Branko Bijeljic, Weng-Hong Chong , Oussama G harbi Stefan Iglauer a nd Martin Blunt

1. Reactive Transport in Acidization andCO2Sequestration :An Experimental Investigation of Calcite Dissolution in Brine Branko Bijeljic, Weng-Hong Chong, Oussama Gharbi Stefan Iglauer and Martin Blunt

2. 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) 77% of total GHG emissions Figure: Global anthropogenic GHG emissions (IPCC, 2007)

3. 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) • CO2 can be stored underground via physical and/or geochemical trapping • Geochemical trapping provides higher trapping security over time Figure: Trapping mechanisms and change of storage security over time (IPCC, 2005)

4. Introduction: Acidization • Increase productivity: • force acid into a carbonate • or sandstone in order to • increase Kand e by • dissolving rock constituents. Dissolution patterns in carbonate acidizing (Freddand Fogler, 1999) Flowrate increases from 0.04cm3/min (a) to 60cm3/min (e)

5. 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)

6. 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 (Golubevet al, 2005) • 1-2 pH units decrease was observed in brine reacted with supercritical CO2 which will affect chemical equilibria of the system (Kazsubaet 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)

7. Research Objectives To understand calcite dissolution in highly saline brine (5% NaCl+1%KCl) To delineate effects of acidity, temperature and surface area on calcite dissolution To investigate the coupled dynamics of calcite dissolution/precipitation and flow though porous medium

8. Sample Description: Rock and Synthetic Brine • 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

9. Batch Experiments: Experimental Procedure Basic Batch Reactor: Thermometer pH meter Fluid sampling point Acidic Brine- Carbonate Mixture Magnet Magnetic stirrer and hot plate Figure: Basic Batch Reactor Apparatus

10. Batch Experiments: HCl-Brine-Carbonate Results Effect of Temperature: 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. • Dissolved Ca2+ concentration shows NO appreciable change with temperature change for all solutions with different initial pH

11. Batch Experiments: HCl-Brine-Carbonate Results Effect of Surface Area: • 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.

12. Batch Experiments: Experimental Procedure • Batch Reactor with CO2 Injection : Flow Control Valve Injection tubing Flow meter Compressed CO2 Basic Batch Reactor Magnetic stirrer and hot plate Figure: Batch reactor with CO2 injection system

13. Batch Experiments: CO2-Brine-Carbonate Results Effect of Surface Area: Effect of Acidity: pH [Ca2+] • Brine saturated with CO2 formed carbonic acid of pH ~4. • pH and dissolved Ca2+ concentration stabilized rapidly (~20min). • Indicateshigh carbonate reactivity towards acidic solutions. • More Ca2+ is dissolved with increasing surface area.

14. Batch Experiments: CO2-Brine-Carbonate Results Effect of Temperature: • 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. pH [Ca2+]

15. Batch Experiments: Comparisons Effect of Surface Area Effect of Temperature Effect of Acidity • 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

16. Column Experiments: Experimental Procedure End caps + wire mesh + filter papers Back pressure valve Effluent Pump Carbonate pack Flow controller Solution Pressure transducers

17. Column Experiments: Results • 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

18. Column Experiments: Calcite Dissolution Assuming the CO2 formed from calcite dissolution forms carbonic acid, the overall reaction is [H+]1> [H+]2> [H+]3 Scenario 1: Calcite Dissolution Dissolution 1> Dissolution 2> Dissolution 3 [Ca2+]1< [Ca2+]1+2< [Ca2+]1+2+3 High H+ Low Ca2+ [H+]1 [H+]2 [H+]3 Dissolution 3 Dissolution 1 Dissolution 2 [Ca2+]1 [Ca2+]1+2+3 Acid Injection Point [Ca2+]1+2 Section 3 Section 2 Section 1

19. Column Experiments: H+ Formation Assuming the CO2 formed from calcite dissolution forms carbonic acid, the overall reaction is pH1> pH2> pH3 [H+]1< [H+]2< [H+]3 Scenario 2: H+ Formation Scenario 1: Calcite Dissolution Formation 1< Formation 2< Formation 3 [Ca2+]1< [Ca2+]1+2< [Ca2+]1+2+3 High H+ Low Ca2+ [H+]1 [H+]2 [H+]3 Formation1 Formation 3 Formation 2 [Ca2+]1 [Ca2+]1+2+3 [Ca2+]1+2 [Ca2+]1 [Ca2+]1+2+3 Acid Injection Point [Ca2+]1+2 Section 3 Section 2 Section 1

20. 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.

21. 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

22. MEAN FLOW DIRECTION X • Pore-scale CT experiments & simulation Geologically equivalent network Micro-CT images

23. Back Up

24. Synthetic Brine Description

25. Apparent Dissolution Rate, R • 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

26. Apparent Solubility Product, Ksp For calcite dissolution in HCl system, we have Assume CO2 formed forms carbonic acid, the overall reaction is Therefore, calcite apparent solubility product is Since [Ca2+] = [HCO3-], we have

27. Apparent Solubility Product, Ksp For calcite dissolution in carbonic acid system, we have Therefore, calcite apparent solubility product is Since 2[Ca2+] = [HCO3-] and [H2CO3] = [H+], we have

28. Effects of Acidity HCl-Brine-Carbonate Mixture Comparisons

29. Effects of Temperature CO2-Brine-Carbonate Mixture HCl-Brine-Carbonate Mixture

30. Effects of Surface Area CO2-Brine-Carbonate Mixture HCl-Brine-Carbonate Mixture