Reactive Transport in Acidization and CO 2 Sequestration : An Experimental Investigation of Calcite Dissolution in Brine Branko Bijeljic, Weng-Hong Chong, Oussama Gharbi Stefan Iglauer and Martin Blunt

Introduction: CO 2 and Global Warming Increase in anthropogenic greenhouse gas (GHG) has profound effects on global warming  CO 2 is the most important anthropogenic GHG CO 2 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 CO 2 and storing it away from the atmosphere for a very long time (IPCC, 2005)  CO 2 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) CO 2 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 to increase K and  by dissolving rock constituents. Dissolution patterns in carbonate acidizing (Fredd and Fogler, 1999) Flowrate increases from 0.04cm 3 /min (a) to 60cm 3 /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 CO 2 sink Carbonate high reactivity may lead to changes in porosity, permeability and storage capacity during CO 2 injection There is a need to establish good understanding of mineral dissolution/precipitation for geological and reservoir model to simulate CO 2 movement and trapping (SPE ATW on CO 2 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 CO 2 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 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 ThermometerpH 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 Effect of Acidity:  The lower the initial solution pH, the more Ca 2+ is leached from the carbonate.  The amount of dissolved Ca 2+ tends to level off to 25ppm when pH is increased.  Dissolved Ca 2+ 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:  Grains with less surface area has less dissolved Ca 2+ 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.

Batch Experiments: Experimental Procedure Figure: Batch reactor with CO 2 injection system Batch Reactor with CO 2 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 Compressed CO 2 Flow Control Valve Basic Batch Reactor

Batch Experiments: CO 2 -Brine-Carbonate Results Effect of Acidity: Effect of Surface Area:  Brine saturated with CO 2 formed carbonic acid of pH ~4.  pH and dissolved Ca 2+ concentration stabilized rapidly (~20 min).  Indicates high carbonate reactivity towards acidic solutions. pH [Ca 2+ ]  More Ca 2+ is dissolved with increasing surface area.

Batch Experiments: CO 2 -Brine-Carbonate Results Effect of Temperature: pH [Ca 2+ ]  Initial pH of the solution increases with increasing temperature  This is due to CO 2 gas being less soluble at higher temperature.  Subsequently, less dissolved Ca 2+ with increasing temperature.

Batch Experiments: Comparisons Effect of Acidity Effect of Temperature Effect of Surface Area  Dissolved Ca 2+ stabilized later in CO 2 -equilibrated brine due to dissolution mechanism differences  CO 2 transformation into H 2 CO 3 is the rate-limiting step  Dissolved Ca 2+ in HCl- brine is more insensitive to temperature  Dissolution in CO 2 -brine strongly influenced by temperature-dependent CO 2 solubility  Increasing dissolved Ca 2+ concentration with increasing surface area is observed for both mixtures

Column Experiments: Experimental Procedure Pack column with crushed carbonate Saturate column Inject acidic brine (pH4) till breakthrough Stop injection and allow 15 minutes stabilization Section column into 8 equal parts Separate fluid from solids Take pH reading and prepare fluid sample End caps + wire mesh + filter papers Effluent Solution Pump Flow controller Pressure transducers Carbonate pack Back pressure valve

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

Scenario 1: Calcite Dissolution Column Experiments: H + Formation Assuming the CO 2 formed from calcite dissolution forms carbonic acid, the overall reaction is Scenario 2: H + Formation Section 1 Section 2Section 3 Acid Injection Point High H + Low Ca 2+ Formation 1 [H + ]1 Formation 2 Formation 3 [H + ]2 [Ca 2+ ]1 [Ca 2+ ]1+2 [H + ]3 [Ca 2+ ]1+2+3 [H + ]1< [H + ]2< [H + ]3 [Ca 2+ ]1< [Ca 2+ ]1+2< [Ca 2+ ]1+2+3 Formation 1< Formation 2< Formation 3 [Ca 2+ ]1 [Ca 2+ ]1+2 [Ca 2+ ]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, S or. 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

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Synthetic Brine Description Synthetic Brine Type HCl-BrineRock-equilibrated Low Salinity Brine Rock-equilibrated High Salinity Brine Salt Content5% NaCl + 1% KCl1% NaCl + 0.2% KCl5% NaCl + 1% KCl Elements Concentration (ppm) Na Na Na K K682.88K Ca4.15Ca32.90Ca29.40 S3.23S1.96S6.34 Si0.71Si6.51Si6.45 Mg0.55Mg0.66Mg1.86 Sr0.03Sr0.05Sr0.09 Ba0.02Ba0.11Ba0.04 FeTracesFe0.47Fe0.38 ZnTracesZnBDLZnBDL

Apparent Dissolution Rate, R To obtain the apparent dissolution rate, R of the reactive system., Change of Ca 2+ concentration in solution against time was plotted to obtain the derivative of concentration-time. The time derivative of Ca 2+ concentration was then corrected for solution volume, V carbonate total surface area, A

Apparent Solubility Product, K sp For calcite dissolution in HCl system, we have Assume CO 2 formed forms carbonic acid, the overall reaction is Therefore, calcite apparent solubility product is Since [Ca 2+ ] = [HCO 3 - ], we have

Apparent Solubility Product, K sp For calcite dissolution in carbonic acid system, we have Therefore, calcite apparent solubility product is Since 2[Ca 2+ ] = [HCO 3 - ] and [H 2 CO 3 ] = [H + ], we have

Effects of Acidity HCl-Brine-Carbonate Mixture Comparisons

Effects of Temperature HCl-Brine-Carbonate Mixture CO2-Brine-Carbonate Mixture pHpH Dissolution Rate, R (mol m -2 s -1 )log (Dissolution Rate) R 1 (25ºC) R 2 (40ºC)R 3 (60ºC) logR 1 (25ºC) logR 2 (40ºC) logR 3 (60ºC)

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