Modeling Supercritical Systems WithTough2: Investigating The Onset of Boiling

at The Geysers ∗

Tom H. Brikowski

The University of Texas at Dallas

August 27, 2001

∗The work upon which this report is based was supported by DOE contract DE-FG07-98ID13677

to The University of Texas at Dallas. All rights reserved, T. Brikowski 2001.

1

Project Summary

• Project goal: investigate the nature and evolution of The Geysers

from the time of magma emplacement, using detailed heat and

water and chemical mass balances (natural state models)

• Main issues:

? nature of the liquid-dominated system, as discernible from the

rock alteration record (fluid inclusions and δ18O )

? nature of the boiling “event” (transition to vapor dominated

conditions)

? nature of interaction between geologic events (e.g. re-

intrusion, faulting), fluid properties, and hydrologic events (e.g.

permeability evolution)

2

Today’s Talk

• Progress prior to May (described in various papers):

? development and preliminary testing of supercritical equation of

state for Tough2 EOS1sc? liquid-only supercritical flow and δ18O -alteration models

• Recent progress:

? EOS1sc redesigned to incorporate NIST-standard numerical

equation of state [NIST(1999)], currently the only flow simulator

with this feature

? relatively robust in testing

? preliminary models of the onset of boiling at The Geysers

3

Why Supercritical?

oTemperature ( C)

Supercritical

Dep

th (

m)

Published Geothermal Temperature−Depth Profiles

500 600400100 200 300

0

−1000

−2000

−3000

−4000

−50000

Kakkonda, Japan

Pure H2O

Boiling Curves

Lardarello, Italy

10% N

aCl

The Geysers

after [Muraoka et al.(2000)]

• Deep drilling magmatic

geothermal systems encounters

these conditions

• Models of non-magmatic

systems often encounter these

conditions at depth in

conductive zones, e.g. the

Basin and Range

[Wisian(2000)]

4

Critical Fluid Properties

• Fluid flow and transport

properties reach strong extrema

at the critical point, profoundly

influencing convection

• Isobaric heat capacity (Cp1◦C )

and isothermal compressibility

(β 1Pa) → +∞ at critical point.

• Extrema extend beyond critical

point along the critical isochore

(ρ = ρcritical)

5

Why Tough2?

Pre

ssur

e (M

Pa)

100

Temperature (degC)

Critical Isochore

Critical Point

EOS1sc

Ton

alite

+ 2

%H

2O S

olid

usTwo−phase Bdy

200 10000

20

40

60

0

80

400 600 800

EOS1

• Flexible (irregular) gridding

• Variety of choices for matrix

solution

• Other capabilities needed for

history matching, reactive

transport modeling, etc.

• As-shipped equation of state

(EOS1) limited to subcritical,

needs revised EOS (EOS1sc)

with extended range

6

Subcritical Tests of EOS1sc

• Comparison of TOUGH2 test problem results using EOS1 and

EOS1sc show excellent match; however EOS1sc run times are

5-50 times longer.

Geothermal 5-Spot Fracture Heat Sweep

7

Extensional Geothermal Systems

350

Dixie Valley Observed and Modeled T−z Profiles

oT ( C)

−8

1

0

−2

−3

−7

−6

−5

−4

−1

400250200 3000 50 100 150

Ele

vatio

n (k

m)

EOS1sc

Observed

EOS1−maxT

• Fluid conditions approach

critical at the base of current

models, model design limited by

capabilities of the reservoir

simulator [Wisian(2000)]

• Application of EOS1sc allows

realistic treatment of the deep

parts of the system, and

simplifies matching of shallow

observations

8

Geysers Models: Location of Cross-Section

9

Geology and Alteration

14

Caprock

12

Felsite

8

Greywacke

14

Fra

nsis

can

Fm

.

Simplified Geology

SB−15d

Hornfels

Top

810

Reservoir

6

Steam

10

of

6000

0

−6000

Elev (m)

δ18 oo

12

Whole−Rock O ( / )o

Elev (m)

−6000

0

6000

after [Moore and Gunderson(1995), Hulen and Moore(1996)]

• Permeability zones

? caprock

? reservoir (lower greywacke

and upper felsite)

? hot intrusive (deep felsite)

• Alteration zones

? minimal in caprock

? widespread moderate

depletion (6-8h) in reservoir

? concentrated strong (8-10h)

along low felsite flank

10

Geysers Supercritical Models

• Hydrothermal flow models that accurately treat critical fluid

properties tend to show strong control by these properties on

the deep system [Brikowski(2001)]

Fluid Velocity Fluid Cp

11

Principal Liquid-Phase Model Results

6

8

10

12

20

18

16

14

4

Vel

oci

ty M

agn

itu

de

(m/s

ec)

Co

olin

g

Hea

tin

g

Hea

t C

apac

ity

(Cp

, 1/d

egC

)Time (yrs)

