GASES
B1.1Introduction ...................................................................................................................................2
B1.2
CO2 compatibility ..........................................................................................................................2
B1.2.1 Buffered formate brines protect against CO2 acidification ...............................2
B1.3 H2S compatibility ......................................................................................................................... 3
B1.3.1 How formates scavenge H2S .......................................................................................3
B1.3.2
H2S scavengers in formates ........................................................................................4
B1.4 O2 compatibility ............................................................................................................................ 4
B1.4.1 Use of O2 scavengers in formate brines ..................................................................5
References ................................................................................................................................................... 5
The Formate Technical Manual is continually updated.
To check if a newer version of this section exists please visit
formatebrines.com/manual
NOTICE AND DISCLAIMER. The data and conclusions contained herein are based on work believed to be reliable; however, CABOT cannot and does not guarantee that similar results
and/or conclusions will be obtained by others. This information is provided as a convenience and for informational purposes only. No guarantee or warranty as to this information, or
any product to which it relates, is given or implied. CABOT DISCLAIMS ALL WARRANTIES EXPRESS OR IMPLIED, INCLUDING MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE AS TO
(i) SUCH INFORMATION, (ii) ANY PRODUCT OR (iii) INTELLECTUAL PROPERTY INFRINGEMENT. In no event is CABOT responsible for, and CABOT does not accept and hereby disclaims liability for,
any damages whatsoever in connection with the use of or reliance on this information or any product to which it relates.
© 2013 Cabot Corporation, MA, USA. All rights reserved. CABOT is a registered trademark of Cabot Corporation.
VERSION 2 – 09/13
FORMATE TECHNICAL MANUAL
C AB O T
B1.1 Introduction
Influx of gases is one of the main causes of corrosion
The carbonate / bicarbonate buffer system provides
failures in completion and packer fluids. The most
strong buffering at two different pH levels:
destructive gases are carbon dioxide (CO2) and hydrogen
sulfide (H2S), which can leak into the wellbore along with
• Higher buffer level at pH = 10.2
pK
−
2−
+
pK
2−
other reservoir gases, and oxygen (O2) that enter the
+
+H→
←
−→ HCO3
3 
←
HCO
1 1CO3 + HCO
(1)
3
fluid during circulation at the surface or, in the case
2
−
−
pK
22−− CO
− HCO−
of packer fluids, during annular pressure bleed-off
where
=CO
10.2
+ H +3←HCO
3→
HCO33
pK a 2 1 pK a 2 CO
33
pK
−
2−
+
operations.
CO
+
H
←
→ HCO3
1
3
pK a1
−
+
2−
−
−
2−
pKH
+ 2 pK − HCO
+
a1
+
←


→
H
CO
2
At
pH
=+ 10.2
( 
) the buffered
HCOcontains
CO3 + H ←

3 →
HCO
1
3
2 solution
3
HCO
H 2−←
a 2 → H2 COCO
3
3 pK
3 3 −
pK
CO
+ H + ←of

→
HCO3 (CO32 − ) and HCO3 −
1the2same
pK
Oxygen and H2S scavengers are commonly used in
amount
carbonate
−
−a2
+ 3 pK
pK a1
2 −H ←
−
COCO

→ −HCO
− 
2
3 +
H3 + ←
→ HH
HCO
halide brines to try and eliminate the problems caused pK1a 2
bicarbonate
(HCO
HCO
3−3 +).
2 CO
3
pK a1 HCO
2−CO
3
2 −3 −pKH CO
pK a1 3 pK
pK
3
2 +HCO
3
−
−a
2
2
+
CO
HCO
+
H
←

3
by these gases. Unfortunately, scavengers do deplete
a2
CO2 −3 + H ←3−→ HCO331 → H2CO3
1
−
+pK a pK a1
CO
HCO
pK
−
−
2
2
2 → H CO
HCO3 +•H Lower
←
during use, and the addition of these scavengers is
buffer
level
CO= 6.35
+3 H −+ ←→HHCO
133a + at pH
2 pK
33
pK 3HCO3
−
2 CO
3
2
2
−
−
1CO ( aqa)1
CO
(
g
)
=
pK
−
2 −3
+pK
H ←→
−
2
CO
g−)HCO
=HCO
CO
COH32CO3 HCO HCO
pK )a1a 2 pK
likely to contribute to deferral rather than removal
+3 32 ( aq
1 of CO3 2+ H+ ←
2 (→
3
H2CO
3−
a1
HCO
2→ H2CO
(2)
2
− 3
3 +−H ←
3
CO
HCO3
pK
the problem.
HCO33
3
a2
H2CO
pK a1
3
+ −( aqpK
a1
2− 2
− )−2 −
pK
−
CO
(
g
=
CO
)
HCO
++HH32+←
H
2 CO
CO3 pK
pK a 2
H22CO
CO
3
←

