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

Phase 3: Test drilling 
This phase is the last one of the exploratory phases. In the beginning of this phase, a drilling program 
is designed to develop a target to confirm the existence, exact location, and potential of the reservoir. 
Usually a set of three to five full size geothermal wells
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are drilled, but, depending on location, 
accessibility and infrastructure at the geothermal field, it might be prudent to start with slim holes 
(holes with diameter under 6 inches/15 cm that can be drilled with lighter equipment (drilling rigs)
than that used for full size wells (with diameter over 8 inches/20 cm). In this context it is also worth 
mentioning that drilling plans have to be revised regularly during the drilling activity due to results from 
well testing. Drilling slim holes for reservoir confirmation, temperature, and chemistry, is becoming 
more attractive as such wells can be drilled to 1,500 meters at approximately 50 percent of the cost of 
a similar depth regular well (Johannesson 2011, personal communication).
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Based on ÍSOR 2009.
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For example, a full size well could be 1.5 to 3.5 km deep and have a bottom hole diameter of 7 to 8 inches. The top (surface) diameter can 
be over 20 inches.
fault and thereby further increase, even multiply, the steam or fluid production of the
geothermal well. Seismic exploration is more effective within sedimentary basins than in
volcanic areas, where the best hydrothermal resources are found. This is sometimes
considered a limitation on the use of seismic exploration for geothermal resources. 

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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 
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Photo of drilling rig Sleipnir from Iceland Drilling Ltd. on borehole WW-03 in Dominica 
(ÍSOR (Iceland Geosurvey
).
Drilling full size wells requires mobilizing heavy equipment of several hundred tons (Figure 2.3),
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transported in many dozen containers. At this stage, no final decision is made as to whether these 
wells will be used as production or reinjection wells, since the developer cannot predict the future 
performance of the wells. New wells might have to be “stimulated” after drilling in order to remove any 
mud or other material that clogs cracks or faults in the rocks. The purpose of stimulation is to increase 
permeability and volume flow of the geothermal fluids or steam into the borehole. Interference tests 
between the different boreholes will show if and how the wells are interconnected. This gives scientists 
a clearer picture of the potential, shape and size of the reservoir in the subsurface, as well as a clearer 
understanding of the potential for premature cooling of production wells. Directional drilling, a cost 
intensive technology from the oil and gas industry, can hit multiple fractures in the same well, thereby 
increasing or even multiplying the well output.