For Mate Manual A5 Crystallization Temperature

8/3/2019 For Mate Manual A5 Crystallization Temperature

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P A G E 1S E C T I O N A 5

Section A5

Crystallization Temperature

NOTICE AND DISCLAIMER. The data and conclusions contained herein are based on work believed to be reliable; however, CABOT cannotand does not guarantee that similar results and/or conclusions will be obtained by others. This inormation is provided as a convenience and orinormational purposes only. No guarantee or warranty as to this inormation, or any product to which it relates, is given or implied. CABOTDISCLAIMS 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 or, and CABOTdoes not accept and hereby disclaims liability or, any damages whatsoever in connection with the use o or reliance on this inormation or anyproduct to which it relates.

2011 Cabot Corporation, M.A.-U.S.A. All rights reserved. CABOT is a registered trademark o Cabot Corporation.

A5.1 Introduction ........................................................................................... 2

A5.2 TCT in ormates what does it mean and why is it so hard to get right? 2A5.3 Crystallization mechanisms in ormate brines ..................................... 2

A5.4 Procedure or TCT determination in ormate brines .............................3

A5.4.1 Selecting and preparing seeding material ......................................... 4

A5.4.2 TCT determination method ............................................................... 4

A5.5 TCT data or ormate brines ................................................................. 5

A5.5.1 TCT in single-salt sodium ormate ..................................................... 6

A5.5.2 TCT in single-salt potassium ormate ............................................... 6

A5.5.3 TCT in single-salt cesium ormates .................................................. 6

A5.5.4 TCT in blended ormate brines ......................................................... 6

A5.6 Pressurized crystallization temperature PCT ...................................... 15

A5.6.1 Introduction ...................................................................................... 15

A5.6.2 Methods or determining PCT in ormate brines ............................. 15

A5.6.3 PCT data or ormate brines ............................................................ 15

A5.7 How to apply TCT / PCT data in the feld ............................................. 18

A5.8 How to lower crystallization temperature o ormate brines ............. 18

A5.8.1 Lowering TCT in single-salt ormate brines ...................................... 18

A5.8.2 Lowering TCT in ormate brines .................................................... 18

Reerences .................................................................................................. 18

The Formate Technical Manual is continually updated.

To check if a newer version of this section exists please visit www.formatebrines.com/manual

CHEMICAL AND PHYSICAL PROPERTIES

F O R M A T E T E C H N I C A L M A N U A LC A B O T S P E C I A L T Y F L U I D S

V E R S I O N 3 0 4 / 1 1 P A G E 1S E C T I O N A 5


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P A G E 2 V E R S I O N 3 0 4 / 1 1

F O R M A T E T E C H N I C A L M A N U A L

S E C T I O N A 5

A5.1 Introduction

Crystallization temperature is an important property

o well construction and intervention uids that are

used in cold weather conditions and / or under high

pressure. True crystallization temperature (TCT) has

historically been used to defne the perormance

ceiling o oilfeld brines and uids. In traditional

oilfeld brines and uids there is typically only a small

dierence (saety margin) between the TCT as

measured in the laboratory and the uids perormance

ceiling in the feld. Formate brines behave very

dierently. With a massive supercooling eect and

the ormation o metastable crystals, the dierence

between TCT as measured in the laboratory and the

perormance ceiling in the feld is enormous, and it is

questionable whether TCT is suitable or measuring

the perormance ceiling o the uid.

Not only is true crystallization temperature (TCT) a

questionable measure o the perormance ceiling oormate brines, but it is also extremely difcult to

measure correctly. Measurements o TCTs in

ormate uids have, over the past years, become a

true stumbling block or many test laboratories.

An assortment o conicting TCT curves and mixing

tables exist in the industry today. For a concentrated

potassium ormate brine, or example, TCT data can

be ound to vary rom -18 to +7C / -4 to +45F.

A5.2TCT in ormates whatdoes it mean and why is itso hard to get right?

The most commonly used procedure or measuring

TCT in oilfeld brines is the API 13J method [1].

In this procedure a brine sample is cooled at a set

rate, oten with a generic seed crystal o barium

sulate, until the onset o precipitation. Once

precipitation starts, a small rise in temperature is

usually seen due to the exothermic nature o the

event. Ater precipitation has been noted the

sample is heated until all crystals have redissolved.

The data recorded rom this procedure includes:

FirstCrystaltoAppear(FCTA):Thetemperature

at which precipitation frst occurs. TrueCrystallizationTemperature(TCT):The

temperature at which the sample spontaneously

rises ater precipitation onset. This point is only

valid i there is less than a 1.5C / 3F dierence

between FCTA and TCT.

LastCrystaltoDissolve(LCTD).Thetemperature

at which no more crystals are present when the

sample is heated.

From a thermodynamic point o view, FCTA, TCT,

and LCTD should be the same. In practice, kinetic

considerations imposed by the method cause

discrepancies.

The API procedure is designed to be reproducible

with minimal training and allows or a rapid sample

throughput or traditional oilfeld brines (halides).

Unortunately, ormate brines were not considered

when this method was developed. The API 13J

guidelines are thereore currently being rewritten

by an API workgroup.

Two problems discovered with the current API

method when applied to ormate brines are

supercooling and ormation o metastable phase

crystals. The enormous amount o supercooling that

takes place in ormate brines makes it impossible

or most laboratories to even reach the low

temperatures required to orm the frst crystals.

TCTs o ormate brines are thereore requently

reported as too low to be measured. When a low

enough temperature is reached, measurements areoten complicated by the ormation o metastable

potassium ormate crystals. In order to produce

meaningul TCT values or ormate brines, it is

thereore crucial to thoroughly understand the

chemistry o these brines and their complex

crystallization behavior.

A5.3 Crystallization mechanismsin ormate brines

A typical TCT curve or a brine system is shown in

Figure 1. This is a phase diagram consisting o

three phase equilibrium lines, an eutectic point, and

a critical point. The phase equilibrium line to the let

represents the brines reezing point. At conditions

along this line, ice crystals are in equilibrium with

the brine. The eutectic point represents the brine

composition (concentration) that gives the lowest

possible TCT. The center equilibrium line represents

the concentration range o the brine where a

hydrated version o the salt crystallizes. Along this

equilibrium line the hydrated salt crystal exists in

equilibrium with the brine. The right equilibrium line

represents the concentration range where dry salt

crystals precipitate. Along this equilibrium line, dry

salt exists in equilibrium with the brine. It is difcultto measure TCT at or just around the eutectic or

critical points. Metastable crystals can orm along

the stippled lines (shown in Figure 1). However,

extrapolating the measured equilibrium lines, which

intersect at the eutectic and critical points, can

generate good TCT curves.

All three ormate brines exhibit very dierent TCT

behavior. Sodium ormate (shown in Figure 4)

behaves just like the salt in Figure 1. Its TCT curve has

one critical point, where hydrated and dry salt orm.


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S E C T I O N A : C H E M I C A L A N D P H Y S I C A L P R O P E R T I E S


The TCT curve o cesium ormate (Figure 6) has

no critical point, which means that only one

salt structure orms. Since cesium ormate is

known to exist as a monohydrate at ambient

conditions, this is the phase equilibrium line or

cesium ormate monohydrate in equilibrium with

cesium ormate brine. The critical point or the

cesium ormate TCT phase diagram is at a

temperature so high that it is impractical to measure.

Potassium ormate (Figure 5) exhibits very unusual

behavior as it precipitates crystals o two dierent

hydrated phases, here reerred to as metastable

and stable phases:

ThemetastablephasegivesalowTCT(around

-10C / 12F) or a concentrated (1.57 g/cm3 /

13.1 lb/gal) brine. This crystal phase is normally

crystallized spontaneously when a certain

degree o supercooling is reached or by seedingwith metastable potassium ormate crystals.

Thethermodynamicallystablephasegivesa

rather high TCT (around 7C / 19F) or a concen-

trated (1.57 g/cm3 / 13.1 lb/gal) brine. These

crystals orm rom the metastable phase crystals

ater some time (hours) in equilibrium with

saturated brine, or by seeding with stable

potassium ormate crystals.

The higher TCT, i.e. the one rom the stable phase,

is thermodynamically correct, and defned as the

scientifcally correct TCT. The TCT o the metastable

phase, on the other hand, oten contains more

useul inormation or many applications.

A5.4 Procedure or TCTdetermination in ormatebrines

With the standard API measuring method [1], the

supercooling eect can be overcome by using very

low cooling rates and cycling the temperature

between TCT and LCTD several times. Using low

cooling rates is not easible, however, when

determining TCT o uids with metastable phases.

A kinetic conversion may take place at any time,

potentially causing the measured TCT to graduallydrit rom the metastable phase TCT to the stable

phase TCT. In many cases, there is no knowledge

about which phase has been measured.

Consequently, Cabot Specialty Fluids (CSF) has

carried out extensive work on crystallization

behavior o ormate brines and on optimizing the

measuring method [2], [3], [4]. From this work, it

has been concluded that it is extremely important to

select the correct type o seeding material, and i

the correct seeding material is selected, the actual

Metastable phase TCT

Eutectic point

Water

Critical point

Ice

Salt-H2O

Salt

Typical TCT curve

Temperature

Brine concentration

Figure 1Typical TCT curve (or phase diagram) o brine, consisting o three phase equilibrium lines, an eutectic point and a critical point.

The phase equilibrium lines represent conditions where three dierent solid crystals exist in equilibrium with the brine. The eutectic point

represents the brine composition (concentration) with the lowest TCT, and the critical point is the point where the phase equilibrium lines

o the two dierent salt structures join.


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S E C T I O N A 5

TCT measurement becomes relatively simple, with

no need or very low cooling rates. By seeding with

crystals o the same kind that crystallize rom the

test brine, one can make consistent and good

measurements, which are not inuenced by

supercooling. For potassium ormate brines and

their blends with metastable phase TCT, the

collection and storage method or seeding crystals

determines which phase is measured.

A5.4.1 Selecting and preparing seeding

material

Selection o seeding material is critical or measuring

TCT in ormate brines. Both the problem o super-

cooling and those associated with metastable phases

orming in potassium ormate and potassium

ormate blends are overcome by selecting a

suitable seeding material.

The rule is simple: Use the same kind o seeding

crystals as those crystallizing rom the test brine.The crystals should be prepared by crystallization

(not drying) and kept in a reezer.

