Reactive Transport in Acidization andCO2 Sequestration :

An Experimental Investigation of Calcite Dissolution in Brine

Branko Bijeljic, Weng-Hong Chong, Oussama Gharbi Stefan Iglauer and Martin Blunt

Introduction: CO2 and Global Warming

• Increase in anthropogenic greenhouse gas (GHG) has profound effects on global warming CO2 is the most important anthropogenic GHG

• CO2 from burning fossil fuel has effective lifetime of tens of thousand years (Archer,2005)

Figure: Global anthropogenic GHG emissions (IPCC, 2007)

77% of total GHG emissions

Introduction: CCS and Storage Security

• Carbon dioxide capture and storage (CCS) is the key emerging technology for anthropogenic GHG mitigation CCS involves capturing of CO2 and storing it away from the

atmosphere for a very long time (IPCC, 2005)

CO2 disposal in deep geological formations is the best option currently available (Bachu, 2000)

Figure: Trapping mechanisms and change of storage security over time (IPCC, 2005)

• CO2 can be stored underground via physical and/or geochemical trapping Geochemical trapping

provides higher trapping security over time

Introduction: Acidization

• Increase productivity: force acid into a carbonate or sandstone in order toincrease K and e by dissolving rock constituents.

Dissolution patterns in carbonate acidizing(Fredd and Fogler, 1999)Flowrate increases from 0.04cm3/min (a) to 60cm3/min (e)

Problem Definition: Importance of Calcite Dissolution

• Carbonate minerals are plentiful in sedimentary rocks and modern sediment (Morse et al, 2002) 60% of known petroleum reserves are located in carbonate

reservoirs (Morse et al, 1990)

High potential as CO2 sink

• Carbonate high reactivity may lead to changes in porosity, permeability and storage capacity during CO2 injection

• There is a need to establish good understanding of mineral dissolution/precipitation for geological and reservoir model to simulate CO2 movement and trapping (SPE ATW on CO2 sequestration, 2006)

Problem Definition: Calcite Dissolution in Brine

• Calcite behavior in highly saline solutions unclear Extensive work only in dilute solutions and seawater

• Acidity plays a key role in mineral dissolution pH of solution in contact with mineral surface is the major

controlling factor of dissolution (Golubev et al, 2005)

1-2 pH units decrease was observed in brine reacted with supercritical CO2 which will affect chemical equilibria of the system (Kazsuba et al, 2003)

• Will precipitation take place post dissolution? Few precipitation experiments were performed by other

researchers Supersaturation does occur in natural water system e.g. lower

Colorado River (USA) (Suarez, 1983)

Research Objectives

1. To understand calcite dissolution in highly saline brine (5% NaCl+1%KCl)

2. To delineate effects of acidity, temperature and surface area on calcite dissolution

3. To investigate the coupled dynamics of calcite dissolution/precipitation and flow though porous medium

Sample Description: Rock and Synthetic Brine

Rock Sample Guiting Limestone Cotswold LimestoneOrigin Guiting Quarry,

Gloucestershire, EnglandCotswold Hill Quarry,

Gloucestershire, EnglandAge Middle Jurassic Rock Group Inferior OoliteDepositional Environment Shallow water marinePorosity 28% 21%Liquid Permeability 2.5 mD 0.2 mDMineralogy Calcite: 90%; Quartz: 6%

Salt: 3%; Feldspar: TracesCalcite: 97%; Quartz: 2%Feldspar: Traces

Grain Size Fine

• Guiting and Cotswold Limestones were used

• Brine is made up of analytical reagent grade NaCl (5%) and KCl (1%) salt in 18.2 MΩ pure water Analytical grade HCl with specific gravity 1.18 is added when

required

Magnetic stirrer and hot plate

Acidic Brine-Carbonate Mixture

Thermometer pH meter

Magnet

Fluid sampling point

Batch Experiments: Experimental Procedure

Figure: Basic Batch Reactor Apparatus

Basic Batch Reactor:

Fill reactor with 400ml HCl-brine

Immerse 5g of crushed carbonate into solution

Agitate mixture

Take fluid samples and pH readings

Batch Experiments: HCl-Brine-Carbonate Results

0 1 2 3 4 50.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

pH

[Ca2

+] (m

ol/L

)

Effect of Acidity:

¨The lower the initial solution pH, the more Ca2+ is leached from the carbonate.

¨The amount of dissolved Ca2+ tends to level off to 25ppm when pH is increased.