→
→ HCO
where
6.35
1 a1 =HCO
3 −33 HCO
4
pK 3a13
−
32)=+
+ ( aq
CO
( aq
H2CO
OCOHCO
=2((aq
H
CO
( aq
)
4
g23)H
=+
)→ H2CO3
HCO
2H
3CO
CO
pK
CO
(
aq
)
+
O
H
)
H
←
2
3
2
3
a
2
2
2
3
1
pK
−
2−
+
→ HCO
1 CO2 (−g ) CO
2−
−
a1 ←
B1.2 CO2 compatibility
=+CO
(pKHaq
) 
3 +
23
2
HCO

→
H2CO(3pKpK)3athe buffered
CO33−solutionHCO
At
pH
=
6.35
contains
3 + H ←
HCO
H
3 3
2
2 CO
a
pK
−
−
23
4
+ CO ( g )
1aq(
=
CO
aq
)
CO
(
)
+
H
O
=
H
CO
(
aq
)
2
2
−
2 3
2
3
+ Hsame
←
→
1 3 CO3 CO
K a 1 2−
2 − KHCO
−
+) and
Carbon dioxide, CO2, is a stable oxide of carbon
the
amount
ofHCO
bicarbonate
pK aaq
CO
+ ( HCO
3 CO
HCO
(aaq

)=+)HH
( aq
) H2CO3
1) )←
4aq
2 ( g) =
2 3(
pK
−−→
1 +HCO
H(a12aq
O+( aq
2)CO
3H
3
a225
3(aq
pK
− 2 CO+3 −(aq
− CO
H
←

→
HCO
))2+−+
5 pK
2
2
3 ( aq)−
32H
pKCO
HCO
+
←

→
H
CO
+
H
←


→
HCO
1
3
2
3 HCO
composed of two double-bonded oxygen atoms. 4
acid
(
HCO
H
CO
pKCO
3 carbonic
3 3 ).
CO
+
H
←


→
1
2
−
−
3
2
a1 ( aq) +
3
3
3
(CO
aq2)( g )HCO
= CO
( aq)
2
2 CO
pK a 2 H−2 O =+ HCO
pK
3 3a1
3 2K a
−
+
Due to its weak bi-polar nature, CO2 is highly soluble in 2 6 HCO
1
46
+
H
←


→
H
CO
H
S
(
g
)
=
H
S
(
aq
)
CO
(
aq
)
+
H
O
=
H
CO
(
aq
)
H
CO
(
aq
)
←


→
HCO
(
aq
)
+
H
(
aq) −
5
3
2H
− (2aq)3222 3CO −(2g ) =
2 22S
3 CO− ( aq
H3 2 S (g ) =CO
3 ) 2−
2
2K a
4 The
HCO
pK a 2CO
− CO HCO +
) + pK
H3 2a1Oof
=pK
H2aCO
( aq
)(aq
1 CO
3 pK
water, with which it reacts to form carbonic acid.3
exact
and
will
vary
somewhat
HCO
H
− 2 ( aq
+levels
3 CO
3
3
a
2
3
H
)
←


→
HCO
(
aq
) + H3 ( aq)
5
2
2
1
2
3
3
+K )Ha ←−→ H2CO
CO2 ( g ) =HCO
CO23( aq
3 +
16 concentration,
−(aq
with
brine
temperature,
and
pressure.
4
H
CO
(
aq
)
←


→
HCO
)
+
H
(
aq
)
H
S
(
g
)
=
H
S
(
aq
)
5
pK
−
+
H2 CO+3 (+aqpK+)a1
a3
HCO
2 2 ( aq
13 CO
2 2 3 pK
2 O3−=−
pK aK)1 a+H22HCO
a1
HCO
+H ←
pK

→
H)22←
CO
1
3→
a6

−
+− +
The consequences of CO2 leakage into a halide-based
S
(
aq
←


)H
H+←
)→) H2CO3
1 aq

aq
( aq
57H3 2 S (aq)H←
3aq
H
S−=→(2HS
g(HCO
)H
=(()aq
SH+(O)aq
4)1 CO
2CO
3 K(
3H
7

