Noble Gas Partitioning in Two-Phase FlowKiran Sathaye, Toti Larson, Marc A. Hesse, Esben PedersonUniversity of Texas at Austin Department of Geological SciencesEmail: kiransathaye@utexas.edu

Summary• Noble gas isotope concentrations are used as tracers

and indicators of subsurface fluid flow and origin

• Motivated by the case of the Bravo Dome magmaticCO2 field in New Mexico, we study the enrichmentof 20Ne and 3He in a natural CO2 injection process

• We observe qualitative agreement between theory,experiments, and field observations of noble gas en-richment during gas injection processes

• Volumes of radiogenic noble gases can be used toinform thermal and tectonic history of regional crust

Gas Injection Theory: Two Phases, N ComponentsTotal Volume Fraction of i : Ci = ci,gasSgas +Kici,gas(1− Sgas) (1)

Fractional flow of gas phase: fgas =kgas/µgas

kgas/µgas + kliq/µliq(2)

Total Fractional Flow of i : Fi = ci,gasfgas +Kici,gas(1− fgas) (3)

1D Equation for Gas Injection:∂Ci

∂t+∂Fi

∂x= 0 i = 1 . . . N − 1 (4)

(N -1) Independent variables: CN = 1−N−1∑1

Ci (5)

• Ki: dimensionless Henry’s law partitioning coefficient

• ci,gas: volume fraction of component i in the gas phase

Noble Gas Volumes: Air-Saturated Water Contribution• Indicates volume of gas interaction with air saturated water

(ASW): Gas cap currently contains roughly 9km3 residual H2O

• All ASW derived gases indicate ASW equilibration volumes of<0.2 residual water volumes - evidence of noble gas sweep

• Noble gas volumes not indicative of ASW volume contacted

Isotope Total Mols Non-Mantle Mols ASW Required Srw Volumes20Ne 6500 3100 0.4 km3 0.0536Ar 1.1 (105) 7.7(104) 1.7 km3 0.284Kr 410 250 0.16km3 0.02

Noble Gas Volumetrics: Radiogenic Contribution• 4He produced as decay product of U and Th in Zircon, Apatite,

Monazite, and Titanite - accumulates in minerals below TC

• 40Ar produced by 40K decay in mica, K-spar

• 40Ar/36Ar Ratio ≈ 3400 = 11Ratm → large crustal component

• Non-mantle 4He/40Ar = 0.9, 4He/40Ar Production Ratio = 5

• Significant excess 40Ar in Bravo Dome gas

Isotope Total Mols Non-Mantle Mols ASW Required Srw Volumes4He 3.3 (108) 2.3 (108) 1.2(105) km3 1.3(104)40Ar 1.1(109) 2.6(108) 20 km3 2.2

Easting (km)

Nor

thin

g (k

m)

4He Mols Per Area (mols/m2)

8 MPa

C’

C

D’

D

0 25 50 750

25

50

75

0

0.1

0.2

0.3

0.4

0.5

Easting (km)

Nor

thin

g (k

m)

40Ar Mols Per Area(mols/m2)

8 MPa

C’

C

D’

D

0 25 50 750

25

50

75

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Hydrocarbon Fractionation and Groundwater

-80 -70 -60 -50 -40 -30 -20100

101

102

103

104

105

CH4/C

2+

13δC CH4 (‰ VPDB)

Osborn et al., PNAS 2011

Fractionation During Migration

Mixing with microbial gas

CH4/C

2+

Hydrocarbon Gas Injection into Residual, N2-Saturated Water

Liters Biogenic-Saturated H2O Degassed

δ1

3C

CH

4 (

‰ V

PD

B)

Thermogenic

Biogenic

δ13C (‰ VPDB) 1L 1atm 25°C

Thermogenic CH4

• Hydrocarbon composition and isotopes are used to distinguishbetween bacterial and thermogenic gas

• Experiment shows composition change during gas injection

• Degassing of bacterial methane form groundwater into migrat-ing gas plume can change C isotope value of methane gas

Theoretical Composition Profiles: (Equation 4)• Solution to equation (4) with boundary conditions for mag-

matic gas injection into subsurface aquifer

• Injected gas: 1% 3He + 99% CO2 (Mantle input)

• Initial Brine: 99.5% H2O + 0.5% 20Ne (Meteoric)

• Volatile noble gases enriched at the front

• 20Ne completely swept from CO2 plume

• Mathematically describes two-stage gas enrichment modelproposed by Gilfillan et al., 2008

Distance (km)

Irreducible water saturation controls CO2 dissolution

Injected helium lls front of plume to due high volatility

100 20 30 40 50 60 70

Observations: Bravo Dome Natural CO2 Field• The Bravo Dome field contains 1.3GtCO2 (22 tcf), with a mag-

matic noble gas isotope signature

• Noble gas isotopes have been used to determine location of gassource and interaction with initial brine

• We used Apatite (U-Th)/He thermochronology to identify agas emplacement age between 1.2Ma and 1.5Ma

[MPa]

(A)

2 4 6 8 10 12 14

B 5 15 25 35 45 55 65 B’500

600

700

800

900

granitic basementbrine

gas anhydrite

elev

atio

n [m

]

(B)

source

C 5 10 15 20 25 30 35 C’450

550

650

750

850

distance along cross-section [km]

anhydrite

brine

elev

atio

n [m

] gas(C)

−60 −40 −20 0 20 40 60 80

20

40

60

80

100

120

140

160

1.7Ma−56ka

9Ma−2.2Ma

easting [km]

nort

hing

[km

]

TexasO

klahoma

Colorado

C’

T1

B

095

Folsom SiteFolsom SiteCapulin volcanoCapulin volcano

B’ B’

New Mexico

C

volcanic ages:

granitic basement

T2

distance along cross-section [km]

Nor

thin

g (k

m)

4He Volume Fraction (ppm)

0

25

50

75

100

200

300

400

Easting (km)

Nor

thin

g (k

m)

3He/4He Ratio (R/Ratm

)

0 25 50 750

25

50

75

1

2

3

4

• CO2/3He ratio is used to infer the amount of local CO2 disso-lution into brine: in total, 22% ± 7% has dissolved

• 40% of the dissolution took place above the gas-water contactin the residual brine: analog to gas injection process

References1. M. Cassidy, Ocurrence and Origin of Free Carbon Dioxide in the Earth’s crust.

PhD Thesis, University of Houston, 2005.

2. S.M.V. Gilfillan, et al. "The noble gas geochemistry of natural CO2 gas reser-voirs from the Colorado Plateau and Rocky Mountain provinces, USA."Geochimica et Cosmochimica Acta 72(2008): 1174-1198.

3. F.M. Orr. Theory of gas injection processes. Tie-Line Publications, 2007.

4. S.G. Osborn., et al. "Methane contamination of drinking water accompany-ing gas-well drilling and hydraulic fracturing." proceedings of the NationalAcademy of Sciences 108.20 (2011): 8172-8176.

5. K.J. Sathaye et al. "Constraints on the magnitude and rate of CO2 dissolu-tion at Bravo Dome natural gas field." Proceedings of the National Academy ofSciences, 2014: 201406076.

Experimental Results: 4 Component Drainage

1501551601651701751801850

0.5

1

1.5

Time Since Injection (min)

Mol

Per

cent

in G

as

CO2

ArgonCH

4

Two-PhaseInterface

1 cm

• Column initially filled with H2O saturated with CH4

• Injection gas: 80% CO2 and 20% Ar

• Dissolved CH4 swept and enriched into first arrival bank

• Argon is 20X less soluble in water than CO2

• Enriched Ar bank reaches the end of the column before CO2

• Experimental results match theory for both initial dissolved gas(CH4) and injected noble gas (Argon)

• Gas composition measured away from source can be very dif-ferent due to fractionation during migration

• After arrival of trace gas banks, gas returns to input mixture

Noble Gas SourcingBallentine, Burgess and Marty Tracing Fluid origin, Transport and Interaction in the CrustRiMG 47 Chapter 13