20 30 40 50 600.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

pH 1 pH 2 pH 3 pH4Temperature ('C)

[Ca2

+] (m

ol/L

)¨Dissolved Ca2+ concentration

shows NO appreciable change with temperature change for all solutions with different initial pH

Effect of Temperature:

Batch Experiments: HCl-Brine-Carbonate Results

Effect of Surface Area:

1.0 1.5 2.0 2.5 3.0 3.5 4.04.0E-07

4.5E-07

5.0E-07

5.5E-07

6.0E-07

6.5E-07

7.0E-07

Surface Area (m2/g)

[Ca2

+] (m

ol/L

)

¨Grains with less surface area has less dissolved Ca2+ than grains with larger surface area

¨The smaller are the particles, the more exposed corners and edges where ions escape are available.

¨Ratio of is not constant indicates that

reaction surface area is NOT equal to total surface area.

[𝐶𝑎2+]𝑇𝑜𝑡𝑎𝑙 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝐴𝑟𝑒𝑎

Batch Experiments: Experimental Procedure

Figure: Batch reactor with CO2 injection system

Batch Reactor with CO2 Injection : Fill reactor with 400ml degassed brine

Inject CO2 at 300ml/min into brine

Immerse 5g of crushed carbonate into solution

Agitate mixture

Take fluid samples and pH readings

Injection tubing

Magnetic stirrer and hot plate

Flow meter

Com

pres

sed

CO2

Flow Control Valve

Basic Batch Reactor

Batch Experiments: CO2-Brine-Carbonate Results

Effect of Acidity: Effect of Surface Area:

0 10 20 30 40 50 603.5

4.0

4.5

5.0

5.5

6.0

0

100

200

300

400

500

Elapsed Time (min)

pH

Ca2

+ C

once

ntra

tion

(ppm

)

380ppm

pH5.8

pH3.8

¨Brine saturated with CO2 formed carbonic acid of pH ~4.

¨pH and dissolved Ca2+ concentration stabilized rapidly (~20 min).

¨ Indicates high carbonate reactivity towards acidic solutions.

pH

[Ca2+]

1.00 1.50 2.00 2.50 3.00 3.50 4.005.0E-06

6.0E-06

7.0E-06

8.0E-06

9.0E-06

1.0E-05

1.1E-05

Surface Area (m2/g)

[Ca2

+] (m

ol/L

)¨More Ca2+ is dissolved with

increasing surface area.

Batch Experiments: CO2-Brine-Carbonate Results

Effect of Temperature:

20 25 30 35 40 45 50 55 60 653.6

3.8

4.0

4.2

4.4

5.0E-06

7.0E-06

9.0E-06

1.1E-05

Temperature ('C)

Initi

al p

H

[Ca2+] (m

ol/L)

pH[Ca2+]

¨ Initial pH of the solution increases with increasing temperature

¨This is due to CO2 gas being less soluble at higher temperature.

¨Subsequently, less dissolved Ca2+ with increasing temperature.

Batch Experiments: Comparisons

Effect of Acidity Effect of Temperature Effect of Surface Area

0 50 100 150 200 250 300-1E-07

-5E-23

1E-07

2E-07

3E-07

CO2 Equilibrated Brine HCl Acidic BrineElapsed Time (s)

d[C

a2+]

/dt (

mol

/L.s

)

¨ Dissolved Ca2+ stabilized later in CO2-equilibrated brine due to dissolution mechanism differences

¨ CO2 transformation into H2CO3 is the rate-limiting step

¨ Dissolved Ca2+ in HCl-brine is more insensitive to temperature

¨ Dissolution in CO2-brine strongly influenced by temperature-dependent CO2 solubility

¨ Increasing dissolved Ca2+ concentration with increasing surface area is observed for both mixtures

25.375 39.9375 59.875-2E-06

5E-20

2E-06

4E-06

6E-06

8E-06

1E-05

CO2 Equilibrated Brine HCl-BrineTemperature ('C)

[Ca2

+] (m

ol/L

)

3.50 2.04 1.290.0E+00

2.0E-06

4.0E-06

6.0E-06

8.0E-06

1.0E-05

1.2E-05

CO2 Equilibrated Brine HCl-BrineSurface Area (m2/g)

[Ca2

+] (m

ol/L

)

Column Experiments: Experimental Procedure

Pack column with crushed carbonate Saturate column

Inject acidic brine (pH4) till breakthrough

End caps + wire mesh + filter papers

Effluent

Solution

Pump

Flow controller Pressure transducers

Carbonate pack

Back pressure valve

Column Experiments: Results

0 5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

7

8

9

0

20

40

60

80

100

120

Distance from Injection Point (cm)

pH

[Ca2

+] (p

pm)

pH column

[Ca2+] column

Dissolved Ca2+ concentration increases along the column

but gradually flattens towards the outlet.