HS
(2CO
aq
)HCO
+aq
((2)aq
−a→
32HCO
+
=)H(Haq
+ 2)
(
g
)
CO
2 CO3 ( aq)
2HCO H(CO
2
pK
H
CO
(
aq
)
←


→
aq
)
+
H
(
aq
)
5
3
2
3
a
2
3
3
6
completion fluid can be catastrophic for the integrity
of COH2 S
that
takes place in completion brines
(g )The
= 1Hacidification
4
2 S (aq)
− )−
2 ( aq) + H2 O = H2 CO+3 ( aq
pK a 1 K a
−
+
−
−S
g )+carbon
=
CO
aq
)MHS
+(g
SH
MHS
(s
)→
1 HS (
H)
CO
pK2a(168
HM
= (→
H(s
(←
aq
)2←
sub-surface equipment and tubulars. Both pitting and 3 8 CO
when
dioxide
gas
(CO
enters
into
the
wellbore
7SH
H
aq

aq− )HCO
+ H)3+
(H
aq+ )( aq) H2CO3
22(HCO
3)H
3 →

HCO
(aq
M
+
→
23)S()aq
pK
5
2)
2 2SCO
a1
6
pK a 1 K a3
−
+
H
S
(
g
)
=
H
S
(
aq
)
2
2
1
stress corrosion cracking (SCC) can occur in CRAs that 3
can( gbe
the( H
following
(aq
) aq
←equations:


→
HS
()aq3−)(+
H
)←

→
HCO
aqH) +( aq
H +)( aq)
5+ CO
2S
CO
) a=described
CO24( aq) 7by
aq
)
+
H
=
H
CO
(
aq
2 CO
3 2(O
2 pK
2
2
3
−
−
+
K
a 11 →
−
+
8
6
M
+
SH
→
MHS
(s
)
7
H
S
(
aq
)
←

HS
(
aq
)
H
(
aq
)
have been exposed to CO2 and halide brines. 5
H2+S
H2 )S (−aq)
2 3 (aq) ←
H4

HCO
(aq
) +(3gpKH)a 1=−( +aq
32 CO
CO
) =)→
aq
)4
(3)
) )=MHS
CO22(79
(gaq
+COHHO22+(2OS
(eaq
) +HS
+3(=
H−6)2=CO
4
=
2CO
H22 (O(−gaq
←+
+COH2+((s
(aq
aq
M→
SH
→
) ))
3H
O
4aq
ea8H
2
9
+ H2 S (aq)
2 + 4H 2+ pK
2−OH2 S (g ) =
1
+7
− H2 S (aq) ←

→
HS
(
aq
)
+
H
(
aq
)
K a1
Buffered formate brines are very different from 6 8
(Saq
) +) H(s25O)+ = H2 CO
( aq
) )←
H42M
S (g+)SH
=COH22→
(MHS
aq
CO

→ HCO3− (aq) + H +(4)
( aq)
3+ (aqpK
−H2 3
−
a1
8
M
+
SH
→
MHS
(s
)
O
+
4
H
+
4
e
=
2HS
H2 O− (aq) + H + ( aq)
9
7
H
S
(
aq
)
←


→
2
halide brines in the way they respond to a CO2
M
+ ) + H −O
4 8 CO
K a 1= H 2CO4−( aq)
+
(
aq
+ ( aq
−a H (5)
+ SH 2 
→ MHS
2
2 (s
3) (aq) +
)pK
O = H CO3 ( aq+)
H2 CO
5
1 =2 2 H O− 2
2 ( aq
3 (aq) ←
OH
+SCO
4H(aq
HHS
e)+
97HCO
6→
)+←4
→ HS
2
2 (aq) + H ( aq)
influx. The difference is largely due to the influence of
H
S3 (g
)− 2=
2 (aq)
+ pK a −
K a1
+ 2 +−
+
−
1 e =82 H O M + SH → MHS(s)
O
+
4
H
+
4
9
H
CO
(
aq
)
←


