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

F I g u r E 2 . 4 
Geothermal Well Head and Silencer
Source | NEA 2011. Orkustofnun, the Icelandic National Energy Agency. Photo of wellheads and silencer at Hellisheiði power plant.
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Production capacities of less than 5 MW per well and sometimes capacities as low as 2-3 MW per well can be considered satisfactory in 
some cases, depending on the project size and other circumstances. However, for utility scale geothermal projects, wells yielding less than 2 
MW of power are usually considered unsuccessful. 
Phase 5: Field development 
Phase 5 marks the beginning of the actual development of the power project and consists of drilling 
production and reinjection wells, and partially constructing the pipelines to connect the wells to the 
plant. Depending on the drilling program, one or more drilling rigs are required to drill the production 
wells necessary to reach the targeted capacity of the power plant. For a utility size geothermal project, 
a commonly used rule of thumb is that every successful production well will provide enough steam or 
fluid to produce 5 MW of electrical power in the power plant.
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However, even in well-explored areas, 
approximately 10 to 30 percent (on average 20 percent) of all drilled wells turn out to be dry or too 
weak to utilize. This reduces the actual average output of every drilled well to 4 MW. 
In addition to production wells, reinjection wells must be drilled to return the geothermal fluids to the 
reservoir. Reinjection of geothermal fluids produces pressure support to the reservoir; nevertheless, 
reinjection must be undertaken in locations where it will not lead to cooling of the geothermal reservoir. 
This requires knowledge of the underground flow patterns, which is gained through construction of the 
conceptual and numerical models of the reservoir and from the numerical reservoir analysis. Design of 
production and reinjection strategies is studied initially through reservoir simulation. 

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C h a p t e r 2
The time needed to drill a geothermal well not only depends on a well’s depth, but also its geology 
(rock) and the capability of the drilling rig used. Shallow fracture areas will require extra cementing 
to fix the well casings (steel pipes) to the surrounding formations to prevent fluid leakage. These 
operations can cause uncertainty about the total time required for the drilling program. In volcanic 
environments, drilling a 2,000-meter deep well with a commercial diameter will take 40 to 50 days, 
on average. The drilling process itself consists of alternating phases of drilling and well casing 
construction and cementing, until the top of the resource is reached. Once the well penetrates the 
geothermal reservoir, permeable slotted liners are used to prevent rocks and debris from getting into 
the wellbore. In addition to casings, materials required for geothermal drilling include drill pipes, drill 
bits, chemicals to add to the drilling fluid or mud, cement, fuel, tools for directional drilling, wellheads, 
valves, etc. 
The following example explains issues related to costs and investments in this phase. If the project 
developer plans to develop a power plant with an installed capacity of 50 MW, it may need 13 wells 
for production. Reinjection might work with half that number, but would depend on the enthalpy and 
chemical composition of the fluids, which is only known after the wells have been tested. Initially, the 
project developer would plan to drill a set of 13 production and 7 reinjection wells (altogether 20 wells). 
B O x 2 . 1
Differences between Drilling for Oil and Geothermal
There are four reasons why drilling and reservoir management in the geothermal sector is different from the oil 
sector: