Geothermal Short Course:
Reservoir Characteristics
Interpreting Temperature and
Pressure Measurements from Wells
Rick Allis
Utah Geological Survey
Penrose Conference, Park City, UT, Oct 19-23, 2013
Themes
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Conductive thermal regimes (and heat flow)
Convective thermal regimes (interpreting geothermal well data)
Importance of water density and pressure data in hot wells
Importance of feedzone control in high permeability wells
Pressure drawdown effects due to production
High permeability effect on deep basin pressure regime
DSTs, and Injectivity Tests as indicators of permeability
Regional heat flow of the conterminous U.S.
(SMU geothermal lab; Blackwell et al., 2011)
Cascades
Yellowstone
Snake River
Plain
Great Basin
Colorado
Rockies
Imperial Valley
Rio Grande
rift
Gulf Coast
Prospective reservoirs with attractive temperature
and hi-permeability stratigraphic target
Heat Flow Measurements – A Critical Component of Geothermal Exploration
However, this map doesn’t have the resolution required for discovery of geothermal
reservoirs (which require adequate temperature, and adequate permeability)
Pavant Butte- Black Rock Desert, Utah:
a high heat-flow basin
Heat Flow (mW/m2) = Thermal Conductivity (W/m°C) x Temperature Gradient (°C/km)
From Gwynn et al., 2013
Thermal
Conductivity is
important!
MIT, (2006)
unconsolidated
basin fill,
mudstone, shale
sandstone,
many
bedrock
lithologies
Heat Flow (mW/m2) = Thermal Conductivity (W/m°C) x Temperature Gradient (°C/km)
Hypothetical stratigraphy
H.F. = 100 mW/m2
Exploration Oil-Well Data:
often the main source of
data on possible
geothermal potential
Correcting Bottom Hole
Temperatures (BHTs)
for the drilling
disturbance can be
tricky. A variety of
methods, but basically,
BHTs are a noisy source
of data; However, they
need to be corrected,
and it can be a tedious
process
The Geothermal
Resource
Oil Exploration
Old Gradient
New Gradient
• Background HF 80-85 mW/m²
= High Heat Flow Basin
• High HF (>85 mW/m²) anomaly
• 350 km² > 150°C at 3 km
• 60 km² > 200°C at 3 km
• Focused heat source
superimposed on basin-scale
thermal regime
• Cooling intrusion?
•Why is southern end of basin
cooler (lower heat flow?)
Heat flow from Wilson and Chapman (mW/m2)
The perils of
outflow plumes!
(Roosevelt HS)
Groundwater chemistry
compiled by D. Cole
• What we see in
downhole temperature
and pressure logs in
deep exploration wells
may only represent the
formation conditions at
a few points!
• However we can still
infer significant
characteristics with
careful detective work!
Logging tool being
prepared for “run”
Deep-Well Temperature and Pressure Logs
• Probably the most important data to have
– Temperature and permeability information is essential
for understanding the reservoir (is there a thermal
resource, and can you sweep the heat out!)
• Logs can be “static” (i.e. shut-in); or flowing – under
injection or production; sometimes this status is not
recorded…
• The best permeability (i.e. best “feed zone”) in the well
dominates the flow regime (shut-in or flowing) – gives
indication of formation pressure
• May see inter-zonal flow as isothermal sections on
temperature logs (sometimes difficult to decide whether
flow is up or down)
Hot water is less dense than cold water: pressure profiles in wells depends on the
fluid state in the well bore, which in the case below is equilibrating with the reservoir
at 500 m depth (and 35 bar).
Note the pressure pivot, and the difference in water “level” in the well.
Pressure (bar)
0
10
20
30
40
0
100
hot water (200 ºC)
200
casing
300
cold water
Depth (m)
400
500
600
700
800
900
1000
pressure control point at
500 m (35 bar)
steam
50
60
70
80
These pressure measurements are
really important for figuring out the
hydrology of the system: is high
permeability extensive and
controlling a large reservoir? Or is the
region of uniform pressure limited?
The Roosevelt reservoir seemed to
be over-pressured with respect to
the ground surface in its natural
state; it may have been in
equilibrium with regional cool
groundwater at about 3 km depth
(10,000 ft. )
The Cove Fort reservoir is underpressured with respect to the
local groundwater, but seems to
be connected to the Twin Peaks
region 10 – 15 km to the north –
there is a dilute warm spring in
that location
Conceptual Model – Awibenkok, Indonesia
(volcano-hosted)
Salak volcano
summit
2000
AWIBENGKOK GEOTHERMAL FIELD
Elevation (masl)
1500
1000
Cl Spring
hydrostatic from
field elevation
500
Reservoir
Temperature
Reservoir Pressure
0
-500
west
wells
northeast
wells
hydrostatic from
adjacent valley floor
-1000
-1500
0
50
100
150
200
250
Pressure(bar g); Temperature (oC)
300
Allis, 1999
350
DownHole Combined Pressure/Temperature
• Major loss zone (and rapid heat up)
at 1410 m depth
0
25
50
Pressure [bara]
75
100
125
150
175
300
350
100
• Minor loss zone at 1200 m depth,
and possibly minor loss zone at well
bottom
Vertical Depth [m]
• Pivot in pressure curves as liquid in
wellbore heats up and becomes less
dense – formation pressure at 1410 m
depth is 121 bar gauge
• Temperature here is at least 300°C
(Possibly conductive gradient down to
about 1000 m depth)
• Note under injection the cool water
level is at about 200 m depth (water
was being poured in); however heated
water column probably has +ve WHP –
implying reservoir outflows as hot
springs
• Note also under injection, the
pressure in the wellbore exceeds likely
formation pressure below pivot
200
300
400
500
600
Heating 1 week
700
800
900
P
under injection
1,000
1,100
T
T
P
1,200
1,300
1,400
1,500
1,600
1,700
1,800
0
50
100
150
200
Temperature [°C]
KA41 PT 20/12/2005 04:50 Well Temperature
KA41 PT 27/12/2005 12:57 Well Temperature
250
KA41 PT 20/12/2005 04:50 Well Pressure
KA41 PT 27/12/2005 12:57 Well Pressure
What would the discharging profile look like for a geothermal
well?