Seeding material should be selected according to

the ollowing guidelines:

Single-saltbrines: For any low-density brines

that precipitate to the let side o the eutectic

point (re. Figure 1), i.e. the part where water

reezes out, no special seeding material is

required. For higher density brines that precipitate

to the right o the eutectic point, where hydrated

or dry salts crystallize, seed with a stable crystal

o this salt (see Figure 3 to Figure 5). Use a

potassium ormate metastable crystal to measure

the metastable phase TCT o a potassium

ormate brine.

Blendedbrines(e.g. a cesium / potassium

ormate blend, Figure 6): For brines to the let o

the eutectic point, where potassium ormate

crystals precipitate frst, seed with a stable

potassium ormate crystal. Alternatively, use a

metastable potassium ormate crystal to measure

the blends metastable TCT. For higher density

brines to the right o the eutectic point, use a

cesium ormate seeding crystal.

Seeding crystals are easiest prepared in the reezer(set at -45C / -49F or lower) according to the

ollowing method:

Placeasampleofbrineinacleanplasticsample

bottle in the reezer.

Addseedingcrystalstothebrineatanytemperature

lower than 10C / 50F below its expected TCT.

For potassium ormate brines, be aware that

seeding with stable phase crystals gives stable

phase crystals and seeding with metastable

crystals gives metastable crystals. The seeding

crystal needs to be o the same type as those

required. Crystallization time varies rom a ew

seconds to a ew hours ater seeding depending

on the samples TCT.

Ifseedingcrystalsarepreparedforthersttime,

i.e. no seeding crystals are available or seeding;

they need to orm rom a brine that crystallizes

along the same phase equilibrium line (re. Figure 1),

but that has a high enough TCT to overcome

supercooling at reezer temperature. For potassium

ormate, whenever crystals are ormed rom such

a spontaneous crystallization process, these

crystals are always metastable. They can then be

converted to stable crystals by leaving them or

some time at higher temperature. This is tricky,

so always keep a sample in the reezer once

success is frst achieved.

Stablephasecrystalsremainstableforever.

Metastable (potassium ormate) crystals should

remain metastable as long as they are kept at low

enough temperature. However, always be

prepared or a situation where these can transormto stable phase crystals.

A5.4.2TCT determination method

The method used by CSF or determining TCT is

based on the API recommended method [1].

Beore TCT measurement can start, determine an

approximate TCT or the sample. This is easiest

completed using the inormation already available in

Figures 4 to 6. By knowing brine type and density,

TCT can be predicted with airly high accuracy.

CSF uses a Grant GR-150 cooling bath controlled by

LabwiseTM sotware. Attached to the bath is a

liquid-cooled sample cup with a stirrer. The test

brine is added directly to this cup. The ollowing

procedure should be ollowed to measure stable

phase TCT:

ProgramtheLabwiseTM temperature controller to

set the frst target temperature at approximately

8C / 14F below approximate TCT.

Whensampletemperatureisaround1C/2F

below its approximate TCT add a seeding crystal.

As soon as the frst crystals appear, note the

temperature (FCTA = First Crystal To Appear). At

this point, the stirring rate should be reduced and

temperature should increase by 1 3C / 2 4Fbeore alling again. The maximum temperature

obtained at this stage is the frst estimation o TCT.

Oncetemperaturestartstodecrease,adjustthe

cooling bath to 2C / 4F above the frst TCT

estimation and switch the cooling system o.

Observe the sample or the Last Crystal To

Dissolve (LCTD) and note temperature. I the

sample has become completely solid, temperature

should rise slowly in small increments until the

sample is completely crystal-ree.


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V E R S I O N 3 0 4 / 1 1



Oncethesampleiscompletelycrystalfree,repeat

this cooling / heating cycle two to three times.

Duringthemeasuringprocess,stirringspeed

should be kept high, but lowered to watch or

signs o FCTA and LCTD.

TCTcanbecalculatedasanaverageofthe

repeat measurements detailed above. Discrepancies

o around 1C / 2F between the results are

normal. However, i one result is signifcantly

lower, this should be rejected.

To measure metastable phase TCT in a potassium

ormate brine or a potassium ormate blend the

same method should be used, with the ollowing

exceptions:

Ametastablecrystalofthebrineitselfshouldbe

used or seeding.

Duringthetemperaturecyclingsteps,crystalscan

start converting to stable phase crystals at any

time. This is represented by a sudden increase in

TCT rom one cycle to another. TCT eventuallystabilizes again at the stable phase TCT. The

correct metastable phase TCT is calculated as an

average o TCTs measured beore this sudden

increase. How likely the brine is to convert to

stable phase during the temperature cycling

depends on the amount o heating applied in

each temperature cycle and brine type or

concentration. The activation energy required to

transorm crystals to the stable phase depends

on brine concentration (or single salt brines) and

brine composition (or blended salts). A typical

temperature plot showing a transition rom

metastable to stable phase potassium ormate

crystals is shown in Figure 2.

A5.5TCT data or ormatebrines

The TCT data presented in this section are a

combination o TCT data measured by CSF and

TCT data taken rom various other sources. All

curves have been verifed with CSFs recommended

test method. As CSFs method involves seeding

with crystals o the ormate brine itsel, these

curves are likely to be rather conservative (read

higher values) than what one could expect rom

other methods.

