modeling supercritical systems with tough2: investigating ...brikowi/...caprock 12 felsite 8...
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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.