Scale - NTNU

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Scale - NTNU

Scale

Precipitation of salts creating problems

NTNU 22.october 2012

Kristian Sandengen

Classification: Internal


Classification: Internal


Agenda

•Introduction

•What is scale?

•How is it formed?

–Typical mechanisms

–The three main types of scale

•Removal of scale

•Examples from real life..

•Summary

Classification: Internal


Conditions

change

Classification: Internal

Challenges

Wax

Asphaltenes

Scale

Hydrates

Emulsions

Corrosion


What is scale?

Classification: Internal

Scale is precipitation of inorganic

salts (minerals) in production

equipment.


Water production

•Virtually all oli/gas wells produce some water

–Condensed water due to high Temp in reservoir

–Formation water from aqueous zone

–Injected water that has reached producer

•Amount of water given as watercut:

–There are wells with 0-99% watercut.

Formasjonsvann Sjøvann

Ion

[mg/l]

[mg/l]

Na 14 800 10 680

K 520 396

Mg 13 1 279

Ca 378 409

Ba 410 8

Sr 228 0

Fe 58 0

Cl 23 600 19 220

SO4 0 2 689

Alkalinity 750 141

Classification: Internal

Perforated

Hydrocarbons

Water

WCUT


V

6

5

V

water

4

water

V

oil


What is Scale?

Scale is: Precipitation of solids

–Minerals dissolved in water

–Change the conditions such that they precipitate

•2 definitions

–Precipitation: When solids precipitate from a solution

Scale: When solids precipitates onto a surface

•Mechanisms for scale formation

–Mixing (typically sea- and formation water)

–Changes in P and T

• Causes changes in the salt solubility

• Secondary effects: water evaporates , CO 2 (g) removed from the water

Classification: Internal


Types of inorganic scale-forming salts

• Sulphates

– Barite (BaSO4 )

– Gypsum (CaSO4 ·2H2O) – Anhydrite (CaSO4 )

– Celestite (SrSO4 )

• “Exotic” scales

– Silicates (Si…)

– Halite (NaCl)

– ?

Not pH dependent

Classification: Internal

• Carbonates

– Calcite (CaCO 3 )

– Iron carbonate (FeCO 3 )

• Sulphides

– Iron sulphide (FeS)

pH dependent


Types of inorganic scale-forming salts

• Sulphates

– Barite (BaSO4 )

– Gypsum (CaSO4 ·2H2O) – Anhydrite (CaSO4 )

– Celestite (SrSO4 )

• “Exotic” scales

– Silicates (Si…)

– Halite (NaCl)

– ?

Not pH dependent

Classification: Internal

i.e. Can not be

removed by acids

• Carbonates

– Calcite (CaCO 3 )

– Iron carbonate (FeCO 3 )

• Sulphides

– Iron sulphide (FeS)

pH dependent

i.e. Can be removed

by acids


When do we get: Sulphates

•Main problem: MIXING!

•When formation water and seawater mix

–In reservoir : Normally not a problem

–In near well bore area : problem

–In well : problem

•A typical example is when seawater breaks through in one

zone

–Seawater and formation water mix in the well

–Sulphates precipitate fast

•Mixing of incompatible waters in the process system

Classification: Internal

Formation

water

Sea water


Formation mechanism: Sulphates

•Reaction:

•Sulphates form normally only in wells/fields with seawater injection

–Formation water normally contain; Ba 2+ , Sr 2+ and Ca 2+

–Seawater has high concentration of SO 4 2-

•Controlled by

–Concentration of scale forming ions i.e. mixing

–Ionic strength

–Temperature

•Not (or very little) dependent on

–Pressure

–Composition of gas and/or oil phase

Classification: Internal

2

2

Ba SO4

BaSO4

s


CaSO 4 and BaSO 4 vs temperature

CaSO 4 (mmol/kgH 2 O)

25

20

15

10

5

0

Classification: Internal

Gypsum

CaSO 4 BaSO4

Anhydrite

Marshall

Bock

Posnjak

Hall

Booth

Partridge

Calculated

0 50 100 150 200 250 300

Temperature ( o C)

0.02

0.015

0.01

0.005

Calculated solubility vs literature data

BaSO 4 (mmol/kgH 2 O)

0

Blount

Templeton

Linke

Calculated

0 50 100 150 200 250 300

Temperature (°C)


Types of inorganic scale-forming salts

• Sulphates

– Barite (BaSO4 )

– Gypsum (CaSO4 ·2H2O) – Anhydrite (CaSO4 )

– Celestite (SrSO4 )

• “Exotic” scales

– Silicates (Si…)

– Halite (NaCl)

– ?