TCT or single-salt brines are shown in Figure 3 to

Figure 6 and Table 1 to Table 3. Figures 4, 5, and 6

also show some measured points representingsupercooling. This is the temperature where the

brine has successully been held or at least two

weeks in the presence o traditional seeding

material and other particles (barite, bentonite, rust,

dust, etc.) without crystallizing. With the use o less

sophisticated measuring techniques that do not use

crystals o the brine itsel or seeding, measured

TCT values can typically be ound anywhere in the

range between the supercooling points and the

TCT curve.

Crystallized potassium formate transferring from metastable to stable phase

Temperature

Time

Stable TCT

Metastable TCT

Figure 2Temperature as a unction o time during a TCT measuring test where the crystallized potassium ormate transers rom a

metastable phase crystal to a stable phase crystal.


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S E C T I O N A 5

A5.5.1 TCT in single-salt sodium ormate

TCT as a unction o uid density or a pure sodium

ormate single-salt brine is presented in Figure 4

and Table 1. The data represent a mixture o

measurements completed by Shell [5] and newer

measurements by Cabot Specialty Fluids and Baroid.

A5.5.2TCT in single-salt potassium

ormate

TCT as a unction o uid density or a pure

potassium ormate single-salt brine is presented in

Figure 5 and Table 2. Freezing point data (the

phase equilibrium line to the let o the eutectic

point) has been taken rom OSCA [6]. Stable and

metastable crystallization temperatures (the phase

equilibrium lines to the right o the eutectic point)

are all measured by CSF [3] according to the

method described above.

A5.5.3TCT in single-salt cesium ormate

TCT as a unction o uid density or a pure cesiumormate single-salt brine is presented in Figure 6

and Table 3. Freezing-point data (the phase

equilibrium line to the let o the eutectic point) has

been taken rom Shell [5]. Crystallization tempera-

tures (the phase equilibrium line to the right o the

eutectic point) are all measured by CSF [2][3]

according to the method described above.

A5.5.4TCT in blended ormate brines

When concentrated cesium ormate and concen-

trated potassium ormate are blended, the blends

TCT depends on the blend ratio. An eutectic point

(minimum TCT) is achieved at around 50 / 50 blend

ratio or, more exactly, at a density o 1.91 g/cm3 /

15.9 lb/gal (see Figure 7 or a blend o 1.57 g/cm3 /

13.1 lb/gal potassium ormate brine and 2.20 g/cm3 /

12.9 lb/gal cesium ormate brine). In winter, blends

are oten made rom more diluted potassium ormate

brine (1.54 g/cm3 / 12.9 lb/gal), which lowers TCT.

Some blends also retain a sae TCT even i water is

removed. The TCT curve in Figure 7 should

thereore not be used as a defnitive guide to TCTs

in blended cesium and potassium ormate blends

supplied by CSF.


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V E R S I O N 3 0 4 / 1 1



Figure 3 True Crystallization Temperature (TCT) or sodium, potassium, and cesium ormate brines.

-80

-60

-40

-20

0

20

40

60

80

100

8 9 10 11 12 13 14 15 16 17 18 19 20

NaFoKFo stable phaseKFo metastable phase

CsFo

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3

NaFo

KFo stable phaseKFo metastable phaseCsFo

TCT of formate brines

TCT of formate brines

Density [lb/gal]

Temperature[C]

Temperature[F]

METRIC

FIELD

Density (g/cm3)

METRIC


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S E C T I O N A 5

Figure 4 True Crystallization Temperature (TCT) or sodium ormate (single salt). The supercooling points represent the temperature

where the uid has been successully kept or at least two weeks in the presence o standard seeding material.

-20

-10

0

10

20

30

40

50

60

8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0 11.2

NaFo

NaFo supercooling

-25

-20

-15

-10

-5

0

5

10

15

20

1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35

NaFo

NaFo supercooling

Density [lb/gal]

Density (g/cm3)

Temperature[C]

Temperature[F]

METRIC

FIELD

TCT sodium formate single salt


FIELD


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Table 1True Crystallization Temperature (TCT) or sodium ormate single salt.

METRIC FIELD

Density TCT Density TCT

[g/cm3] [C] [lb/gal] [F]

1.00 0.0 8.34 32.0

1.01 -0.7 8.40 31.21.02 -1.6 8.50 29.4

1.03 -2.5 8.60 27.5

1.04 -3.4 8.70 25.5

1.05 -4.4 8.80 23.3

1.06 -5.4 8.90 21.0

1.07 -6.5 9.00 18.4

1.08 -7.8 9.10 15.5

1.09 -9.1 9.20 12.3

1.10 -10.6 9.30 8.8

1.11 -12.1 9.40 4.9

1.12 -13.9 9.50 0.6

1.13 -15.7 9.60 -4.2

1.14 -17.8 9.70 -5.9

1.15 -20.0 9.80 3.8

1.16 -22.5 9.90 12.5

1.17 -18.2 10.00 20.2

1.18 -13.9 10.10 26.9

1.19 -10.0 10.20 32.7

1.20 -6.4 10.30 37.4

1.21 -3.3 10.40 41.2

1.22 -0.5 10.50 44.0

1.23 1.9 10.60 45.7

1.24 3.9 10.70 47.6

1.25 5.5 10.80 54.2

1.26 6.7 10.90 60.0

1.27 7.6 11.00 65.2

1.28 8.0 11.10 69.8

1.29 10.7

1.30 13.6

1.31 16.2

1.32 18.61.33 20.7


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S E C T I O N A 5

Figure 5 True Crystallization Temperature (TCT) or potassium ormate (single salt). The stable phase TCT is measured by seeding with

stable phase potassium ormate crystals. The metastable phase TCT is measured by seeding with metastable phase potassium ormate

crystals. The supercooling points indicate the temperature where the uid has been successully kept or at least two weeks in the

presence o standard seeding material.