→
HCO
(
aq
)
+
H
(
aq
)
5
2
2
7
H
S
(
aq
)
←


→
HS
(
aq
)
+
H
(
aq
)
the carbonate / bicarbonate pH buffer.
Depending
on pH in+the3 brine
system, dissolved CO2
2
3
62
−
H2 S9(g ) = HO
S2 K(+aq
) 8 + 4−e−M= +2+HSH
2+
a 1 4H
+ → MHSK(s
2O
) CO ) −
− HCO
a(H
in
the
brine
as
either
carbonic
1
H2 CO
(
aq
)
←


→
(
aq
)
+
H
5 + 9 remains
3
O
+
4
H
+
4
e
=
2
H
O
3
3
H
COa 1 3( aq
(aq) )acid
←


→2 HCO
(aq) + H + ( aq)
5
2
2
−
2pK
−
+ 3
6
- S (aq) ←
S
(
g
)
=
H
S
(
aq
)
M + SHH
→
MHS
(s
)
B1.2.1 Buffered formate brines protect against8
7
H

→
HS
(
aq
)
+
H
(
aq
2
2
or bicarbonate ( HCO32) according
to equation 5. As )
+
−
O
+
4
H
+
4
e
=
2H2 Oequilibrium
9
6
CO2 acidification
2
pK
a
−
+
H
S
(
g
)
=
H
S
(
aq
)
more
CO) 2←
gas
brine,
1enters into
2
H
(g+))the
= H−2 S (aq)
7
H22 S (aq

→ HS 6
(+aq)the
+H
2 (Saq
−+
O
4
H
+ (s
4e)portion
= 2H2 Oexists
8 to9the
M +left
SH2and
→aMHS
Formate brines used in field applications are buffered
gradually
moves
larger
pK a 1
−
+
+
−
7
H
S
(
aq
)
←


→
HS
(
aq
)
+
H
(
aq
)
O
+
4
H
+
4
e
=
2
H
O
9
by the addition of potassium or sodium carbonate
as
This
pH drop, and allows
82
M2 +carbonic
+ SH −→pK2acid.
MHS
(s
) − makes the
a1
+
pK a 1
7
H
S