Not pH dependent

Classification: Internal

• Carbonates

– Calcite (CaCO 3 )

– Iron carbonate (FeCO 3 )

• Sulphides

– Iron sulphide (FeS)

pH dependent


Formation mechanism: Carbonates

Changes in P and T

•Reaction:

•Carbonates can form in all systems containing CO 2 and scale forming

ions: Ca 2+ , Fe 2+ , Mg 2+ , Sr 2+ ,Ba 2+

•Depends on many parameters

– Concentration of scale forming ions

–Temperature

–CO 2 pressure

– pH/alkalinity

– Ionic strength

Classification: Internal

2

2

Ca CO3

CaCO3

s


The CO 2-Carbonate system

•CO 2 phase distribution

•CO 2 solubility in water

•CO 2 dissociation

– 1st

– 2nd

2

2

•Precipitation CO CaCO s P tot reduced

Temp increase

Classification: Internal

HCO

oil CO gas CO2 2

gas CO water CO2 2


aq H O HCO H

CO2 2

3


3

2


CO3

H

Ca 3

3

P CO2 =y CO2 P tot


When do we get: Carbonates

Changes in P and T

•Pressure reduction

–When pressure is reduced, CO 2 pressure is also reduced i.e. carbonates may

form

–Typically: Large draw down, choke, flash tank

•Temperature increase

–CaCO 3 has lower solubility at high temp

–Typically: Heat exchangers

•Mixing of different formation waters

–E.g. Injection of high pH fluid (e.g. H 2 S scavanger)

Classification: Internal


Scale theory at Åsgard

Down-hole mixing+temperature

T=144˚C

T=149˚C

T=154˚C

•Water from shallow zone – GARN

•Production from deeper zones

TOFTE/TILJE

•Warm fluid from TILJE/TOFTE

•Water is heated relative to GARN

temperature

Scale risk

*Vassenden, Classification: Internal F.; Gustavsen, O.; Nielsen, F.M.; Rian, M.; Haldoupsis, A.J.; SPE

94578, Int. symp. on oilfield scale, Aberdeen, 2005


Types of inorganic scale-forming salts

• Sulphates

– Barite (BaSO4 )

– Gypsum (CaSO4 ·2H2O) – Anhydrite (CaSO4 )

– Celestite (SrSO4 )

• “Exotic” scales

– Silicates (Si…)

– Halite (NaCl)

– ?

Not pH dependent

Classification: Internal

• Carbonates

– Calcite (CaCO 3 )

– Iron carbonate (FeCO 3 )

• Sulphides

– Iron sulphide (FeS)

pH dependent


Large Changes in P and T

water evaporation!!

•Example of Fields

–Kristin

–Morvin (Åsgard Tie-in)

•Proposed* that the inflow

from reservoir to well can be

modelled as an isenthalpic

process (constant H)

– Joule-Thomson effect

*Vassenden, F.; Gustavsen, O.; Nielsen, F.M.; Rian, M.; Haldoupsis, A.J.;

SPE 94578, Int. symp. on oilfield scale, Aberdeen, 2005

Classification: Internal

Temperature change [C]

6

4

2

0

-2

-4

-6

-8

-10

Temperature change

free water

200 300 400 500 600 700 800 900

Pressure [bar]

1.2

1

0.8

0.6

0.4

0.2

0

water volume [relative]

Multiscale 7.0 calculation


How to avoid scale?

•1. Avoid scale forming conditions

•If 1) is impossible use scale inhibitors:

–Chemicals that attaches to the mineral and inhibits further growth

• «push away» ions so that they cannot come to the mineral surface

Classification: Internal


Examples (on the blackboard)

•Kristin field

–High drawdown in a HPHT field

•Åsgard water treatment

Scale in line to caisson

Classification: Internal


What have we learned?

Classification: Internal

Mechanism

Type of scale P T mixing

Sulphate

Typically BaSO 4

Carbonates

Typically CaCO 3

Chlorides

Typically NaCl

No

Could

be

YES!!

YES!! Yes Yes Acid

YES

(large dP)

Evaporation

Could

be

No Water

Removal Characteristic

EDTA chemical

(poor efficiency)

Mechanical!!

Low solubility

Often radioactive

due to RaSO 4

Highly soluble!

Could be radioactive

Contains all sorts of

scale, not just NaCl.


Classification: Internal


CaCO 3 vs temperature and P CO2

CaCO 3 (mmole/kgH2O)

20

15

10

5

0

Classification: Internal

75°C

100°C

125°C

150°C

200°C

0 10 20 30 40 50 60 70 80

CO pressure (bar)

2

Calculated solubility vs literature data

CaCO 3 (mmole/kgH2O)

8

7

6

5

4

3

2

1

0

1 atm

4 atm

12 atm

62 atm

Calculated

100 150 200

Temperature (°C)

250 300


CaCO 3 vs Ionic strength

CaCO 3 (mmole/kgH2O)

5

4

3

2

1

0

Classification: Internal

10°C

25°C

60°C

0 1 2 3 4 5 6

NaCl (mole/kgH2O)

CaCO 3 (mmole/kgH2O)

Calculated solubility vs literature data

5

4

3

2

1

0

10°C

25°C

60°C

0 1 2 3 4 5 6

KCl (mole/kgH2O)

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