Case Study:

Modeling potential effects of leakage from CO2 sequestration on

shallow groundwater chemistry

Problem Statement

• CO2 sequestration by injection into depleted oil reservoir (~3 km deep)

• Overlain by shallow aquifer (<300 m) used for municipal supply (USDW – underground source of drinking water)

• Many abandoned oil wells; potential preferential pathways for upward CO2 leakage

• What are water quality effects on shallow groundwater?

• How to monitor for evidence of leakage?

Modeling Approach

• MODFLOW Groundwater Flow Model• PHREEQC Geochemistry Model• PHT3D Reactive Transport Model

Groundwater Flow Model• Confined sand aquifer

(150-200 m deep)• 27.6x42.3 km domain

(~1,100 km2)• 100x100x75 m grid blocks• Fixed head boundary

based on USGS water level data

• Production at pumping wells based on USGS data

• Calibrated to water levels in wells

Initial Conditions - Groundwater Chemistry

• Shallow groundwater– Based on regional and site

data for major ions, trace elements

– Pre-conditioned by reacting with aquifer minerals until steady state

• CO2-charged brine (leaking fluid)– Based on formation water

analyses– Set at CO2 saturation at

hydrostatic pressure (~16 bars at top of 400 ft sand)

Parameter Shallow Groundwater

CO2–Charged Brine Units

Temperature 15 15 deg. CpH 6.956 3.535 -pe 2.565 0.996 -DIC 7.36 27.35 mmol/kgw

Calcium 1.06 290. mmol/kgw

Magnesium 0.60 37.7 mmol/kgw

Sodium 2.60 1952. mmol/kgw

Potassium 0.285 15.5 mmol/kgw

Silica 0.213 1.10 mmol/kgw

Chloride 0.138 2671. mmol/kgw

Sulfate 0.134 0.650 mmol/kgw

Aluminum 4.19E-07 0.280 mmol/kgw

Barium 1.33E-03 0.400 mmol/kgw

Cadmium 2.51E-05 - mmol/kgw

Copper 7.78E-05 5.25E-03 mmol/kgw

Iron 0.0108 8.60 mmol/kgw

Manganese 3.62E-03 0.280 mmol/kgw

Lead 4.61E-05 - mmol/kgw

Zinc 0.0131 0.097 mmol/kgw

Initial Conditions – Aquifer Mineralogy

• Based on core and outcrop samples

• High content of calcite, Fe-oxides found in outcrop samples, but not in core samples (weathering products?)

• Illite/smectite and Fe-oxides important for TE sorption (cationexchange and surface complexation)

PhaseAbundance

(mole/L porous medium)

Quartz 357K-feldspar 19.83K-mica (illite) 1.0Kaolinite 4.52Calcite 0 – 10Ferrihydrite(Fe-oxyhydroxide) 0.1

Gibbsite 0*Chalcedony 0*Siderite 0*Rhodochrosite 0**possible secondary mineral

Mineral Dissolution Kinetics• Rate laws include dependence on temperature, mineral

saturation, pH, and CO2 concentration• Kinetic parameters for quartz, K-feldspar from Palandri and

Kharaka (2004)

Sorption–Desorption Reactions

• Cation exchange on clay minerals (illite/smectite)

• Surface complexation of trace metals on Fe-oxide (Dzombak & Morel, 1990)

Mz+ + z X- ↔ MXz

Mz+ + S≡OH ↔ S≡OMz-1

Test Case

• Areal leakage of CO2-charged brine into 150-200 m sand aquifers

• Assumed brine leakage rate equivalent to 0.1% of CO2 injection rate

• Monitor brine arrival and changes in water quality at nearest pumping well

• Examine effect of presence/absence of CO2“bubble” in shallow aquifer on water quality

Chloride Breakthrough

0.0001

0.001

0.01

0.1

1

0 50 100 150 200

Cl (M

)

Time (yrs)

obs wellleak areaSMCL

Observation well

Leak area• Chloride arrival at nearest

observation well predicted at ~70 years after start of leakage

• Exceedance of SMCL predicted at ~100 years

pH and Lead Breakthrough

• Acidic pH front breaks through gradually (more rapidly if CO2 “bubble” present in leak area)

• Lower pH induces desorption of adsorbed trace metals

• Lead MCL exceedancepredicted at ~100 years with no bubble in leak area or ~80 years with CO2 bubble

4

5

6

7

8

0 50 100 150 200

pH

Time (yrs)

brine only, leak areabrine only, obs wellbrine + CO2, leak areabrine + CO2, obs well

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 50 100 150 200

Pb (M

)

Time (yrs)

brine only, leak area brine only, obs wellbrine + CO2, leak area brine + CO2, obs wellMCL

pH_brnco2.mv

pH_brn.mv

Trace Metals Breakthrough

• CO2 “bubble” scenario

• Timing and magnitude of increases in dissolved metals are metal-specific

• First arrival ranges from 70 to 100 years

• Predicted sequence of arrival of MCL/SMCL exceedances at observation well: Cd, Zn, Ba, Cu

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 50 100 150 200

Mn

(M)

Time (yrs)

obs wellleak areaSMCL

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

0 50 100 150 200

Zn (M

)

Time (yrs)

obs wellleak areaSMCL

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

0 50 100 150 200

Cu (M

)

Time (yrs)

obs wellleak areaMCL

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 50 100 150 200

Ba (M

)

Time (yrs)

obs wellleak areaMCL

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

0 50 100 150 200

Al (M

)Time (yrs)

obs wellleak areaSMCL

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

0 50 100 150 200

Cd (M

)

Time (yrs)

obs wellleak areaMCL