Noble Gas Partitioning in Two-Phase Flow Kiran Sathaye, Toti Larson, Marc A. Hesse, Esben Pederson University of Texas at Austin Department of Geological Sciences Email: [email protected] 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 magmatic CO 2 field in New Mexico, we study the enrichment of 20 Ne and 3 He in a natural CO 2 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 to inform thermal and tectonic history of regional crust Gas Injection Theory: Two Phases, N Components Total Volume Fraction of i : C i = c i,gas S gas + K i c i,gas (1 - S gas ) (1) Fractional flow of gas phase: f gas = k gas /μ gas k gas /μ gas + k liq /μ liq (2) Total Fractional Flow of i : F i = c i,gas f gas + K i c i,gas (1 - f gas ) (3) 1D Equation for Gas Injection: ∂C i ∂t + ∂F i ∂x =0 i =1 ...N - 1 (4) (N -1) Independent variables: C N =1 - N -1 X 1 C i (5) • K i : dimensionless Henry’s law partitioning coefficient • c i,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 9km 3 residual H 2 O • 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 S rw Volumes 20 Ne 6500 3100 0.4 km 3 0.05 36 Ar 1.1 (10 5 ) 7.7(10 4 ) 1.7 km 3 0.2 84 Kr 410 250 0.16km 3 0.02 Noble Gas Volumetrics: Radiogenic Contribution • 4 He produced as decay product of U and Th in Zircon, Apatite, Monazite, and Titanite - accumulates in minerals below T C • 40 Ar produced by 40 K decay in mica, K-spar • 40 Ar/ 36 Ar Ratio ≈ 3400 = 11R atm → large crustal component • Non-mantle 4 He/ 40 Ar = 0.9, 4 He/ 40 Ar Production Ratio = 5 • Significant excess 40 Ar in Bravo Dome gas Isotope Total Mols Non-Mantle Mols ASW Required S rw Volumes 4 He 3.3 (10 8 ) 2.3 (10 8 ) 1.2(10 5 ) km 3 1.3(10 4 ) 40 Ar 1.1(10 9 ) 2.6(10 8 ) 20 km 3 2.2 Easting (km) Northing (km) 4 He Mols Per Area (mols/m 2 ) 8 MPa C’ C D’ D 0 25 50 75 0 25 50 75 0 0.1 0.2 0.3 0.4 0.5 Easting (km) Northing (km) 40 Ar Mols Per Area(mols/m 2 ) 8 MPa C’ C D’ D 0 25 50 75 0 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 -20 10 0 10 1 10 2 10 3 10 4 10 5 CH 4 /C 2+ 13 δC CH 4 (‰ VPDB) Osborn et al., PNAS 2011 Fractionation During Migration Mixing with microbial gas CH 4 /C 2+ Hydrocarbon Gas Injection into Residual, N 2 -Saturated Water Liters Biogenic-Saturated H 2 O Degassed δ 13 C CH 4 (‰ VPDB) Thermogenic Biogenic δ 13 C (‰ VPDB) 1L 1atm 25°C Thermogenic CH 4 • Hydrocarbon composition and isotopes are used to distinguish between 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% 3 He + 99% CO 2 (Mantle input) • Initial Brine: 99.5% H 2 O + 0.5% 20 Ne (Meteoric) • Volatile noble gases enriched at the front • 20 Ne completely swept from CO 2 plume • Mathematically describes two-stage gas enrichment model proposed by Gilfillan et al., 2008 Distance (km) Irreducible water saturation controls CO 2 dissolution Injected helium lls front of plume to due high volatility 10 0 20 30 40 50 60 70 Observations: Bravo Dome Natural CO 2 Field • The Bravo Dome field contains 1.3GtCO 2 (22 tcf), with a mag- matic noble gas isotope signature • Noble gas isotopes have been used to determine location of gas source and interaction with initial brine • We used Apatite (U-Th)/He thermochronology to identify a gas 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 basement brine gas anhydrite elevation [m] (B) source C 5 10 15 20 25 30 35 C’ 450 550 650 750 850 distance along cross-section [km] anhydrite brine elevation [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] northing [km] Texas Oklahoma Colorado C’ T1 B 0 95 Folsom Site Folsom Site Capulin volcano Capulin volcano B’ B’ New Mexico C volcanic ages: granitic basement T2 distance along cross-section [km] Northing (km) 4 He Volume Fraction (ppm) 0 25 50 75 100 200 300 400 Easting (km) Northing (km) 3 He/ 4 He Ratio (R/R atm ) 0 25 50 75 0 25 50 75 1 2 3 4 • CO 2 / 3 He ratio is used to infer the amount of local CO 2 disso- lution into brine: in total, 22% ± 7% has dissolved • 40% of the dissolution took place above the gas-water contact in the residual brine: analog to gas injection process References 1. 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 CO 2 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 National Academy of Sciences 108.20 (2011): 8172-8176. 5. K.J. Sathaye et al. "Constraints on the magnitude and rate of CO 2 dissolu- tion at Bravo Dome natural gas field." Proceedings of the National Academy of Sciences, 2014: 201406076. Experimental Results: 4 Component Drainage 150 155 160 165 170 175 180 185 0 0.5 1 1.5 Time Since Injection (min) Mol Percent in Gas CO 2 Argon CH 4 Two-Phase Interface 1 cm • Column initially filled with H 2 O saturated with CH 4 • Injection gas: 80% CO 2 and 20% Ar • Dissolved CH 4 swept and enriched into first arrival bank • Argon is 20X less soluble in water than CO 2 • Enriched Ar bank reaches the end of the column before CO 2 • Experimental results match theory for both initial dissolved gas (CH 4 ) 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 Sourcing Ballentine, Burgess and Marty Tracing Fluid origin, Transport and Interaction in the Crust RiMG 47 Chapter 13