Significant increase of pH near the inlet but gradual

decrease towards the outlet

Column Experiments: Calcite Dissolution

𝑪𝒂𝑪𝑶𝟑 + 𝐇+ ↔ 𝑪𝒂𝟐+ + 𝑯𝑪𝑶𝟑−

Assuming the CO2 formed from calcite dissolution forms carbonic acid, the overall reaction is

Scenario 1: Calcite Dissolution

Section 1 Section 2 Section 3Acid Injection

Point

High H+

Low Ca2+

Dissolution 1

[H+]1Dissolution

2Dissolution

3

[H+]2

[Ca2+]1 [Ca2+]1+2

[H+]3

[Ca2+]1+2+3

[Ca2+]1< [Ca2+]1+2< [Ca2+]1+2+3

Dissolution 1> Dissolution 2> Dissolution 3

[H+]1> [H+]2> [H+]3

Scenario 1: Calcite Dissolution

Column Experiments: H+ Formation

𝑪𝒂𝑪𝑶𝟑 + 𝐇+ ↔ 𝑪𝒂𝟐+ + 𝑯𝑪𝑶𝟑−

Assuming the CO2 formed from calcite dissolution forms carbonic acid, the overall reaction is

Scenario 2: H+ Formation

Section 1 Section 2 Section 3Acid Injection

Point

High H+

Low Ca2+ Formation1

[H+]1 Formation 2

Formation 3

[H+]2

[Ca2+]1 [Ca2+]1+2

[H+]3

[Ca2+]1+2+3

[H+]1< [H+]2< [H+]3

[Ca2+]1< [Ca2+]1+2< [Ca2+]1+2+3

Formation 1< Formation 2< Formation 3

[Ca2+]1 [Ca2+]1+2 [Ca2+]1+2+3

pH1> pH2> pH3

Conclusions• Dissolution increases with increasing acidity but tends to

stabilize at circumneutral pH• The temperature range under investigation (25-60ºC) shows a

weak effect on dissolution• An increase in total surface area increases the dissolution• The acidity of solution has more impact on dissolution than

surface area and temperature• For the column experiment, most significant dissolution occurs

near the inlet and the least near the outlet• pH values increase dramatically near the column inlet due to

high dissolution. The gradual decrease in pH along the column is due to the backward reaction (i.e. formation of H+) is favoured.

Recommendations

• Dissolution experiments using actual formation brine.• Dissolution experiments with other types of sedimentary

carbonate rocks, e.g. aragonite, dolomite.• Column experiments with different injected fluid pH,

flow rate, grain sizes, rock type and residence time.• Column experiments with carbonate pack with residual

oil saturation, Sor.• Coreflooding experiment at high pressure elevated

temperature conditions.• Pore scale CT scan experiments on acidization• Modeling advection/diffusion/reaction and with CTRW

based direct/network simulation

MEAN FLOW DIRECTION X

Micro-CT images

Geologically equivalent network

Pore-scale CT experiments & simulation

Back Up

Synthetic Brine Description

Synthetic Brine Type

HCl-Brine Rock-equilibrated Low Salinity Brine

Rock-equilibrated High Salinity Brine

Salt Content 5% NaCl + 1% KCl 1% NaCl + 0.2% KCl 5% NaCl + 1% KClElements Concentration (ppm)

Na 11025.31 Na 2827.49 Na 9431.57K 3423.23 K 682.88 K 3143.55

Ca 4.15 Ca 32.90 Ca 29.40S 3.23 S 1.96 S 6.34Si 0.71 Si 6.51 Si 6.45Mg 0.55 Mg 0.66 Mg 1.86Sr 0.03 Sr 0.05 Sr 0.09Ba 0.02 Ba 0.11 Ba 0.04Fe Traces Fe 0.47 Fe 0.38Zn Traces Zn BDL Zn BDL