TCT potassium formate single salt

TCT stable phase

TCT stable phase extrapolated

TCT metastable phase

TCT metastable phase extrapolated

KFo supercooling

-80

-70

-50

-60

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0

-60

-50

-40

-30

-20

-10

0

10

20

30

1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65

TCT stable phase

TCT stable phase extrapolated

TCT metastable phase

TCT metastable phase extrapolated

KFo supercooling

Density [lb/gal]

Density [g/cm3]

Temperature[C]

Temperature[F]

METRIC

FIELD

TCT potassium formate single salt


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P A G E 1 1S E C T I O N A 5

Table 2True Crystallization Temperature (TCT) or potassium ormate single salt.

METRIC FIELD

DensityTCT

(stable)TCT (metastable) Density

TCT(stable)

TCT (metastable)

[g/cm3] [C] [C] [lb/gal] [F] [F]

1.00 0.0 8.34 30.8

1.01 -1.2 8.40 30.1

1.02 -1.7 8.50 28.9

1.03 -2.4 8.60 27.5

1.04 -3.1 8.70 25.9

1.05 -3.9 8.80 24.1

1.06 -4.8 8.90 22.1

1.07 -5.8 9.00 20.0

1.08 -6.8 9.10 17.7

1.09 -7.9 9.20 15.2

1.10 -9.1 9.30 12.5

1.11 -10.4 9.40 9.7

1.12 -11.7 9.50 6.7

1.13 -13.1 9.60 3.5

1.14 -14.6 9.70 0.2

1.15 -16.1 9.80 -3.3

1.16 -17.7 9.90 -6.9

1.17 -19.3 10.00 -10.7

1.18 -21.0 10.10 -14.71.19 -22.7 10.20 -18.8

1.20 -24.6 10.30 -23.0

1.21 -26.4 10.40 -27.4

1.22 -28.3 10.50 -31.9

1.23 -30.3 10.60 -36.6

1.24 -32.3 10.70 -41.3

1.25 -34.4 10.80 -46.3

1.26 -36.5 10.90 -51.3

1.27 -38.6 11.00 -56.5

1.28 -40.8 11.10 -61.8

1.29 -43.0 11.20 -67.2

1.30 -45.3 11.30 -58.6

1.31 -47.5 11.40 -49.4

1.32 -49.9 11.50 -40.7

1.33 -52.2 11.60 -32.31.34 -54.6 11.70 -24.3 -71.9

1.35 -52.1 11.80 -16.8 -63.0

1.36 -47.8 11.90 -9.6 -54.5

1.37 -43.6 12.00 -2.9 -46.4

1.38 -39.6 12.10 3.4 -38.8

1.39 -35.7 12.20 9.3 -31.6

1.40 -32.0 -58.6 12.30 14.8 -24.9

1.41 -28.5 -54.4 12.40 19.9 -18.6

1.42 -25.1 -50.3 12.50 24.6 -12.7

1.43 -21.8 -46.5 12.60 28.9 -7.3

1.44 -18.8 -42.8 12.70 32.8 -2.4

1.45 -15.9 -39.3 12.80 36.2 2.2

1.46 -13.1 -36.0 12.90 39.3 6.3

1.47 -10.5 -32.8 13.00 42.0 9.9

1.48 -8.1 -29.8 13.10 44.2 13.1

1.49 -5.8 -27.0 13.20 46.0 15.91.50 -3.7 -24.3 13.30 47.4 22.8

1.51 -1.7 -21.8 13.40 65.6 41.2

1.52 0.1 -19.5 13.50 89.3 59.7

1.53 1.8 -17.3

1.54 3.3 -15.4

1.55 4.6 -13.5

1.56 5.8 -11.9

1.57 6.8 -10.4

1.58 7.7 -9.1

1.59 8.4 -8.0

1.60 12.4 0.4

1.61 23.4 14.2


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S E C T I O N A 5

Figure 6True Crystallization Temperature (TCT) or cesium ormate (single salt). The supercooling points indicate the temperature where

the uid has been successully kept or at least two weeks in the presence o standard seeding material.

TCT cesium formate single salt

-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

8 9 10 11 12 13 14 15 16 17 18 19 20 21

TCT CsFo

TCT CsFo extrapolated

CsFo supercooling

-70

-60

-50

-40

-30

-20

-10

0

10

20

30

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5

TCT CsFo

TCT CsFo extrapolated

CsFo supercooling

Density [lb/gal]

Density [g/cm3]

Temperature[C]

Temperature[F]

METRIC

FIELD

TCT cesium formate single salt


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V E R S I O N 3 0 4 / 1 1



Table 3 True Crystallization Temperature (TCT) or cesium ormate single salt.

