5. ¿What’s in this lecture for you?
• You might be part of the team that eventually has to
transform the huge volumen of data into a number
(reserves size)
• You might have to assess the work and results of a
third party, understand the reasons for her proposals
• Understand the overall goal of your work in geotermal
exploration. How will my work be used to make
decisions?, how can I facilitate effective use of
information?
• Understand uncertainty magnitude in this kind of
assessment. Error bars in results
11. The underlying concept is the specific
heat capacity
𝑐 =
∆𝑄
𝑚∆𝑇
∆𝑇 =
∆𝑄
𝑚𝑐
Scienceblogs.scom
12. The energy we use comes mainly from
the hot rock
Example
• Volume 5 km3 (5x109 m3)
• Saturated with liquid water
• Porosity 0.1
• Initial temperature 250 °C
• Final temperature 180 °C
13. The energy we use comes mainly from
the hot rock
Qw = mw cw (Tr – T0)
= Vw φ ρw cw (Tr – T0)
= (5x109 m3)(0.1)(800 kg/m3)(4.82 kJ/kg°C)(250-180 °C)
= 1.35x1014 kJ
QR = VR (1-φ) ρR cR (Tr – T0)
= (5x109 m3)(1-0.1)(2700 kg/m3)(0.9 kJ/kg°C)(250-180 °C)
= 7.65x1014 kJ
Heat from water
Heat from fock
14. The energy we use comes mainly from
the hot rock
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Heat from water Heat from rock
Storedenergy
15. A 100 MWe plant will "empty" the
reservoir 2+ times
Si h=1085 kJ/kg (liquido), Psep = 7 bar-g, xsep=0.17:
Vfluid = (5x109 m3)(0.1) = 5x108 m3
Required steam (2 kg/s-MWe)(100 MWe) = 200 kg/s
Required extraction (200 kg/s)/0.17/(800 kg/m3) = 1.47
m3/s
In 25 years operation, will be required:
Vreq = (1.47 m3/s)(25a)(365d)(24h)(3600s) = 11.6x108 m3
Vreq/Vfluid = 2.32
(Assuming reservoir remains liquid)
16. It is not technically feasible to extract
reservoir energy "to the last drop"
• Imperfect heat sweep, non-uniform, not all
the hot reservoir is permeable/accessible
• Numerical simulations in ideal conditions
(intergranular heat sweep) estimate
theoretical maximum close to 50%
• Qwh = Qr+w Rf Recovery factor
• In practice significantly lower (<20%)
17. It is not technically possible to convert
all thermal energy to electrical
• Maximum theoretical efficiency in any thermal
machine given by Carnot. Real efficiencies
significantly lower (f(h), 5~15%).
η = 1 −
𝑇 𝑐𝑜𝑙𝑑
𝑇ℎ𝑜𝑡
1 −
40 + 273
250 + 274
= 0.4
18. La energía aprovechable se “dosed” en
un período (años) arbitrario
𝑀𝑊𝑒 =
𝑄 𝑟𝑤 𝑅𝑓 η
𝐹 𝑡
F: Capacity factor
t: time [seconds]
Effective operational
time of power plant
All geoscientific data reduced to
2 parameters!
19. Assume hypothetical reservoir and
utilization with these characteristics:
Assume Carnot efficiency
What power plant size would you
recommend?
Area A = 8 km2 Thickness h = 1
km
Tr = 280 °C
T0 = 180 °C Rho rock = 2600
kg/m3
C rock= 0.9
kJ/kg°C
Rho water= 750
kg/m3
C water = 4.8
kJ/kg°C
Rf = 0.15
Fp = 0.9 t = 30 year Tcold = 40 °C
Φ = 0.1
Formulas:
Qw = VR φ ρw cw (Tr – T0)
QR = VR (1-φ) ρR cR (Tr – T0)
Qrw = Qr + Qw
η = 1 −
𝑇 𝑐𝑜𝑙𝑑
𝑇ℎ𝑜𝑡
𝑀𝑊𝑒 =
𝑄 𝑟𝑤
𝑅𝑓 η
1000 𝐹 𝑝
𝑡
20. Qw = (8x1x109 m3)(0.1)(750 kg/m3)(4.82 kJ/kg°C)(280-180 °C)
= 2.892 x 1014 kJ
Qr = (8x1x109 m3)(1-0.1)(2600 kg/m3)(0.9 kJ/kg°C)(280-180 °C)
= 16.85 x 1014 kJ
Qrw = 2.892 x 1014 kJ + 16.85 x 1014 kJ
= 19.74 x 1014 kJ
η = 1 −
40+273
280+273
= 0.43
𝑀𝑊𝑒 =
(19.74 𝑥 1014) (0.15) (0.43)
1000(0.9)(30)(365)(24)(3600)
= 149 MWe
21. Stored heat method assumes no heat
recharge
• Can be valid, but not always. Think of heat
extraction rate vs. natural heat recharge rate.
• Conservative assumption
23. Area: surface features, geophysical
anomalies, well temperature contours
Fumarole
Contour 5 ohm-mChloride
spring
24. Thickness: conductive layer (MT),
measured well temperatures, drilling
cost
50 °C
220-260 °C
Caprock
Reservoir
Expensive
drilling!
2500 m
25. Thickness: conductive layer (MT),
measured well temperatures, drilling cost
Reservoir
Reservoir
T < 180 °C???
T < 180 °C
26. Recovery factor is (perhaps) the most
uncertain parameter
Porosity φ
Rf Linear
Interpolation
Rf = 2.5φ
Nathenson, 1975. Numerical
simulation heat sweep.
Φ=0.2, Rf~0.5
No porosity:
No intergranular sweep
Starting point.
May be
modified at
discretion
Φ=f(depth)
28. T0: reservoir temperature, conversion
technology
NetMW
180 220 250
Temperature °C
Sanyal et al., 2007
100 160 220
Temperature °C
NetMW
0816
NetMW
059
Self-flowing (flash) Pumped (binary)
180 °C 125-140 °C
51. Sometimes, estimates can be
conservative or realistic...
Berlin
1982:
55 MWe (numérical)
150 MWe (volum.)
1993:
Proven 55~65 MWe
Probable 100 MWe
2003:
90 MWe OK
(numérical)
52. … But some challenges are difficult to
anticipate…
P90: 87 MWe
P50: 140 MWe
105 MWe
(flow tests)
Bjornsson, 2008
U1
U2
U3B
Reality:
• Scaling problems
• Reservoir cooling
Momotombo
53. … Therefore, staged development has
advantages (and disadvantages)
Steingrimsson et al., 2005
54. Other reserves estimation methods
exist, but not standard
• Heat flow method correlates reserves with natural surface heat
discharges.
• Lumped parameter model are a super-simplified version of numerical
simulation, needs production data as well, but less inputs
• Decline curves have been used mainly in vapor-dominated reservoirs
• Areal analogy method gives an interesting (realistic) reference point.
• Plane fracture method (Bodvarsson, 1974) useful in reservoirs mostly
confined to large fractures/faults
55. Summary
• 2 standard methods for reserves estimation area stored heat (with Monte Carlo)
and numerical reservoir simulation
• Stored heat is quick, simplified (over-simplified?), all exploration data reduced to 2
parameters
• Numerical simulation more detailed, more parameters, requires production data
• Other methods available, application not standard
• The base for most methods is the conceptual model
• All methods have significant uncertainties >> staged development