Predicting Mineral Transformations in Wet Supercritical CO2: The Critical Role of Water

Andrew R. FelmyEugene S. IltonAndre AnderkoKevin M. RossoJa Hun KwakJian Zhi Hu

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Outline

Background Importance of low water content CO2 solutions in geologic sequestrationMineral reaction mechanisms

Parameterization of the MSE model (OLI)Experimental Studies (PNNL)Comparison with the thermodynamic modeling

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Background: CO2 Disposed as an Anhydrous Supercritical Fluid can Become Water Saturated During or After Disposal in the Subsurface

This “wet” CO2 is the most likely to come into contact with the overlying caprock

Less denseHigh diffusivityLow viscosity

“Wet” CO2 also very reactive High effective PCO2Water chemical potential same as bulk water if saturated

Virtually unstudied“Half the story is missing” – McGrail et al. (2008)

Reaction mechanism different from aqueous solution

Mineral transformation rather than dissolution – no solvation energy

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Figure 1. Diagram showing a typical plume of injection dry CO2. Adapted from Nordbotten and Celia (2006).

Background: Water Interactions with Mineral SurfacesExamples:

Water in the scCO2 tends to strongly associate with the mineral surface and potentially condense to form a surface film which can result in mineral dissolution.

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As dry scCO2 is pumped through the subsurface the electrolytes present in the solution (brines) can become highly concentrated as water dissolves into the dry scCO2 creating highly reactive solutions.

Background: Thermodynamic Modeling

Thermodynamic modeling requirementsDifferent solvents (aqueous, supercritical CO2) Range of temperatures and pressures High electrolyte concentration possible

Subsurface brines and/or solution drying when reacting with anhydrous CO2

Mixed-solvent electrolyte (MSE) model specifically designed for such applications

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MSE Model Parameterization

Model parameterized for the aqueous carbonate system Na-K-Ca-Mg-Cl-HCO3-CO3-H2O systems, including all possible ternaries up to 300°C

Recently updated the database for the Mg-Cl-HCO3-CO2 system Extensive mineral database (geochem)CO2-water systems on both sides of the phase diagram (CO2-rich and H2O-rich) from 0 to 300°C and 300 atm pressure

No data for any components in the CO2 rich phase except for the water solubility at saturation

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MSE Model Parameterization (recent updates)

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MSE Model Parameterization (recent updates)

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MSE Model Parameterization (recent updates)

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Experimental Studies

Water equilibria in scCO2

Measurements of water contents in scCO2 in equilibrium with different electrolyte solutions as a function of T&P

Tests both the aqueous phase model (CO2- electrolyte interactions at high CO2) and the activity model for water in scCO2

Mineral reactivity in scCO2 as a function of water content.Divalent orthosilicates (Mg2SiO4, Ca2SiO4, …)Important in reservoir environments (e.g. basalts)Transformation reactions thermodynamically favorable

Form stable divalent carbonates (CaCO3, MgCO3, …)Reactions occur on a measurable time scale Initial studies focus on forsterite (Mg2SiO4)

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Near –IR measurements of water in scCO2(example calibration curve)

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Water Concentrations in scCO2

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T (°C) P (atm) Pure water (exp)

Pure water(calc)

Saturated CaCl2 (exp)

Saturated CaCl2 (calc)

40 90 0.046 (M) 0.071 (M) 0.017 (M) 0.011 (M)0.0042 (X) 0.0040 (X) 0.0016(X) 0.00062 (X)

M = moles/l X = mole fraction

Forsterite Studies

Supercritical scCO2 conditions (80°C, 75 atm)Variable water contents

Anhydrous scCO2

Aqueous solution (no CO2)scCO2 plus variable amounts of liquid water (excess of water saturation in scCO2) scCO2 plus variable water (below water saturation in scCO2)

Experimental probes29Si, 13C NMRSEM, TEM images of products and reactantsXPS of O, Si, and C on the surfaceTPD of H2O and CO2 release

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NMR Characterization of Reacted Solids in the Presence and Absence of Liquid Water

-84.8 (Q1)-91.8(Q2)

-150-100-50050ppm

-102(Q3)

-111.6(Q4)

×32

×32

×32

* *×32

Mg2SiO4 SSBs

scCO2no H2O

LiquidH2O Only

-61.9

Initial Forsterite

Liquid H2O + scCO2

No reaction observed

Reaction proceeds all the way to formation of amorphous silica (Q4)

Q bond notation Q0: no SiOSi, 4SiOHQ1: 1:SiOSi, 3 SiOHQ4: 4 SiOSi

Reaction proceeds only to formation of aqueous species (Q0, Q1) or surface hydroxylation (Q1, Q2)

(29Si MAS NMR Spectra (20 Hours Reaction Time)

13C NMR of Reacted Solids in scCO2 plus Liquid Water

150200ppm

164.7162.5

170ppm

dypingite standard(Mg5 (CO3)4 (OH)2 5H2O)

7 Day Reactionmagnesite only found

20 Hours Reactionmixed hydroxy carbonate formation + magnesite

13C MAS NMR (1g Mg2SiO4 + 1g H2O)

150200ppm

150200ppm

TEM Images from Different Spots to Highlight Phase Differences (4 days reaction time)

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Mg2SiO4

MgCO3

SiO2SiO

OMg

Partially reacted Amorphoussilica

Magnesite (d)

