Predicting Mineral Transformations in Wet Supercritical CO 2 : The Critical Role of Water Andrew R. Felmy Eugene S. Ilton Andre Anderko Kevin M. Rosso Ja Hun Kwak Jian Zhi Hu 1
Outline Background Importance of low water content CO 2 solutions in geologic sequestration Mineral reaction mechanisms Parameterization of the MSE model (OLI) Experimental Studies (PNNL) Comparison with the thermodynamic modeling 2
Background: CO 2 Disposed as an Anhydrous Supercritical Fluid can Become Water Saturated During or After Disposal in the Subsurface 3 This wet CO 2 is the most likely to come into contact with the overlying caprock Less dense High diffusivity Low viscosity Wet CO 2 also very reactive High effective P CO2 Water 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 Figure 1. Diagram showing a typical plume of injection dry CO 2. Adapted from Nordbotten and Celia (2006).
Background: Water Interactions with Mineral Surfaces Examples: Water in the scco 2 tends to strongly associate with the mineral surface and potentially condense to form a surface film which can result in mineral dissolution. As dry scco 2 is pumped through the subsurface the electrolytes present in the solution (brines) can become highly concentrated as water dissolves into the dry scco 2 creating highly reactive solutions. 4
Background: Thermodynamic Modeling Thermodynamic modeling requirements Different solvents (aqueous, supercritical CO 2 ) Range of temperatures and pressures High electrolyte concentration possible Subsurface brines and/or solution drying when reacting with anhydrous CO 2 Mixed-solvent electrolyte (MSE) model specifically designed for such applications 5
MSE Model Parameterization Model parameterized for the aqueous carbonate system Na-K-Ca- Mg-Cl-HCO 3 -CO 3 -H 2 O systems, including all possible ternaries up to 300 C Recently updated the database for the Mg-Cl-HCO 3 -CO 2 system Extensive mineral database (geochem) CO 2 -water systems on both sides of the phase diagram (CO 2 -rich and H 2 O-rich) from 0 to 300 C and 300 atm pressure No data for any components in the CO 2 rich phase except for the water solubility at saturation 6
7 MSE Model Parameterization (recent updates)
8 MSE Model Parameterization (recent updates)
9 MSE Model Parameterization (recent updates)
Experimental Studies Water equilibria in scco 2 Measurements of water contents in scco 2 in equilibrium with different electrolyte solutions as a function of T&P Tests both the aqueous phase model (CO 2 -electrolyte interactions at high CO 2 ) and the activity model for water in scco 2 Mineral reactivity in scco 2 as a function of water content. Divalent orthosilicates (Mg 2 SiO 4, Ca 2 SiO 4, ) Important in reservoir environments (e.g. basalts) Transformation reactions thermodynamically favorable Form stable divalent carbonates (CaCO 3, MgCO 3, ) Reactions occur on a measurable time scale Initial studies focus on forsterite (Mg 2 SiO 4 ) 10
11 Near IR measurements of water in scco 2 (example calibration curve)
Water Concentrations in scco 2 T ( C) P (atm) Pure water (exp) Pure water (calc) Saturated CaCl 2 (exp) Saturated CaCl 2 (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 12
Forsterite Studies Supercritical scco 2 conditions (80 C, 75 atm) Variable water contents Anhydrous scco 2 Aqueous solution (no CO 2 ) scco 2 plus variable amounts of liquid water (excess of water saturation in scco 2 ) scco 2 plus variable water (below water saturation in scco 2 ) Experimental probes 29 Si, 13 C NMR SEM, TEM images of products and reactants XPS of O, Si, and C on the surface TPD of H 2 O and CO 2 release 13
NMR Characterization of Reacted Solids in the Presence and Absence of Liquid Water ( 29 Si MAS NMR Spectra (20 Hours Reaction Time) Initial Forsterite SSBs * -61.9 * Mg 2 SiO 4 32 Q bond notation Q0: no SiOSi, 4SiOH Q1: 1:SiOSi, 3 SiOH Q4: 4 SiOSi scco 2 no H 2 O Liquid H 2 O + scco 2-102 (Q3) -111.6 (Q4) 32 32 No reaction observed Reaction proceeds all the way to formation of amorphous silica (Q4) Liquid H 2 O Only 50 0-50 ppm -84.8 (Q1) -91.8 (Q2) -100-150 32 Reaction proceeds only to formation of aqueous species (Q0, Q1) or surface hydroxylation (Q1, Q2)
