Now, in the year 2007, ten high-temperature
geothermal areas in Iceland have been explored by
deep drilling and the generation is 420 MWe and 450
MWt are at six sites. The drilling effort has accelerated
in the last few years as the geothermal industry has
found a new market in selling electricity to large
aluminium smelters. Previously the local market for
district heating, electricity and steam to industry was
the driving force.
Drilling into high-temperature geothermal reservoirs in
Iceland started in 1940. Some 200 HT wells have been
drilled which fit the definition of exceeding 200°C at
1000 m depth. Intensive drilling started half a century
ago and now four drilling rigs are active drilling such
wells to depths of 2000-3000 m, half of them
directional. These wells are of two basic designs, either
having 9-5/8“ or 13-3/8“ production casing. There are a
total of four overlapping casing strings, each one
cemented in place, and the open hole either has a
slotted liner or is „barefoot“. The depth of each casing
string overlaps roughly about 1/3 of the intended depth
for the interval being drilled for safety. Most wells are
very productive, capable of yielding 5-15 MWe of
electricity, but the best ones 20 MWe. The latest power
plants, at Hellisheiði 90 MWe and at Reykjanes 100
MWe, require only 6 and 10 production wells
respectively. All or part of the wastewater is reinjected
into wells at all sites, each one capable of accepting up
to 250 l/s.
Initially, there was not much knowledge available on HT
drilling initially, but over the years the technology has
improved and lessons have been learned. The
presentation will focus on the successes as well as the
most serious difficulties that relate specifically to the
geothermal environment such as: blow-outs inside the
well or outside, casing damages, wellhead failures,
cementing, material selection, coring. Case histories are
presented. The successes in geothermal drilling have to
do with much faster drilling and good well outputs. The
time to drill a well has been reduced by the use of
modern drilling rigs and equipment, mud motors and
aerated drilling fluids and less time is spent on
cementing loss zones. The structure of the drilling
contracts, standardization of well design and
professionalism of the drilling crews are important
factors in reducing the time to drill a well. A drilling
contract was awarded with 50 wells, based on
international tendering. Geothermal utilization in
Iceland has thus become an industry where
reproducabel results are obtained and risks have been
reduced. It now takes some 35 days to drill down to
2500 m, including the time for rig transport.
The HT wells typically encounter temperatures well
above 300°C without drilling problems, even with mud
motors and MWD equipment, as the well temperature
stays below 100°C due to mud/water circulation. There
is rapid heating-up once he circulation stops but there
is usually a time-window within which logging tools can
be run. As the rigs are reaching ever greater depths
the temperature can be expected to increase and now
the Iceland Deep Drilling Project (IDDP) will target
zones where supercritical temperatures are to be
expected (>374°C for fresh water, higher for saline).
Drilling in temperatures of 400-500°C poses a new
challenge.
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