Cementing of casing strings in geothermal wells is one
of the most critical operations to insure safety and long
life of the wells. In high temperature wells there are
usually three or four casing strings that are cemented
for the full length, the deepest ones typically in Iceland
from surface and reaching down to 700-1300 m. The
purpose of the cement is to support the casing, isolate
the formation and provide corrosion protection. If the
cement bond is inadequate the casing thermal
expansion, normally constrained by good cementing,
will result in elongation that may exceed what the
wellhead is designed for. If the cement is poor steam
seepage to surface outside the well may become a
problem. A special case is collapse in the casing due to
external pressure that occurs if water or poor cement
becomes trapped in the annulus but this can also occur
deep in the well during the cementing operation.
The cementing operations in Iceland are carried out by
the drilling contractors and their drilling crews. The
inner-string cementing method is the rule and
frequently topping up when no returns are received or
the cement level goes down. This is due to losses of
cement to the formation or through normal losses or
through fracturing. The loss of circulation policy, while
drilling the cased portion of the wells in Iceland, is only
to stop the drilling and cement large loss zones (>10
l/s). The smaller losses are drilled past and attempts
made to heal with loss of circulation material (LCM). If a
large loss zone is near the target depth, they are not
cemented. The first cementing attempt is to pump the
cement slurry through the drill string (inner-string
method) and up the annulus to surface. Based on
Cement Bond Logs (CBL) the cement is topped-up by
squeeze cementing or through a „spaghetti“ string
where two small- diameter pipes extend to the top of
cement. In case of large loss zones the well is as a first
step cemented up to the loss zone and the loss kept
open by clean water pumped simultaneously down the
annulus. After a short time for the original cement to
set, the next step is to „squeeze cement“ by pumping
the cement down to the loss zone. Repeated CBL logs
have shown this procedure to result in continuous
cement support for the whole casing string.
The conventional procedure places great pressure on
the formation, with the danger of creating new
fractures. Another problem is the collapse force on the
casing from the slurry having a density o f 1.65 g/cm3.
In most cases this collapse force during inner-string
cementing is what determines the required casing
thickness. Most wells have relatively large losses when
it comes time to pump the cement slurry (5-20 l/s).
Adding LCM and the large expanded perlite particles
frequently does a good job of healing the losses,
especially the ones caused by thermal fractures that
tend to open up while the well is being cooled and
running of the casing.
To overcome many of these problems two tests were
made at Reykjanes in 2004 with what is referred to as
top-down cementing or reverse circulation cementing.
Then all of the cement slurry is pumped down the
annulus and the displaced water allowed to flow up
inside the casing and out through a cement head on
the top of the casing. This flow is throttled by two
choke valves to match the cement slurry volume being
pumped. Water meters were placed on the return
flowlines and there is also a totalizer on the cement
pump to allow proper control of the chokes. There is
therefore no „free fall“ of the slurry and a backpressure
built gradually up at the cement head from the weight
of the cement column, as is to be expected in this U-
tube arrangement. The cement slurry is pumped to the
annulus through the kill-lines on the wellhead with the
annular BOP´s closed in order that no air is sucked in. A
casing shoe without a float valve is used to allow return
flow and no drill string was used as in the inner-string
method. A milk tracer is used to give notice that the
cement had reached the casing shoe. Reversing of the
pressure build-up at the wellhead also reveals when
the cement has reached the shoe and starts traveling
up inside the casing. Both of these experiments were
successful and required small topping up jobs. For a
very short period early in the cementing job, until the
choke was properly adjusted, there was more water
coming out through the choke valves than cement
slurry being pumped in. Balance was quickly regained
so the vacuum break did not last long (suction in the
annulus). As the pressure on the wellhead goes up as
the job progresses the choke needs to be adjusted to
insure that the volumes are in balance. Instrumentation
is required to allow proper flow control. A CBL log
showed good cement bond, except for a short section
in the annulus between the two casings. As there can
be no external water in this part and the cement
density was adequate, no explanation has been found.
This may be an inherent problem with the way the CBL
is made, for example the casing was not pressurized
while the log was run.
Longer casings will have to be run in the future and
loss zones are frequently encountered. Therefore it is
likely that the reverse circulation method will find wider
use. The main benefits for geothermal wells are the
following:
• Wells with loss zones can be cemented as
gravity aids the flow and less pressure is exerted on
the formation.
• Less collapse pressure exerted on the casing
as compared to inner-string cementing. This is because
the cement column pressure is partly being balanced
inside the casing by the wellhead pressure. The wells
can thus be designed with thinner walled casings.
• Long casing strings can be cemented without
reverting to 2-stage cementing.
• Cement slurry will be in the annulus between
the two casing strings. The slurry tail can be of high
density to insure that there be no free water which can
result in a casing collapse.
• No float equipment (float shoe and float
collar) is required.
• Less time spent on cementing as the drill
string does not have to be tripped in for an inner-string
job.
• Low pressure cement slurry pumps can be
used as there is no pressure build-up (no pressure or
suction in the annulus).
• There is less heating-up of the cement slurry,
allowing longer pumping times or less use of retarders.
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