METRIC FIELD

Density TCT Density TCT

[g/cm3] [C] [lb/gal] [F]

1.00 0.0 8.34 32.0

1.05 -1.0 8.5 31.4

1.10 -2.3 9.0 29.01.15 -3.8 9.5 25.9

1.20 -5.5 10.0 22.2

1.25 -7.5 10.5 17.8

1.30 -9.8 11.0 12.8

1.35 -12.3 11.5 7.2

1.40 -15.0 12.0 1.0

1.45 -18.0 12.5 -5.9

1.50 -21.2 13.0 -13.4

1.55 -24.7 13.5 -21.6

1.60 -28.4 14.0 -30.4

1.65 -32.4 14.5 -39.8

1.70 -36.6 15.0 -49.8

1.75 -41.0 15.5 -60.5

1.80 -45.7 16.0 -71.8

1.85 -50.6 16.2 -70.41.90 -55.8 16.3 -61.9

1.92 -58.0 16.4 -53.9

1.94 -57.4 16.5 -46.2

1.96 -49.6 16.6 -38.9

1.98 -42.5 16.7 -32.0

2.00 -35.9 16.8 -25.5

2.02 -30.0 16.9 -19.3

2.04 -24.5 17.0 -13.5

2.06 -19.6 17.1 -8.0

2.08 -15.1 17.2 -2.8

2.10 -11.0 17.3 2.1

2.12 -7.2 17.4 6.8

2.14 -3.8 17.5 11.2

2.16 -0.7 17.6 15.3

2.18 2.2 17.7 19.3

2.20 4.9 17.8 23.02.22 7.4 17.9 26.5

2.24 9.9 18.0 29.9

2.26 12.2 18.1 33.1

2.28 14.5 18.2 36.2

2.30 16.9 18.3 39.1

2.32 19.2 18.4 41.9

2.34 21.7 18.5 44.7

2.36 24.3 18.6 47.3

2.38 27.1 18.7 49.9

2.40 30.1 18.8 52.4

18.9 55.0

19.0 57.5

19.1 60.0

19.2 62.5

19.3 65.019.4 67.6

19.5 70.3

19.6 73.1

19.7 75.9

19.8 78.9

19.9 82.0

20.0 85.2


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S E C T I O N A 5

Figure 7 True Crystallization Temperature (TCT) or a blend o 1.57 g/cm3 / 13.10 lb/gal potassium ormate and a 2.20 g/cm3 / 18.36 lb/gal

cesium ormate. The stable phase TCT is measured by seeding with stable phase potassium and cesium ormate crystals, whilst the

metastable phase TCT is measured by seeding with metastable potassium ormate crystals. Blends o potassium and cesium ormate

used in the feld can contain more or less water than this standard blend, so feld brine TCTs might be higher or lower than shown in

this plot.

12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 17.0 17.5 18.0 18.5 19.016.5

-30

-20

-10

0

10

20

30

40

50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.25

Density [lb/gal]

Density [g/cm3]

Temperature[C]

Temperature[F]

METRIC

FIELD

TCT and supercooling data for blended Cs and K formate

TCT and supercooling data for blended Cs and K formate

CsKFo stable phaseCsKFo metastable phaseCsFo

KFo stable phaseKFo metastable phaseCsKFo supercooling

CsKFo stable phaseCsKFo metastable phaseCsFoKFo stable phaseKFo metastable phase

CsKFo supercooling

-40


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V E R S I O N 3 0 4 / 1 1



A5.6 Pressurized crystallizationtemperature PCT

A5.6.1 Introduction

In deep-water environments, crystallization can

become a serious problem. High pressure and low

temperature can cause the salts in high-density brine

solutions to become more susceptible to crystallization.

Extremely high pressure and low temperatures are

normally encountered at the mud line and during

pressure testing o equipment. Pressure as high as

16,000 to 18,000 psi is not unusual. Thereore, it is o

great importance to know the crystallization

temperature or the uid at realistic pressure conditions.

There are two major problems associated with PCT

measurements in general. The frst problem is the lack

o a dependable standardized method. The second

problem is the poor availability o high-pressure

testing equipment to identiy the TCT under dynamic

conditions o pressure and temperature. For ormatebrines, with the additional difculties o extreme

supercooling and existence o metastable phases,

these measurements become extremely complicated.

A5.6.2 Methods or determining PCT in

ormate brines

PCT measurements o ormate brines have been

carried out at two test laboratories: Westport

Technology Center International and Baroid. The

test methods that have been used to determine

PCT in ormates are:

Westport Technology Center International:

acoustic method

Westport has chosen an acoustic method or

determination o PCT. This technique was chosen

due to serious limitations in standard determination

techniques, such as visual detection, temperature-

time-plot, and volume change. The equipment can

measure down to -30C / -22F. The pressure

range is rom 0.07 to 140 MPa / 10 to 20,000 psi,

and the sample volume is rom 5 to 350 mL. Both

the arrival time o the acoustic wave and attenuation

o the wave amplitude are unctions o the number

o solid particulate in the brine solution.

To ensure temperature and compositionhomogeneity, the cell is rocked back and orth,

which also helps reduce the supercooling eects.