13 C NMR of Reacted Solids in scco 2 plus Liquid Water 13 C MAS NMR (1g Mg 2 SiO 4 + 1g H 2 O) 164.7 162.5 20 Hours Reaction mixed hydroxy carbonate formation + magnesite 200 170ppm ppm 150 7 Day Reaction magnesite only found 200 ppm 150 dypingite standard (Mg 5 (CO 3 ) 4 (OH) 2 5H 2 O) 200 ppm 150
TEM Images from Different Spots to Highlight Phase Differences (4 days reaction time) Partially reacted Mg 2 SiO 4 Amorphous silica Magnesite O Si SiO 2 (d) O Mg MgCO 3 16
Conversion of Forsterite to Reaction Products Conversion (%) 80 70 60 50 40 30 20 10 0 (1g H 2 O, 80 C, 75 atm) 67% 47% 8% 0 50 100 150 200 Reaction time (h) 17
Forsterite Reactivity Model (NMR results) In the absence of water no reactivity Initial stages in the presence of water Mg 2 SiO 4 + H 2 O 2Mg 2+ + 4OH - + H 4 SiO 4 (aq)/h 3 SiO 4 - Solution becomes basic in the absence of CO 2 Reactivity with scco 2 (initial stages) 5Mg 2 SiO 4 + 22H 2 O + 8CO 2 2Mg 5 (CO 3 ) 4 (OH) 2.5H 2 O + 5H 4 SiO 4 Indicates that the initial near surface could be slightly basic Intermediates consume significant water Undersaturated with amorphous silica Kwak et al. 2010; J. Phys. Chem. C 114, 4126-4134 18
Forsterite Reactivity Model (NMR results) Reactivity with scco 2 (later stages) H 4 SiO 4 (aq) SiO 2 (am) + 2H 2 O Mg 5 (CO 3 ) 4 (OH) 2.5H 2 O + CO 2 5MgCO 3 + 6H 2 O Mg 2+ + 2OH - + CO 2 MgCO 3 + H 2 O Liberates water from initially formed intermediates Can enhance further reactivity 19
Reactivity at Lower Water Content 1 H 29 Si CP-MAS % water saturation in scco 2 1g Mg 2 SiO 4 + (H 2 O/mineral) % H 2 O + scco 2 +80 C for 4 days 15% added 45% added 74% added 100% saturated 149% added 50 0-61.5 F -50 ppm -78.8 0 1 3 4 2-102 -111.6-100 -84.8-91.8-150 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 0:Q0,1:Q1, 2:Q2, 3:Q3, 4:Q4, F:Mg 2 SiO 4
Reactivity at Lower Water Contents 13 C-SP-MAS Using 99% 13 C CO 2 Amorphous carbonate species 15% added 45% added 74% added 100% (in tube) 149% added -170.8-166.4 200 190 180 170 160-164.1 dypingite 16 150 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
Forsterite Reactivity as a Function of Time at Different Water Contents 5 4 3 2 1 0 149% Initial Water Saturation (1%) 50 40 30 20 10 0 371% Initial Water Saturation (2.5%) 0 20 40 60 0 10 20 30 40 50 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. 22
Thermodynamic Modeling (Phase equilibria) H 2 O added (g) % Initial water saturation H 2 0 film (nm) Phase equilibria (MSE) Solid Phases (exp) Solid Phases (MSE) Solid Phases (MSE) SiO 2 (c), Talc suppressed 1.0 14,900 994 Aq, V, S SiO 2 (am), MgCO 3 MgCO 3, Quartz MgCO 3, SiO 2 (am) 0.5 7430 493 Aq V, S SiO 2 (am), MgCO 3 MgCO 3, Quartz MgCO 3, SiO 2 (am) 0.1 1410 88 Aq, V, S SiO 2 (am), MgCO 3 MgCO 3, Quartz MgCO 3, SiO 2 (am) 0.05 743 43 Aq, V, S SiO 2 (am), MgCO 3, Q3 species 0.01 149 3 V, S SiO 2 (am), MgCO 3, Mg 5 (OH) 2.4/5H 2 O, Q3 species 0.005(6) 74 - V,S Carbonate (am), low coordinated Si MgCO 3, Quartz (V,S) MgCO 3, SiO 2 (am), Mg 5 (OH) 2 (CO 3 ) 4. 4H 2 O MgCO 3, Quartz (V,S) MgCO 3, SiO 2 (am) MgCO 3, Quartz (V,S) MgCO 3, SiO 2 (am) 0.003 45 - nc Carbonate (am), low coordinated Si 0.001 15 - nc Carbonate (am), low coordinated Si nc nc nc nc 23
Summary Mineral reactivity in wet scco 2 is an important and largely uninvestigated issue in CO 2 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 term reactivity in low water content environments. Hydrated reaction products or reaction intermediates can consume water and limit reactivity Anhydrous 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 in these low water content environments. MSE is ideally suited for such applications 24
Scientific Progress Identifying the nature and thickness of water film formation on mineral surfaces. Formation of HCO 3- /H 2 CO 3 in water films 0.20 0.15 T = 50 C P = 180 atm Excess Water Induced Liquid Water Film 3 hr 6 hr 9 hr 12 hr 15 hr 18 hr 21 hr 24 hr Water Removed in situ FTIR, in situ NMR Absorbance 0.10 0.05 95% Saturation 55% Saturation 0.00 0% Saturation 32002800 1850 1650 1450 1250 1050 850 25 Wavenumber / cm -1
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