Pressure (bar)
0
10
20
30
40
50
60
70
80
90
0
well 28-3, Roosevelt Field, UT
100
200
static 5/10
steam
300
casing
Depth (m)
400
500
600
Steam zone
pressure control
700
liquid
800
900
1000
1100
1200
Somewhere down
here, liquid zone
pressure control
100
Example of a discharging geothermal well
Pressure (bar)
0
10
20
30
40
50
60
70
80
90
0
well 28-3, Roosevelt Field, UT
100
200
static 5/10
steam
300
flowing 8/09
Depth (m)
400
casing
500
600
700
800
900
1000
1100
1200
depth of first boiling
(flash) about 690 m
liquid
Hypothetical
pump
100
What happens when we draw down a cold water
well?
Pressure (bar g)
0
10
20
30
40
50
60
0
undisturbed
pumped
100
Depth (m)
Draw down = 5 bar (50 m)
200
casing
300
400
reservoir
500
What about a geothermal well?
Groundwater Analogy:
Shape of cone of depression
depends on the permeability of
the aquifer (reservoir)
Roosevelt Geothermal
Field: 25 years of
production, recently at
35 MWe level.
About 75% of the
produced mass in
injected back into the
reservoir.
There has been pressure
decline of about 35 bar
(500 psi), and an increase
in steaming ground
activity.
Pressure (bar g)
0
10
20
30
40
50
60
70
80
90
0
250
Natural State
1000
3-1
casing
2000
steam zone pressure decline
750
3000
1000
deep liquid
pressure decline
4000
1250
1500
0
250
500
750
Pressure (psi g)
1000
1250
Depth (feet)
500
Depth (m)
Roosevelt system has
developed a steam
cap due to the
production-induced
pressure decline, and
it now is an attractive
target for increased
power generation
3-1 1975
3-1 05/02/1988
3-1 05/31/2001
3-1 11/09/2010
58-3 06/30/2008
100
0
Pressure (psi g)
0
200
400
600
800
1000
1200
1400
1600
2000
Roosevelt Hot Springs – Blundell
Power Plant (Pacificorp Energy)
ground level at plant
5720
Roosevelt hot
springs
52-21
Elevation (m asl)
1000
Initial Pressure Regime
(hot hydrostatic)
4720
72-16
35-3
82-33
3-1
Initial reservoir pressure (1976 - 1984)
(slope = 0.82 bar/m ≈ 240 C water)
3720
27-3
2720
54-3
9-1
500
1720
720
13-10
82-33
0
14-2
-280
52-21
12-35
25-15
-500
-1280
-2280
-1000
0
Pressure (bar g)
50
100
150
200
Pressure (bar g)
0
50
100
150
200
250
2000
Roosevelt hot springs and Opal Mound
54-3
5720
3-1
52-21
1500
4720
Initial reservoir pressure (1976 - 1984)
9-1
3720
Elevation (m asl)
58-3
28-3
500
13-10
2720
54-3
1720
58-3
720
71-10*
0
-280
-1280
-500
* based runs in
2008 and 2010,
both with problems
-2280
-1000
-3280
0
500
1000
1500
2000
Pressure (psi g)
2500
3000
3500
4000
Elevation (ft asl)
25-15
1000
Final Pressure Regime
(hot hydrostatic; but
drawn down by 35 bars, or
500 psi)
-3280
250
Elevation (ft asl)
1500
Drill Stem Tests:
Standard test in
oil exploration
drilling to
detect
permeability,
pore fluid
composition,
static
pressure…
Example of a drill stem test from well Rocky Ridge 33-1 drilled by Python Ag LLC in 2010 (report
accessed at http://oilgas.ogm.utah.gov/wellfiles/027/4302750001.pdf ). In this case the interval
tested is between 6648 and 6818 feet depth, the formation pressure derived from shut-in
pressures is 2810 psi (absolute), and the temperature at that depth is 195°F (91°C).
Pressure trend with depth
showing normally pressured
gradient (0.43 psi/foot),
significant overpressures at
depth, and comparison with a
hot hydrostatic trend in a
geothermal system with a
temperature of 220°C (428°F)
(modified from Nelson, 2003).
Two “apparent pressure” points
are shown, which can be
excluded when considering the
trend from the other data.
In both basins in The Rockies, high permeability and normal (hydrostatic)
pressures were found in the Leadville carbonate at 5 – 7 km depth.
Temperatures were in the range of 200 – 240°C at these depths.
1989
Piceance Basin, Colorado
Wilson et al., 1998
Wind River Basin, Wyoming
20 – 40 MMcf/day
68% C2H6, 20% CO2, 12% H2S
68% C2H6, 20% CO2, 12% H2S