Geothermal handbook: PlanninG and Financing power Generation t e c h n I c a L r e p o r t 2 / 2

A N N E x 2 , T A B L E 1
 
Optimal Decisions for Different Test Costs and Well Success Probabilities
PROBABILITy OF 
SUCCESS
COST OF TEST
OPTIMAL DECISION
NET ECONOMIC BENEFIT
0.2
50
Test
10
0.6
50
No test, invest
140
0.2
65
No test, no invest
0
0.6
30
Test
150
0.2
30
Test
30
Source | Robert Bacon.

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129
A N N E x 2 , B O x 1
Drilling Success Rate for Geothermal Wells
Sanyal and Morrow (2012) estimated that at least 4,000 geothermal wells have been drilled to date globally. They 
had sufficient information to analyze 2,528 geothermal wells in 52 fields in 14 countries and found that the mean 
success rate was 68 percent. Analysis of results for individual fields showed that the mean success rate stabilized 
fairly quickly after broad fluctuations for the first few trials. For example, in the Kamojang field in Indonesia, where 
a large number of wells have been drilled, the success rate settled down after 5 or 6 wells and then slowly rose to 
an asymptotic value around 70 percent as shown below in a figure taken from a study by Geothermex prepared for 
the World Bank in 2010.
Average Drilling Success Rate vs. Number of Wells Drilled in Kamojang 
Field, Indonesia 
Source | World Bank/PPIAF, 2010.
Sanyal and Morrow also present aggregate statistics field by field as shown below, indicating that a substantial 
majority of fields produced success rates above 50 percent.
Source | Sanyal and Morrow 2012.
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0
0 
10 
20 
30 
40 
50 
60
70 
80 
A
verage Dr
ill
in
g 
S
uc
ce
ss
R
at
e (%)
Number of Wells Drilled
SUCCESS RATE
NUMBER OF FIELDS
SUCCESS RATE
NUMBER OF FIELDS
33% - 40%
4
71% - 80%
10
41% - 50%
5
81% - 90%
8
51% - 60%
9
91% - 100%
5
61% - 70%
11
A n n e x 3

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130
G e o t h e r m a l H a n d b o o k : P l a n n i n g a n d F i n a n c i n g P o w e r G e n e r a t i o n 
A N N E x 3
A N I L L u S T r A T I v E C A S E O F g O v E r N M E N T C O S T - S h A r I N g O F