→ HS
) ) ←
and potassium or sodium bicarbonate. Typical
unbuffered
brines
to acidify.
2+ (aq) ←
7(aq) + +HH2(Saq
(−aq
→ HS − (aq) + H + ( aq)
−
8
M
+
SH
→
MHS
(s
)
O
+
4
H
+
4
e
=
2
H
O
3
9
2
2
recommended levels are 17 to 34 kg/m / 6 to 12 lb/bbl
+
−
+ → MHS
8
M + +different
SH −→ MHS
(s)
of carbonate or a blend of carbonate and bicarbonate 9
Figure
brine
OM2 ++ 4SH
H1 demonstrates
+ 4e− = (s
2H) 2 O8how three
+
−
(see Part A of the manual). The main purpose of this
systems
react to a CO2 influx:
O
9
2 + 4H + 4e = 2 H2 O
buffer is to provide an alkaline pH and to prevent the
+
O + 4H + 4e− = 2H2 O9
O2 brines
+ 4H + are
+ 4e − = 2H2 O
pH from fluctuating as a consequence of acid or base 9
•2Conventional divalent
halide
influxes into the brine.
incompatible with carbonate-based pH buffers
due to the precipitation of divalent carbonate salts
A buffered solution is defined as a solution that resists
(CaCO3, ZnCO3) that inevitably occurs when these
a change in its pH when hydrogen ions (H+) or hydroxide
two classes of chemicals are mixed together. These
ions (OH-) are added. The ability to resist changes in
divalent brines have a naturally low pH (2 – 6), and an
pH comes about by the buffer’s ability to consume
influx of CO2 may cause a further drop in pH. The CO2
hydrogen ions (H+) and / or hydroxide ions (OH-).
will largely be converted to carbonic acid, which is
very corrosive.
a2
a2
a2
a2
a2
a2
a2
a2
a2
a2
a2
a2
a2
a2
PAGE 2
SECTION B1
a2
VER S IO N
2
–
09 / 13
SECTION B: COMPATIBILITIES AND INTERACTIONS
C A B O T
pH in various brine systems as a function of CO2 influx volume
12
11
Buffered formate
10
pH>6.35:
CO2 mainly converted to
bicarbonate (HCO3-), which
does not promote corrosion
9
pH 8
Unbuffered formate
7
6
5
4
0
pH<6.35:
CO2 mainly converted to
carbonic acid (H2CO3),
Calcium bromide
which promotes corrosion
50
10 0
150
200
250
300
350
400
450
BBL gas influx / BBL buffered formate brine (2% CO2 , 21°C / 70°F, 1 atm)
500
Increasing time of CO2 influx
Figure 1 pH as a function of CO2 influx in a typical halide brine, an unbuffered formate brine, and a buffered formate brine.
• Buffered formate brines are capable of
absorbing large amounts of CO2. Unless the influx is
unusually large, the brine maintains a pH (at around the
upper buffer level), which is high enough to prevent
carbonic acid being present in the fluid. With a large
influx of CO2, pH drops down to the lower buffer level
(pH = 6.35) where it stabilizes. Measurements of pH in
formate brines exposed to various amounts of CO2 have
confirmed that pH never drops below 6 – 6.5. This pH is
still close to neutral, meaning that this brine system can
not be ‘acidified’ to any great extent by exposure to any
amount of CO2.
• Unbuffered formate brines: The pH of these brine
systems behave very much like halide brines when
exposed to CO2 gas. However, they do have a higher 1
initial pH, and the pH drop is limited as formate brine 1
is a buffer in itself (pKa = 3.75). The use of unbuffered
formate brines is not recommended.
2
2
In order for the pH to drop below this second buffer
level of 6.0 – 6.5, an acid needs to be added that is
stronger than the acid formed, i.e. carbonic acid. As
any CO2 gas influx into the buffered solution dissolves
3
and converts to carbonic acid, a CO2 influx is therefore 3
incapable of pulling the pH much below this second
buffer level.
4
4
To summarize for buffered formate brines:
•For small and medium size influxes of CO2, the buffer 5
fully absorbs the gas influx, and pH remains high, i.e. 5
above 8.
6
• Only for very large influxes of CO2, does the pH drop to 6
6 – 6.5 and some carbonic acid exists in equilibrium
with bicarbonate. The pH never drops below this level. 7
7
8
V ERSION 2 – 0 9/ 13
8
9
9
There are no scavengers for this gas that are
compatible with conventional high-density halide
brines, which puts formate brines in a unique position
over traditional halide brines in any well application
where CO2 is a potential threat.
B1.3 H2S compatibility
Hydrogen sulfide, chemically the sulfur analog of water,
is deadly poisonous. Concentrations above 600 ppm
can be fatal in three to five minutes. Hydrogen sulfide is
also the most corrosive gas encountered in the oilfield.
Concentrations of 50 ppm causes highly stressed,
high-strength steel to fail in only minutes. Small
pK
−
2−
+
concentrations
of HCO
H2S 3in
drilling fluids can reduce drill
CO
→
3 + H ←
pK
−
2−
+
CO
+
H
←


→
HCO
pipe
3 life by a factor of 3ten.
2−
−
CO3
HCO3
pK a 2
2−
−
COthe
HCO3 or packer fluid either from
pK
H2Sa 2can enter
completion
3
pK a1
−
+
HCO
+
H
←


→
H
CO
the reservoir
(along
with
3−
2
3CO2) or from decomposition of
pK a1
HCO
H + ←
→
H2CO3 used as corrosion inhibitors
3 +
sulfur
containing
additives
−
in halide
brines,
example
A number
HCOfor
H2thiocyanates.
CO3
pK
3−
a1
HCO
H2CO3 well equipment in
pK
of recent
failures3 of sub­-surface
a1
halide brines have been caused by H2S formed from
CO
g ) = CO2decomposition
( aq)
the2 (thermal
of sulfur-based corrosion
CO2 ( g ) = CO2 ( aq)
inhibitors [1][2].
a2
a2
CO2 ( aq)How
+ H2formates
O = H2 CO3scavenge
( aq)
B1.3.1
H2S
CO2 ( aq) + H2 O = H2 CO3 ( aq)
H2S is a very weak acid with pKa1 of about 7.
K a1
− aqueous
+ solution, the
When
H2 CO3 (introduced
aq) ←
→into
HCOan
3 (aq) + H ( aq)
K a1
−
+
following
is3created:
H2 CO3 (aqequilibrium
) ←
→ HCO
(aq) + H ( aq)
H2 S (g ) = H2 S (aq)
H2 S (g ) = H2 S (aq)
(6)
pK
a1
H2 S (aq) ←