The acoustic cell sits in a controlled temperature

chamber with a circulation system that provides

uniorm temperature distribution and cooling rates.

Baroid: Fiber optics technique

Baroid uses the ollowing methods or determining

crystallization:

Visual(beroptics)

Volumechange

Temperatureinectionpoint

The cell volume is 70 mL. The cell is equipped with a

stir disk. Testing starts at 10,000 psi and decreases

to the base line o 100 psi in 2,500 psi increments.

The test includes our cycles at each pressure

stage to check or supercooling eects. Each test

takes 16 to 21 hours with 0.05g o 5 micron marble

used as seeding agent.

A5.6.3 PCT data or ormate brines

By using the two measurement methods described

above some limited PCT values have been determined.

PCT data or a 2.195 g/cm3 / 18.3 lb/gal

cesium ormate brine with and without

0.5% KCl

TCT was measured as a unction o pressure or a

2.195 g/cm3 / 18.3 lb/gal buered cesium ormate

brine with and without the addition o 5% KCl.

KClwas added to reduce TCT. The tests were

completed at Westport Technology Center according

to its acoustic technique described above. Theresults are listed in Table 4 and plotted in Figure 8.

PCT data or various ormate brines

and blends

Baroid has carried out a number o PCT tests on

ormate uids by using their fber optics detection

technique mentioned above.

TCT as a unction o pressure (up to 20,000 psi)

was measured on buered, saturated cesium

ormate brine (2.18 g/cm3 / 18.18 lb/gal). The test

results are shown in Table 5 and plotted in Figure 8.

Similar tests were carried out on several buered

ormate brines: 1.32 g/cm3 / 11.01 lb/gal sodium

ormate, 1.58 g/cm3 / 13.17 lb/gal potassium

ormate, 2.18 g/cm3 / 18.18 lb/gal cesium ormate,

2.2 g/cm3 / 18.34 lb/gal cesium ormate, and 1.52

g/cm3 / 12.67 lb/gal potassium / cesium ormate.

From Figure 8 it can be seen that there is very

good consistency between the two methods o

PCT measurements. Also, by comparing the TCTs

when no pressure is applied, it is ound that the

TCTs measured with these instruments are similar

to the ones measured in standard TCT tests.

For cesium ormate and cesium / potassium

ormate blends, the ollowing rule o thumb applies:

Cs&Cs/K formate:

Increase in TCT ~ 1F per 1,000 psi

pressure increase.


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P A G E 1 6 V E R S I O N 3 0 4 / 1 1


S E C T I O N A 5

Pressure PCT 2.195 g/cm3 / 18.3 lb/gal CsFoPCT 2.195 g/cm3 / 18.3 lb/gal CsFo

+5%KCl

[MPa] [psi] [C] [F] [C] [F]

0.14 21 7.2 41.0 -4.7 23.6

20.7 3,000 -2.3 27.834.5 5,000 7.5 42.048.3 7,000 -1.9 28.668.9 10,000 10.2 50.4 -0.33 31.4

Table 4TCT as a unction o pressure or a buered 2.195 g/cm3 / 18.3 lb/gal cesium ormate brine with and without 5% KCl. Measured

at Westport Technology Center International.

Table 5TCT as a unction o pressure or a variety o ormate brines and blends. The measurements have been carried out by Baroid. The

1.52 g/cm3 / 12.7 lb/gal potassium / cesium ormate blend has been designed specifcally to lower TCT / PCT.

Pressure PCT

[MPa] [psi] [C] [F]

1.32 g/cm3 / 11.0 lb/gal

NaFo

0.69 100 10.39 61.217.24 2,500 12.78 62.334.47 5,000 12.33 63.051.71 7,500 16.28 63.5

68.95 10,000 17.06 65.5

1.58 g/cm3 / 13.2 lb/gal

KFo

0.69 100 -8.3 17.017.24 2,500 -4.3 24.234.47 5,000 -2.0 28.451.71 7,500 0.56 33.068.95 10,000 1.96 35.5

1.52 g/cm3 / 12.7 lb/gal

KCsFo

0.69 100 -6.03 21.217.24 2,500 -4.67 23.634.47 5,000 -3.42 25.951.71 7,500 -2.18 28.168.95 10,000 -2.40 27.7

2.20 g/cm3 / 18.3 lb/gal

CsFo buffered

0.69 100 5.3 41.517.24 2,500 6.7 44.034.47 5,000 8.2 46.751.71 7,500 9.6 49.368.95 10,000 11.2 52.2

2.18 g/cm3 / 18.2 lb/gal

CsFo

132.5 19,221 15.5 59.9117.8 17,086 13.9 57.1102.2 14,830 12.6 54.786.7 12,574 11.0 51.871.4 10,352 9.2 48.568.2 9,888 10.8 51.451.2 7,433 8.4 47.234.3 4,969 7.2 44.917.1 2,476 5.7 42.30.56 81 4.2 39.6


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V E R S I O N 3 0 4 / 1 1



Figure 8TCT as a unction o pressure or a variety o ormate brines and blends.