Conversion of Forsterite to Reaction Products

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0

10

20

30

40

50

60

70

80

0 50 100 150 200

Reaction time (h)

Con

vers

ion

(%)

67%

47%

8%

(1g H2O, 80°C, 75 atm)

Forsterite Reactivity Model (NMR results)

In the absence of water no reactivityInitial stages in the presence of water

Mg2SiO4 + H2O → 2Mg2+ + 4OH- + H4SiO4(aq)/H3SiO4-

Solution becomes basic in the absence of CO2

Reactivity with scCO2 (initial stages) 5Mg2SiO4 + 22H2O + 8CO2 → 2Mg5(CO3)4(OH)2.5H2O + 5H4SiO4

Indicates that the initial near surface could be slightly basicIntermediates consume significant waterUndersaturated with amorphous silica

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Kwak et al. 2010; J. Phys. Chem. C 114, 4126-4134

Forsterite Reactivity Model (NMR results)

Reactivity with scCO2 (later stages)H4SiO4 (aq) → SiO2(am) + 2H2O Mg5(CO3)4(OH)2.5H2O + CO2 → 5MgCO3 + 6H2OMg2+ + 2OH- + CO2 → MgCO3 + H2O

Liberates water from initially formed intermediatesCan enhance further reactivity

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Reactivity at Lower Water Content

15% added

74% added

149% added

-150-100-50050ppm

100% saturatedF

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3 4

1g M

g 2S

iO4

+ (H

2O/m

iner

al) %

H2O

+ sc

CO

2+8

0°C

for 4

day

s% water saturation in scCO2

45% added

-61.

5

-78.

8

-84.8-91.8

-102

-111

.6

0:Q0,1:Q1, 2:Q2, 3:Q3, 4:Q4, F:Mg2SiO4

At low water content reaction occurs but silica species have low Si-O-Si coordination

Exact structures not yet identified

Small amount of liquid water induces amorphous silica formation

1H – 29Si CP-MAS

0

Reactivity at Lower Water Contents

150200 160170180190

×16dypingite

Amorphous carbonate species

-164.1

-166

.4-170

.8

15% added

45% added

100%(in tube)

74% added

149% added

Low water contents carbonate surface species form but the NMR spectra cannot be resolved.

Small amount of initial water reaction intermediates form along with some magnesite

13C-SP-MAS Using 99% 13C CO2

Forsterite Reactivity as a Function of Time at Different Water Contents

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0 20 40 60

0

1

2

3

4

5

149% Initial Water Saturation (1%)

0 10 20 30 40 50

0

10

20

30

40

50371% Initial Water Saturation (2.5%)

At 175% initial saturation only a small amount of actual liquid water (4mg). This water is rapidly consumed and the reaction stops. If only slightly more liquid water initially present (19mg) reaction continues and at least 3 moles of forsterite react for every more of initial liquid water.

Thermodynamic Modeling (Phase equilibria)

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H2O added (g)

% Initial water saturation

H20 film (nm)

Phase equilibria(MSE)

Solid Phases(exp)

Solid Phases (MSE)

Solid Phases (MSE) – SiO2(c), Talc suppressed

1.0 14,900 994 Aq, V, S SiO2(am), MgCO3 MgCO3, Quartz MgCO3, SiO2(am)

0.5 7430 493 Aq V, S SiO2(am), MgCO3 MgCO3, Quartz MgCO3, SiO2(am)

0.1 1410 88 Aq, V, S SiO2(am), MgCO3 MgCO3, Quartz MgCO3, SiO2(am)

0.05 743 43 Aq, V, S SiO2(am), MgCO3, Q3 species

MgCO3, Quartz (V,S) MgCO3, SiO2(am), Mg5(OH)2(CO3)4

.

4H2O

0.01 149 3 V, S SiO2(am), MgCO3, Mg5(OH)2.4/5H2O, Q3 species

MgCO3, Quartz (V,S) MgCO3, SiO2(am)

0.005(6) 74 - V,S Carbonate (am), low coordinated Si

MgCO3, Quartz (V,S) MgCO3, SiO2(am)

0.003 45 - nc Carbonate (am), low coordinated Si

nc nc

0.001 15 - nc Carbonate (am), low coordinated Si

nc nc

Summary

Mineral reactivity in wet scCO2 is an important and largely uninvestigated issue in CO2 sequestration.The presence of a liquid water film and the nature of that film are critically important to mineral reactivity.Formation of reaction products is a critical aspect of long termreactivity in low water content environments.

Hydrated reaction products or reaction intermediates can consume water and limit reactivityAnhydrous reaction products (magnesite and amorphous silica) can result in the release or recycling of water and greatly enhance further reactivity

Thermodynamic models are needed to predict mineral reactivity inthese low water content environments.

MSE is ideally suited for such applications

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Scientific Progress

Identifying the nature and thickness of water film formation on mineral surfaces. Formation of HCO3

-/H2CO3in water films

in situ FTIR, in situ NMR

25 Wavenumber / cm-18501050125014501650185028003200

Abs

orba

nce

0.00

0.05

0.10

0.15

0.20

95% Saturation

Excess WaterInduced Liquid Water Film

55% Saturation

0% Saturation

3 hr6 hr9 hr12 hr15 hr18 hr21 hr24 hrWater Removed

T = 50°CP = 180 atm

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