1e−11

1e−09

1e−10

150100500 200 450400350300250

Velocity

Cp

• Zone of critical conditions

“drives” the pre-boiling flow

system at The Geysers

• System cools in approximately

500 Kyr, despite low reservoir

permeability (10−17m2)

• Alteration distribution indicates

persistent deep horizontal

permeability throughout

liquid-dominated stage

12

Model Grid

• Coarse grid, 43x21 elements sized 150m x 250 m

• Assume very high permeability reservoir (k = 1 md)

• Seek to encourage boiling by developing isothermal low pressure

zone throughout reservoir, similar to present-day conditions

13

Liquid-Dominated Stage

• Model begins with intrusion of

felsite at 890◦C

• Reservoir rapidly develops

several vigorous convection cells

• Strongest cell upwells over the

apex of the felsite, near the

location of well SB-15d

• red line is location of P-T

section on next slide

14

Liquid PT Path

Critical Point

10

5

100 200 300 400 500

15

0

35

30

25

20

0 600 900800700

Reservoir

T ( C)

P−T Profile

P (

MP

a)

Two−P

hase Boundary

Top

Reservoir Base

o

P−T Profile and Paths, Central Upflow Zone • Points in the upflow zone (and

near the felsite contact

elsewhere) migrate rapidly

toward the two-phase boundary,

critical point, or critical

isochore

• Fluid packets follow a path

down the critical isochore, past

critical point and then

alongside the 2-phase boundary

(liquid-stable)

15

Formation of Steam “Bubbles”

• Take closeup view of reservoir above apex of felsite intrusion

• Base of upflow zone moves onto 2-phase boundary, forming a steam packet(10% saturation “bubble”, shown in green)

• This causes large P perturbation (owing to steam expansion), disrupting upflowzone

• Eventually steam packets advect upward to top of reservoir

16

Steady Boiling

• To force continuous boiling, system must suddenly lose pressure

(fracturing or drilling), or be reheated (reintrusion)

• Preliminary tests show extreme fracturing required, else metastable

“simmering” conditions persist

• To date only unnatural pressure reduction using wells successfully

drives upflow column to full steam saturation

17

Implications for Geysers

• Deep reservoir behaves much like the roots of a true geyser

• Episodic boiling occurs over extended period, potentially advecting with the flowfield

• These episodes initiated at felsite contact by perturbations toward the fluidcritical point

• During the metastable period, profound oscillations in flow and fracturing willoccur

• These oscillations are recorded in the rock record at The Geysers, includingmineral and alteration zoning, paragenetic sequences, and episodic fracturing

• System requires a significant “kick” to break out of this metastable state

18

Summary

Critical Point

10

5

100 200 300 400 500

15

0

35

30

25

20

0 600 900800700

Reservoir

T ( C)

P−T Profile

P (

MP

a)

Two−P

hase Boundary

Top

Reservoir Base

o

P−T Profile and Paths, Central Upflow Zone • Is “simmering” a long-lived

transition state to traditional

boiling?

? Base of upflow zones in

magmatic systems likely to

be at critical point conditions

? Near-critical fluid properties

encourage this behavior in

high-permeability systems

• Tools like EOS1sc are now

available to investigate such

high P-T phenomena

19

References

[Brikowski(2001)] Brikowski, T. H., 2001. Deep fluid circulation and isotopic alteration in The Geysers geothermal system: Profile models.Geothermics 30 (2-3), 333–47.

[Hulen and Moore(1996)] Hulen, J. B., Moore, J. N., 1996. A Comparison of Geothermometers for The Geysers Coring Project, California -Implications for Paleotemperature Mapping and Evolution of The Geysers Hydrothermal System. Geotherm. Resour. Council Transact. 20,307–314.

[Moore and Gunderson(1995)] Moore, J. N., Gunderson, R. P., 1995. Fluid inclusion and isotopic systematics of an evolving magmatic-hydrothermal system. Geochim. et Cosmo. Acta 59 (19), 3887–3908.

[Muraoka et al.(2000)] Muraoka, H., Yasukawa, K., Kimbara, K., 2000. Current state of development of deep geothermal resources in theworld and implications to the future. In: Iglesias, E., Blackwell, D., Hunt, T., Lund, J., Tmanyu, S. (eds.), Proceedings of the WorldGeothermal Congress 2000, International Geothermal Association, pp. 1479–1484.

[NIST(1999)] NIST, 1999. NIST/ASME STEAM PROPERTIES DATABASE: VERSION 2.2. NIST Standard Reference Database 10, U.S.National Institute of Standards and Testing, URL http://www.nist.gov/srd/nist10.htm.

[Wisian(2000)] Wisian, K. W., 2000. Insights into Extensional Geothermal Systems from Numerical Modeling. Geotherm. Resour. CouncilTransact. 24, 281–286.