Modeling Shallow Pore Water Chemistry above Marine Gas Hydrate Systems

Sayantan Chatterjee, Gerald Dickens, Gaurav Bhatnagar, Walter Chapman, Brandon Dugan, Glen Snyder, George Hirasaki

Rice University, Houston, Texas, USA

April 23, 2012

Rice UniversityConsortium on Processes

in Porous MediaDE-FC26-06NT 42960

Torres et al., Earth Planet. Sci. Lett., (2004)

2

Gas hydratesCage structure “Ice that burns” Core sample

Source: USGS

Source: USGS

- Clathrates- Ice-like solids- Guest molecules (e.g., CH4) encapsulated in H2O cages

Stability- High pressure- Low temperature- Low salinity

Occurrence- Marine sediments along continental margins - Permafrost regions

Motivation

Potential energy resource

Subsea geohazard Global climate change

McIver, AAPG (1982) Westbrook et al., Geo. Res. Lett., (2009)

A fundamental understanding of the dynamics of gas hydrate systems

Soil1400

Gas hydrates (on shore and

offshore), 10,000

Other, 67

Peat, 500

Land Biota, 830

Dissolved organic matter in water, 980

Unit 1015 g carbonRecoverable and

Non-recoverable fossil fuels (coal, oil, natural gas), 5,000

Hydrate dissociation due to burial below the GHSZ

Free gas recycling

Geologic time (Myr)

Concentration (mM)

Subsidence Subsidence

Hydrate layer extending downwards

Solubility

Hydrate

Free gas

Dissolved gas

Organic carbon

CH4

SO42- reduction zone

Base of GHSZ

GHSZ

Temperature (oC)

Dep

th

Seafloor

Geotherm

CH

4 3-ph

ase

equ

ilibriu

m

0 10 20 30

Sedimentation fluid flux

External fluid flux

0 100 200 300

T0

Phase relationships

Steady state Transient state Components

TOCao

Sediment flux

SO42-H

ydro

ther

m

Tran

sien

t st

ate

Ste

ady

stat

e

Schematic of hydrate formation and burial

Bhatnagar et al., Am. J. Sci., (2007); Chatterjee et al., (2012) to be submitted

5

Methods to quantify gas hydrate amount and distribution

Dickens., Org. Geochem., (2001)

Free gas

994 995 997

Paull et al., ODP Init. Repts., 164, (1996)

Boswell et al., Mar. Pet. Geo., (2011)

Gas hydrate systems and the SMT

6Bhatnagar et al., Geo. Res. Lett., (2008)

SMT: Sulfate Methane Transition

SMT depth inversely proportional to upward methane flux

7

Borowski et al., Geology, (1996) Paull et al., Geo-Mar. Lett., (2005)

Gas hydrate bearing sediment

Gas hydrate free sediment

Fault line SMT Chemosyntheticcommunity

De

pth

be

low

se

afl

oo

r

SMT

SMT

CH4 solubility and phase equilibrium curves

Sedimentation and compaction Mass balance equations:

• Sediment• Water• Organic carbon

• Methane (CH4)

• Sulfate (SO42-)

• Bicarbonate (HCO3-)

• Calcium (Ca2+)

• Carbon isotopes of CH4 and HCO3-

CH4 sources:

• In situ methanogenesis (biogenic)• Deep external sources (thermogenic)

Advection, diffusion and reaction

Key model features

8

PDEs solved using finite-difference method

(Explicit and implicit schemes)

Geologic sites known/inferred for gas hydrates

Chatterjee et al., J. Geophys. Res., (2011)

Modeling pore water chemistry at 3 sites

Chatterjee et al., J. Geophys. Res., (2011)

Flux balance across the SMT using steady-state simulations

Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted

iiD z

c

At the SMT:

Diffusive flux

Advective flux = 0

A 1:1 flux balance across SMT implies dominant AOM at the SMT

Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted

24 4 3 2( ) CH aq SO HCO HS H O

flux

flux

Anaerobic Oxidation of Methane (AOM)

Paull et al., Geo-Mar. Lett., (2005)

SMT depth: A useful proxy

13

Gas hydrate bearing sediment

Gas hydrate free sediment

Fault line SMT Chemosyntheticcommunity

SMT dept

h

Net fluid flux

Top of

gas hydrate

Hydrate

saturation <Sh>

Bhatnagar et al., Geochem. Geophys. Geosyst., (2011)

Pe 1

<S

h>

Top

of

hy

dra

te /

SM

T

Normalized Scaled SMT depth

Scaled SMT depth

De

pth

be

low

se

afl

oo

r

SMT depth Hydrate saturation

SMT depth Top of gas hydrate

“Rule of ten”

Top

of

hy

dra

te /

SM

T14

Seafloor and geologic

parameters

Base of hydrate stability

Local SMT depth

Top of gas hydrate

Local fluid flux

Local hydrate

saturation

Modeling to quantify hydrate amount and distribution

De

pth

Base of GHSZ

Dep

th Geotherm

T3 P

Hyd

roth

erm

Scaled SMT depth

“Rule of ten”

