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Using Groundwater Geochemistry for Leak Detection at Carbon
Sequestration SitesChangbing Yang & Katherine Romanak
Rebecca Smyth & Susan Hovorka
Gulf Coast Carbon CenterBureau of Economic Geology University of Texas at Austin
Collaborators and Sponsors
www.gulfcoastcarbon.org
Shallow GroundwaterMonitoring
Demonstrate that CO2has been stored with respect to the near-surface.
Monitor resources that need protection.
Courtesy of Curt Oldenburg
Biosphere
Geochemical Monitoring of Shallow Aquifers
Parameters sensitive enough to provide a geochemical shift larger than background variability.
Parameters that can be globally applied to any aquifer type to avoid costly hydrochemical characterization studies.
+
Predictions
CO2 dissolution into groundwaterDecrease in pH Carbonate mineral dissolution
Potential mobilization of trace metals (Fe, Mn, Pb, Zn, U, As, Cd, Sr,…)
Monitoring parameters: trace metals, Ca2+, HCO3
-, pH, DIC, carbon isotopes
H2O + CaCO3 + CO2 → 2HCO3- + Ca2+
-16 -12 -8 -4 0 4 8 12 16Time (days)
2
4
6
8
10
pH
DockumOgallalaHickoryCranfield
-16 -12 -8 -4 0 4 8 12 16Time (days)
0
1
2
3
4
5
6
7
Ca
(mm
ol/L
)
DockumOgallalaCranfieldHickory
Water-Rock-CO2Interaction Experiments
pH Ca
Trace metals of different rock samples show different and very complicated behaviors.
-16 -12 -8 -4 0 4 8 12 16Time (days)
10-1
100
101
102
As (
ppb)
DockumOgallalaHickoryCranfield
EPA DWS
-16 -12 -8 -4 0 4 8 12 16Time (days)
10-1
100
101
102
103
104
Fe (p
pb)
DockumOgallalaHickoryCranfield
EPA DWS
As Fe
Water-Rock-CO2Interaction Experiments
~150 MMt CO2 injected~75 MMt CO2 recovered
SACROC Oilfield, West Texas
Injection since 1972
Field Cases: CO2-EOR Oilfields
LegendHistoric production well
Injection well
Groundwater well
LegendHistoric production well
Injection well
Groundwater well
Cranfield Oilfield, Mississippi
Injection since 2008
~ 2 MMt CO2 injected~ 100 Historic 1950’s-era P&A wells
•No obvious change in pH, HCO3
- or metals over time•No damage to shallow groundwater from prolonged CO2-EOR.
Field Case: SACROC Oilfield
0.0001
0.0010
0.0100
0.1000
1.0000
10.0000
1980 1990 2000 201years
Conc
netra
tion
(mg/
L)
Mn
5
5.5
6
6.5
7
7.5
8
6/1/08 12/18/08 7/6/09 1/22/10 8/10/10 2/26/11Date
pH
0
50
100
150
200
6/1/08 12/18/08 7/6/09 1/22/10 8/10/10 2/26/11Date
Alk
alin
ity (m
g/L)
pH Alkalinity
Field Case: Cranfield Oilfield
0
0.2
0.4
0.6
0.8
6/1/08 12/18/08 7/6/09 1/22/10 8/10/10 2/26/11Date
Mn
0
0.5
1
1.5
2
2.5
3
6/1/08 12/18/08 7/6/09 1/22/10 8/10/10 2/26/11Date
Pb
Mn Pb
• HCO3 shows an overall increase with log-PCO2 (Log-CO2 pressure)• pH shows good linear relation with log-PCO2• This suggests that CO2 may dominate pH and carbonate parameters
of groundwater at the Cranfield aquifers
Log-PCO2Log-PCO2
Cranfield, Mississippi
Understanding Geochemical Processes
Importance of Geochemical Context
Mixing creates a complex carbonate system.
Data trends to follow dedolomitization rather than calcite dissolution as hypothesized.
Carbonate system driven by Ca2+ input not CO2
SACROC, Oilfield
0
200
400
600
800
0 100 200Ca2+ (mg/L)
HC
O3-
(mg/
L)
02468
101214
0.0 0.2 0.4 0.6 0.8Si (mmol/L)
HC
O3-
(mm
ol/L
)
5
105
205
305
405
505
605
6.0 7.0 8.0 9.0pH
HC
O3-
(mg/
L)
-30
-25
-20
-15
-10
-5
0
0 200 400HCO3- (mg/L)
dC13
(%o)
SACROCCranfield
Understanding Geochemical Processes
Sensitivity of Groundwater Chemistry to CO2
Less Ca2+ input →
System is sensitive to CO2
6 mmoles brings signal above noiseHigher CO2/Ca ratios approximate calcite dissolution
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20HCO3 (mmoles)
Ca
(mm
oles
)
0.1 0.5 12
10
calcite dissolution
Sensitivity of Groundwater Chemistry to CO2
DIC
Ca2+
HCO3-
pH
Concluding Remarks
Site-specific context is critical for identifying leakage signals using geochemical parameters.
Geochemical parameters can be used for leak detection only if geochemical processes which dominate groundwater chemistry in aquifers are well understood. Information of aquifer mineralogy is also important.
Blind use of carbonate parameters may lead to wrong interpretations regarding leakage; however, DIC shows promise as a global parameter.
Geochemical modeling and sensitivity analysis could help on selection of site-specific parameters for monitoring CO2 leakage.
Thank you!
0.1
1
10
100
1000
-4 -3 -2 -1 0 1 2Log-PCO2
DIC
(mm
ol/L
)
MeasuredPredicted
3
5
7
9
-4 -3 -2 -1 0 1 2Log-PCO2
pH
MeasuredPredicted
Cranfield aquifer
Sensitivity of groundwater chemistry to CO2 leak
• Carbonate mineral saturation indices show that carbonate minerals (calcite and dolomite) are under saturated with respect to groundwater
• Preliminary XRD and SEM analyses indicate that the shallow aquifer sediments consist of mainly silicate minerals
Mineralogy of the sediments
Cranfield, Mississippi
Understanding geochemical processes is important
Dedolomitization
Ca 2+ + CaMg(CO3)2 ↔ 2CaCO3 + Mg2+
Natural: mixing with Permian CaSO4 waters Land use: Infiltration of NaCl brines with cation
exchange
SACROC, western Texas
Understanding geochemical processes is important
Quartz (60-80%) > K-feldspar (10-20%) > albite > dolomite > calcitesmectite and illite clays coating grains
Aquifer mineralogy analysis
Mixing
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