Stimulation operations are an integral part of the completion programs of 
geothermal wells drilled in the basaltic environment of Iceland, for high-
enthalpy as well as low-enthalpy wells. These are most commonly applied for a 
few hours to a few days, while the drill rig is still on location, although much 
longer stimulation operations have also been conducted. The purpose of the 
operations is to enhance the output of the wells either by improving near-well 
permeability that has been reduced by the drilling operation itself or to open up 
hydrological connections to permeable zones not intersected by the well in 
question. The method most often applied to low-enthalpy wells involves 
applying massive high-pressure water injection, often through inflatable open-
hole packers. This method is not as commonly applied as was the case 2-3 
decades ago, partly because air-lift aided drilling has reduced the need for such 
stimulations. This method still has great potential in particular cases, however. 
The method most often applied to high-temperature production wells involves 
cyclic cooling and heating, through intermittent cold water injection, aimed at 
thermal shocking and fracturing. This method has proven to be effective; 
especially when the stimulation period can be extended for several weeks after 
the drill rig has been removed. The stimulation of wells through acidizing is not 
commonly applied in geothermal wells in Iceland, however. The stimulation 
operations often result in well injectivity/productivity being improved by a factor 
of 2-3, while in some instances no improvement is observed. Emphasis is placed 
on careful reservoir monitoring during stimulation operations in Iceland.

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