Pe1<Sh>

Loca

l flui

d flu

x

Conclusions

15

A 1:1 flux balance across the SMT implies dominant AOM at the SMT

SMT depth is a direct proxy to relate upward methane flux and hydrate saturation

An empirical “rule of ten” established to relate SMT depth and top of hydrate

Developed a model to evaluate hydrate amount and distribution

Back up slides

16

Pore water chemistry and reaction zones

17

Sulfate reduction zone

Methanogenesiszone

Snyder et al., J. Geochem. Explor., (2007)

Pore water chemistry data: Sites 1244 and KC151

Chatterjee et al., J. Geophys. Res., (2011)18

Pore water chemistry data: Site 1230

Chatterjee et al., (2012) to be submitted19

Concentration crossplot of DIC and SO42-

20Chatterjee et al., J. Geophys. Res., (2011)

Steady state profiles: Site 1244

21Chatterjee et al., J. Geophys. Res., (2011)

Steady state profiles: Site KC151

22Chatterjee et al., J. Geophys. Res., (2011)

Physical property data: Site 1230

Chatterjee et al., (2012) to be submitted23

Pre

ssur

e (M

Pa)

Evidence of a 4.3 Myr hiatus implies Site 1230 is in transience

24

Hiatus

Seafloor 2.4 Myr ago

Pre-hiatus steady state profiles: Site 1230

25Chatterjee et al., (2012) to be submitted

Transient state profiles: Site 1230

26Chatterjee et al., (2012) to be submitted

pH and activity correction

27Chatterjee et al., (2012) to be submitted

Geotherm correction

28Chatterjee et al., (2012) to be submitted

Effect of DaAOM

29Chatterjee et al., J. Geophys. Res., (2011)

Effect of DaPOC

30Chatterjee et al., J. Geophys. Res., (2011)

2:1 concentration crossplot

31Chatterjee et al., J. Geophys. Res., (2011)

Concentration crossplot: Site 1244

Da = 0.22; Cb,ext = 27

32Chatterjee et al., J. Geophys. Res., (2011)

Da = 1; Cb,ext = 50

Concentration crossplot: Site KC151

33Chatterjee et al., J. Geophys. Res., (2011)

Concentration crossplot: Site 1230

34Chatterjee et al., (2012) to be submitted

Concentration crossplots CANNOT determine stoichiometry

Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted

Site 685/1230

Flux crossplot: Site 1244

Chatterjee et al., J. Geophys. Res., (2011)36

Flux crossplot: Site 1230

Chatterjee et al., (2012) to be submitted37

1. Anaerobic Oxidation of Methane (AOM)

(1:1)

2. Organoclastic Sulfate Reduction (OSR)

(2:1)

Two potential causes for SMT

38

= Dissolved Inorganic Carbon (DIC) ~ Alkalinity

∆ = change from seawater

AOM

SO42-, Alkalinity (mM)

Dep

th (

mbs

f)

∆ (

Alk

+C

a+M

g)

∆SO4

39

OSR

Site 1244

22 4 32 ( ) 2CH O s SO HCO HS H 2

4 4 3 2( )CH aq SO HCO HS H O

Kastner et al., Fire in the ice, (2008)

Arguments for OSR: Stoichiometry and d13C of DIC

Dep

th (

mbs

f)

d13CDIC (‰)

OSR (2:1); δ13CDIC ≈ -25‰ AOM (1:1); δ13CDIC ≈ -60‰

Dep

th (

mbs

f)

∆ (

Alk

+C

a+M

g)

40

Site 1244

22 4 32 ( ) 2CH O s SO HCO HS H 2

4 4 3 2( )CH aq SO HCO HS H O

Dickens and Snyder., Fire in the ice, (2009)

Dep

th (

mbs

f)

Counterarguments for AOM and methanogenesis

Methanogenesis; δ13CDIC ≈ 10‰

Flux units mol/m2kyr

2 2 4 32 CH O H O CH HCO H

SO42-, Alkalinity (mM) ∆SO4

+10‰ (methanogenesis)

-60‰ (AOM)

Deep DIC flux is enriched in 13C

OSR (2:1); δ13CDIC ≈ -25‰ AOM (1:1); δ13CDIC ≈ -60‰

d13CDIC (‰)

13C enriched DIC flux from depth impacts alkalinity and d13C of DIC at SMT

Chatterjee et al., J. Geophys. Res., (2011)

24 4 3 2( ) CH aq SO HCO HS H O

2 2 3 42 ( ) CH O s H O HCO CH H

AOM (δ13CDIC ≈ -60‰)

Methanogenesis (δ13CDIC ≈ 10‰)

