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