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

0 2 000 4 000 6 000 8 000 10 000 12 000 14 000 16 000 18 000 20 000

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

1416

18

20

22

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Pressure [psi]

Pressure [MPa]

TCT

[C]

TCT

[F]

METRIC

FIELD

PCT in various formate brines and blends

1.32 g/cm3 NaFo (Baroid)

2.20 g/cm3 CsFo-buffered (Baroid)

2.12 g/cm3 CsFo (Westport)

2.18 g/cm3 CsFo (Baroid)

2.20 g/cm3 CsFo + 5% KCl (Westport)

1.52 g/cm3 KCsFo (Baroid)

1.58 g/cm3 KFo (Baroid)

PCT in various formate brines and blends

11.0 lb/gal NaFo (Baroid)

18.3 lb/gal CsFo-buffered (Baroid)

18.3 lb/gal CsFo (Westport)

18.2 lb/gal CsFo (Baroid)

18.3 lb/gal CsFo + 5% KCl (Westport)

12.7 lb/gal KCsFo (Baroid)

13.2 lb/gal KFo (Baroid)


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S E C T I O N A 5

A5.7 How to apply TCT / PCTdata in the feld

Although the scientifcally correct uid TCT is the

thermodynamically stable one, i.e. the highest one

measured, this might not be the most suitable TCT

value to use when ormulating drilling and completion

uids. As ormates supercool more than other

brines, and potassium ormate brines and their

blends precipitate metastable phase crystals, it is

impossible to apply measured TCT data in the

same way as in other brines.

Storage o potassium ormate brine (and ormate

blends with a high potassium ormate content) in

tanks is a good example o this. In the absence o a

stable-phase potassium ormate seeding crystal, a

thermodynamically stable phase cannot be ormed

beore metastable crystals exist in the uid. Thereore,

this uid can be stored saely at temperatures

down to the TCT o the metastable phase, or evenlower due to supercooling. In periods when

temperatures outside the tank go beyond the TCT

o the metastable phase, metastable crystals may

orm locally at the sides o the tank, although the

bulk temperature in the uid inside the tank is

signifcantly above this temperature. The extent o

this crystallization is limited, but within a ew hours

the transormation to thermodynamically stable

crystals can occur. These stable phase crystals

serve as seeding crystals or thermodynamically

stable-phase crystals to orm in the whole storage

tank, assuming bulk temperature inside the tank is

below the TCT o the stable phase. Crystallization

will be substantial as the uid can be regarded as

heavily supersaturated with respect to this type o

crystallization. In order to dissolve these crystals

again, the temperature obviously needs to be

above the LCTD value o the stable phase, which is

signifcantly higher than the LCTD temperature o

the metastable phase.

Thereore, potassium ormates can saely be

stored down to the lower TCT o the metastable

phase, and even lower due to supercooling.

However, one should keep in mind that localized

cooling and crystallization at the wall o the

container can have drastic consequences or theextent o crystallization and the ability to dissolve

the crystals aterwards.

A5.8 How to lower crystallizationtemperature o ormatebrines

In some applications it might be desirable to lower

the crystallization temperature o ormate brines

and blends.

A5.8.1 Lowering TCT in single-salt

ormate brines

TCT can be lowered in single-salt ormate brines by

adding chloride ions. The lowering o TCT by adding

15% and 20% potassium chloride to a sodium

ormate brine has been demonstrated by Shell [5],

and is shown in Figure 9.

Care should be taken, however, when adding

chloride ions to a ormate brine. Chloride is known

to cause localized corrosion problems, and is hard

to remove.

A5.8.2 Lowering TCT in ormate blends

In certain deepwater applications there is need or

ormate brine with the typical density o single-salt

potassium ormate brine, but with lower TCT than

can be obtained by this single-salt brine alone. In

this case, a blended potassium / cesium ormate

brine can be ormulated with some additional water

added.

Reerences

[1] API RP 13J: Testing o Heavy Brines.

[2] Obi, A.S.: Measurements o True Crystallisation

Temperature in High Density Caesium Brines used

in Drilling Fluids, MSc Thesis, Robert Gordon

University, Aberdeen, September 2008.

[3] Chrenowski, M.: TCT Behaviour o Formate

Drilling and Completion Fluids, MSc Thesis, Robert

Gordon University, Aberdeen, September 2009.

[4] TCT Cesium Potassium Formate Blends, Lab

Report LR-406, Cabot Operations & Technical

Support Laboratory, Aberdeen, September 2010.

[5] Howard, S.K., Houben, R.J.H., Oort, E. van, and

Francis, P.A.: Formate drilling and completion uids

Technical Manual, Shell Report SIEP 96-5091, 1996.

[6] OSCA report: Crystallization Temperatures or

Potassium Formate Brines Formate Brine Project

Task #1, March 1995.


19/19


V E R S I O N 3 0 4 / 1 1



Figure 9TCT in single-salt sodium ormate brine. Eect o adding 15% and 20% KClto a sodium ormate brine [5].


-20

-10

0

10

20

30

40

50

60

8.2 8.4 8.6 8.8 9.0 9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0 11.2

NaFo

NaFo +15% KCl

NaFo +20% KCl


-30

-25

-20

-15

-10

-5

0

5

10

15

20

1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35

NaFo

NaFo +15% KCl

NaFo +20% KCl

Temperature[C]

Temperature[F]

METRIC

FIELD

Density [lb/gal]

Density [g/cm3]

For Mate Manual A5 Crystallization Temperature

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