OSR (δ13CDIC ≈ -25‰)2

2 4 32 ( ) 2CH O s SO HCO HS H

42

OSR dominated systems

CH4 and SO42- DIC (or HCO3

-) Ca2+ δ13C in DIC

BHSZ

Distinct zones of local fluid flux

43

Hig

h lo

cal

flu

id f

lux

Low local fluid flux

Seafloor

Low local fluid flux

Local SMT depends on local fluid flux

SMT depth

Top of gas hydrate

Low flux in sediment High flux in fracture

Low flux in sediment High flux in fracture

BHSZ

PeLocal = - 29

PeLocal = - 0.85

No

rmal

ized

dep

th

Normalized concentration

BHSZ

<Sh>Local = 22%<Sh>Local = 6%

No

rmal

ized

dep

th

Hydrate and free gas

No

rmal

ized

dep

thN

orm

aliz

ed d

epth

Hydrate and free gas

Normalized concentration

44

Generalized model to quantify amount and distribution of gas hydrates

45

Pe1<Sh>

Bhatnagar et al., Am. J. Sci., (2007)

Biogenic sources only Biogenic sources and external flux

Net fluid flux (Pe1) and org C input at seafloor

Hydrate saturation

Net fluid flux (Pe1 + Pe2)Hydrate

saturation

Kinetic and equilibrium reaction model

Methanogenesis reaction:

AOM reaction at the SMT:

POC driven sulfate consumption above the SMT:

Calcite precipitation reaction:

46

δ13C definition

The isotopic carbon composition (δ13C ) in any sample is defined

The isotope ratios usually reported in per mille, relative to an standard Pee-Dee-Belemnite (PDB) marine carbonate

47

Methanogenesis reaction:

AOM reaction of biogenic methane at the SMT:

Organoclastic sulfate consumption:

Calcite precipitation reaction:

Reactions with corresponding δ13C values

48

1-D organic carbon mass balance

Dimensionless mass balance

49

4

1

  ,

,

1(1 )  

1 1(1 )      

1

sed

POC s o lsw POC s

m SO m eqb

Pe Ut z

M cDDa S Da

D Mc

c

4

 

1     1  

1(1 )   (   )(   )

sed sed s

lsed w seP d

Sw

OsOC

vt z

S cM

1-D methane mass balance

Dimensionless mass balance

50

1  (1 ) l

h g m h gg

h m g mhS S c S c S c

t

1 1

1 2

1 1

1 

1

1 

s sh h gh w h h mm h g m g

lPeU PeU

Pe Pe S c S c S ccz

4 4

4

,

,

1 1    1  2

lCH CH s o l lm

w sed w AOM m sPOC SO m eqb

M M ccS Da S Da

z z M Mc c

c

1-D sulfate mass balance

Dimensionless mass balance

51

1

1 2

1

1

1 1 1  

s hh w h s

ll l s s

w s wm

PeU cc Pe Pe S c

t z zS c

D z

DS

1 11       

1 2l l ls

w AOM m s w sed POC sm

cD

S Da c Da cSD

1-D DIC mass balance

Dimensionless mass balance

52

1

1 2

1 1  

1

1

s hh w

l lb h bw

PeUc Pe Pe S cS c

t z

3   3

4  

, ,

, ,

1 1  1  2  

lHCO m eqb HCO s o l lb b

w sed w AOM m sm POC b o SO b o

M c M cDS Da S Da c c

D z z M c M c

c

3 3

4

, ,

, ,

1 1   1       

  (1 )HCO s o CaCOCa ols

w sed POC s wSO b o m b o

M c ccDS Da c S

c tM c D

1-D calcium mass balance

Dimensionless mass balance

53

1

1 2

1 1

1     

1sl lw C

hh w h Ca a

PeUPe Pe S cS c c

t z

31 1

 l

CaCOCa Caw w

m

cD cS S

D z z t

1-D δ13Cmethane mass balance

Dimensionless mass balance

54

4

131  (1 ) l

h g m h h m g g mh g

CHS S CS c c S ct

4

1 1 131 2

1 1

1 1

1   ls sh h g

h w h h m h g m g CHm

PeU PeUPe P c

ze S c S c S c C

4

4

4

1313

,

(1    1  2

)CHC

lm CH

w sedPOC

H meth

c MDa

z MCS

z

C

4

4

4

1,

,

31C

CH s o l lw AOM m s

SO mH

eqb

M cS Da

M cc c C

.

1-D δ13CDIC mass balanceDimensionless mass balance

55

3 3

113 131 2

1 1  

1

1

s hHCO h w h b HC

lb O

lwS c

PeUc C Pe Pe S c C

zt

3

3

3  

1313

,,

,

(1  1  2  

)HCOHCO me

lb HCO m eqbb

w sedm P

thOC b o

M cDS Da

D z z M c

c CC

4

 

3

3

3

4

4

13

13,

,

,

,

,

1

1 1      

  (1 )

HCO s o l lw AOM m s

SO b o

HCO s o lsw sed POC s

SO b o m

CH

HCO POC

M cS Da c c

M c

M c DS Da c

M c

C

CD

3 3,

,

13( 1 )  CaCOCa o

wb o

HCOccS

C

tc