Apparent Dissolution Rate, R

𝑅= 𝑉𝐴𝑑𝑐𝑑𝑡

• To obtain the apparent dissolution rate, R of the reactive system., • Change of Ca2+ concentration in solution against time was plotted to obtain

the derivative of concentration-time. • The time derivative of Ca2+ concentration was then corrected for

• solution volume, V • carbonate total surface area, A

Apparent Solubility Product, Ksp

𝐶𝑎𝐶𝑂3 + 2𝐻+ ↔ 𝐶𝑎2+ + 𝐶𝑂2 + 𝐻2𝑂 𝐶𝑂2 + 𝐻2𝑂 ↔ 𝐻2𝐶𝑂3 𝐶𝑎𝐶𝑂3 + 𝐻2𝐶𝑂3 ↔ 𝐶𝑎2+ + 2𝐻𝐶𝑂3−

For calcite dissolution in HCl system, we have

𝐶𝑎𝐶𝑂3 + H+ ↔ 𝐶𝑎2+ + 2𝐻𝐶𝑂3−

Assume CO2 formed forms carbonic acid, the overall reaction is

𝐾𝑠𝑝,𝐶𝑎𝐶𝑂3 = [𝐶𝑎2+]𝑇[𝐻𝐶𝑂3−]𝑇[H+]T Therefore, calcite apparent solubility product is

𝐾𝑠𝑝,𝐶𝑎𝐶𝑂3 = [𝐶𝑎2+]𝑇2[H+]T Since [Ca2+] = [HCO3-], we have

Apparent Solubility Product, Ksp

For calcite dissolution in carbonic acid system, we have

Therefore, calcite apparent solubility product is

𝐶𝑎𝐶𝑂3 + 𝐻2𝐶𝑂3 ↔ 𝐶𝑎2+ + 2𝐻𝐶𝑂3− 𝐻2𝐶𝑂3 ↔ 𝐻+ + 𝐻𝐶𝑂3−

𝐾𝑠𝑝,𝐶𝑎𝐶𝑂3 = [𝐶𝑎2+]𝑇[𝐻𝐶𝑂3−]𝑇2[H2CO3]T

𝐾𝑠𝑝,𝐶𝑎𝐶𝑂3 = 4[𝐶𝑎2+]𝑇3[H+]T Since 2[Ca2+] = [HCO3

-] and [H2CO3] = [H+], we have

Effects of Acidity

HCl-Brine-Carbonate Mixture

0 1 2 3 4 5

-9.0

-8.5

-8.0

-7.5

-7.0

pH

log

R

Comparisons

Effects of Temperature

HCl-Brine-Carbonate Mixture CO2-Brine-Carbonate Mixture pH

Dissolution Rate, R (mol m-2 s-1) log (Dissolution Rate)R1

(25ºC)R2 (40ºC) R3 (60ºC) logR1

(25ºC)logR2 (40ºC)

logR3 (60ºC)

1 4.4310-08 4.6210-08 4.7110-08 -7.35 -7.34 -7.332 6.4710-09 7.0410-09 7.6210-09 -8.19 -8.15 -8.123 1.6110-09 1.8010-09 1.7210-09 -8.79 -8.74 -8.764 1.2010-09 1.2910-09 1.4510-09 -8.92 -8.89 -8.84

20 25 30 35 40 45 50 55 60 65-4.0

-3.8

-3.6

-3.4

-3.2

-3.0

-2.8

pH 1 Linear (pH 1)pH 3 Linear (pH 3)pH 2 pH 4

Temperature ('C)

log

Ksp 20 25 30 35 40 45 50 55 60 65

-8.18-8.18-8.17-8.17-8.16-8.16-8.16-8.15-8.15-8.14-8.14-8.14-8.13-8.13

-9.2

-9.0

-8.8

-8.6

-8.4

Temperature ('C)

log

R

log Ksp

Effects of Surface Area

HCl-Brine-Carbonate Mixture CO2-Brine-Carbonate Mixture

1.00 1.50 2.00 2.50 3.00 3.50 4.00-9.40

-9.30

-9.20

-9.10

-9.00

-8.90

-3.4

-3.3

-3.2

-3.1

-3.0

-2.9

Surface Area (m2/g)

log

R

log Ksp

1.00 1.50 2.00 2.50 3.00 3.50 4.00-9.0

-8.8

-8.6

-8.4

-8.2

-8.0

-7.8

-9.8

-9.6

-9.4

-9.2

-9.0

-8.8

-8.6

-8.4

-8.2

-8.0

Surface Area (m2/g)

log

R

log Ksp