Stimulation operations are commonly 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. The purpose 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 methods most often used involve applying
massive and/or high-pressure water injection, often through open-hole packers, or
intermittent cold water injection into high-temperature wells, aimed at causing
cracking through thermal shocking. The stimulation of wells through acidizing is in
its infancy, however. Stimulation operations are most commonly applied for a few
hours to a few days while the drill rig is still on location. In some instances
stimulation operations have been conducted for several months. The stimulation
operations often result in well injectivity, or 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 but seismic
monitoring is not commonly applied. Yet, examples are available where long-term
water injection has caused a marked change in seismic activity as well situations
where long-term high-pressure injection has caused no microseismic activity at all.
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