→ HS − (aq) + H + ( aq) (7)
pK a 1
H2 S (aq) ←→ HS − (aq) + H + ( aq)
M + + SH −→ MHS(s)
PAGE 3
M + + SH −→ MHS(s) S E C T I O N B 1
O2 + 4H + + 4e− = 2H2 O
O2 + 4H + + 4e− = 2H2 O
FORMATE TECHNICAL MANUAL
C AB O T
Therefore, in an alkaline aqueous solution, the
dissolved H2S gas will largely exist as bisulfide (HS-).
1
2
Fortunately, formate brines used as completion fluids
are heavily buffered with soluble carbonate. This buffer 1
acts
like+an H
S scavenger
by converting any H2S that
pK2
−
2−
COsolubilized

HCO
3 + H ←
3
is
in→
the
brine
phase to HS-. In buffered
formate brines 2at− pH 9.5 – 10.5,
− the majority of any H2S 2
HCO
pK a 2 influx isCO
3
3
gas
converted
to
bisulfide
ions unless the
influx− brings
with
pK a1 it a large volume of CO2 that makes
+
HCO + H ←→ H2CO3
brine3 pH drop.
a2
−
7
HCObenefit,
H2CO3 brines do not require 3
pK aan
3
As
additional
formate
1
corrosion inhibitors of any kind, which negates a potent
man-made
source of hydrogen sulfide.
CO
2 ( g ) = CO2 ( aq)
4
B1.3.2 H2S scavengers in formates
The
CO2 (carbonate
aq) + H2 O /=bicarbonate
H2 CO3 ( aq) buffer that is added
5
to formate brines when they are used as well
construction fluids provides useful protection against
K a1
6
H22SCOcorrosion.

→ alkaline
HCO3− (aqpH
) +helps
H + ( aq
The
to) push the
3 (aq) ←
chemical equilibrium (Equation 7) towards the less
H2 S (g ) =bisulfide
H2 S (aq)(HS-). Additionally, in the alkaline pH
harmful
range, the large amount of alkali metal ions (Na+, K+, Cs+) 7
present in the
brine helps to scavenge the
pK a 1 formate
−
8
H2 S (aq) ←

→ HS
(aq) + H +reaction:
( aq)
bisulfide
through
the following
8
M + + SH −→ MHS(s)
3
4
5
6
(8)
where M = Na, K, or Cs.
9
O2 + 4H + + 4e− = 2H2 O
The capacity of the carbonate / bicarbonate buffer
is enormous (as demonstrated in Figure 1), and large
amounts of acid gas can be converted to HCO3- and
HS- before the pH starts dropping. However, if the
carbonate portion of the buffer is overwhelmed, its
scavenging effect is lost and H2S gas comes back
out of solution. It is unlikely that the buffer will be overwhelmed during field use, but to reduce the risk
associated with this, an H2S scavenger could be added.
The inclusion of H2S scavengers has additional benefits
over the use of buffer alone as scavengers tie up sulfide
long term rather than just changing the equilibrium.
Furthermore, use of an additional H2S scavenger also
removes bisulfide from formate brine.
Adding an H2S scavenger to buffered formate brine
therefore provides double protection against this very
harmful gas.
A zinc-free, iron-based H2S scavenger, Ironite Sponge®,
has been tested in formate brines, and is shown to
have some positive effects in scavenging H2S. However,
Ironite Sponge® is a solid, which limits its application in
clear completion brines.
PAGE 4
SECTION B1
9
Another iron-based scavenger, compatible with high
concentration formate brines, is iron gluconate [3], a
Fe(II) complex, which is water-soluble at high pH. In
addition to being
solids free, this scavenger has the
pK
−
2−
+
CO3 + Hbenefit
←of
→reacting
HCO3 very rapidly on a quantitative
added
basis with sulfide.
Three pounds
per barrel of iron
2−
−
CO3been tested
HCOin
pK a 2
3 buffered 2.2 g/cm3 /
gluconate
has
18.3 lb/gal
cesium
pK a1 formate brine (pH = 11). The added
−
HCO3 + H + ←
→ H CO
scavenger
was shown2 to3be compatible with the brine;
it dissolved completely
within five minutes without any
−
H2CO3
pK a1
change
in pH.HCO3
a2
A third
iron-based scavenger that may be compatible
CO
2 ( g ) = CO2 ( aq)
with formate brines is iron oxalate. Compatibility testing
still needs to be carried out with this scavenger.
CO2 ( aq) + H2 O = H2 CO3 ( aq)
Another group of zinc-free H2S scavengers, which
is expected to be compatible with formates are the
K a1
−
H2 CO3 (aq) ←organic

→ HCO
H + ( aq
) up sulfur in an
electrophilic
inhibitors
bind
3 (aq) + that
organic form. These have the advantage that they do
H2 S form
(g ) =any
H2 Ssolids
(aq) when reacting with H2S.
not
pK
B1.4
2 compatibility
H2 S (aq)O←
→ HS − (aq) + H + ( aq)
a1
Dissolved
oxygen from the atmosphere is present in all
M + + SH −→ MHS(s)
oilfield brines. Oxygen is a strong oxidizing agent, which
means that it is reduced in redox reactions according to:
O2 + 4H + + 4e− = 2H2 O
(9)
The strong oxidizing properties of oxygen makes this
gas corrosive to materials and commonly used organic
additives.
Concentrated formate brines have beneficial properties
that protect against damage caused by oxygen:
1. Low solubility of oxygen in formate brines:
The solubility of oxygen in low-salinity aqueous
solutions at surface temperature and pressure is
about 9 ppm. Solubility decreases in high-salinity
formate brines and at elevated temperatures [4],
as shown in Figure 2.
2. Formate brines are antioxidants:
Formate is an anti-oxidant and free radical scavenger.
As these are properties of the formate ion itself,
which are present in massive quantities in highdensity formate brines, they will never be depleted.
The product of this oxygen scavenging
is bicarbonate.
Halide brines have no anti-oxidant properties. Therefore
if oxygen is not removed from halide-based drilling
and completion fluids, the soluble oxygen can cause
serious problems with metals, elastomers, plastics,
VER S IO N
2
–
09 / 13
SECTION B: COMPATIBILITIES AND INTERACTIONS
C A B O T
Solubility of O2 in formate brines
10
9
8
Solubility [ppm]
7
6
5
4
Solubility
[ppm]
3
2
1
0
0
10
20
30
40
50
60
70
80
Formate concentration [% weight]
Figure 2 Solubility of oxygen in potassium formate at 21°C / 70°F.
References
and additives. For this reason, it is essential to add an
oxygen scavenger to halide brines. These scavengers
are generally quite effective until they become depleted
(consumed) or degraded, at which point further
contamination with oxygen could cause a problem. A
recent well tubular failure [5] was caused by oxygen
(air) ingress into a CaCl2 packer fluid during an annular
pressure bleed-off operation. In this instance, the
oxygen scavenger present in the brine was apparently
unable to cope with the new influx of oxygen.
B1.4.1 Use of O2 scavengers in formate brines
Due to their strong antioxidizing properties, it has never
been thought necessary to scavenge soluble oxygen
from concentrated formate brines. Furthermore, it isn’t
normal practice to add oxygen scavengers to formate
brines prior to field use.
There are, however, some indications that the addition
of fast-reacting antioxidants and oxygen scavengers
to high-concentration formate brines can contribute to
further polymer stabilization at high temperatures [6].
[1] Stevens, R. et al.: “Oilfield Environment-Induced
Stress Corrosion Cracking of CRAs in Completion
Brines”, SPE 90188, September 2004.
[2] Mack, R. et al.: “Stress Corrosion Cracking of a Cold
Worked 22Cr Duplex Stainless Steel Production Tubing
in a High Density Clear Brine CaCl2 Packer Fluid,”
NACE 02067, 2002.
[3] Davidson, E. and Hall, J.: “An EnvironmentallyFriendly,
Highly effective Hydrogen Sulphide Scavenger for
Drilling Fluids,” SPE 84313, October 2003.
[4] HYCOOL information sheet.
[5] Mowat, D.E.: “Erskine Field HPHT Workover and
Tubing Corrosion Failure Investigation,” SPE/IDAC 67779,
March 2001.
[6] Messler, D., Kippie, D., Broach, M.: “A Potassium
Formate Milling Fluid Breaks the 400° Fahrenheit Barrier
in Deep Tuscaloosa Coiled Tubing Clean-out,”
SPE 86503, Lafayette 2004.
Low-density formate brines, containing more water
than formate, may not offer the same protection
against oxygen as high-density brines, and will likely
benefit from the addition of an oxygen scavenger.
Sodium ascorbate and sodium erythorbate have been
proposed as effective oxygen scavengers in formate
brines.
V ERSION
2
–
0 9/ 13
SECTION B1
PAGE 5