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Drilling cost effectiveness and feasibility of high-temperature drilling - Reykjavik, Iceland, Workshop4
Drilling cost effectiveness and feasibility of high-temperature drilling - Reykjavik, Iceland, Workshop4
1-5 July 2007 ISOR
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Top-down cementing of geothermal wells in Iceland
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 
•	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 
•	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.
Id: 36
Place: ISOR
Grensasvegur 9
Room: Vidgelmir
Starting date:
03-Jul-2007   09:40
Duration: 20'
Primary Authors: Mr. BIRKISSON, Sturla (Iceland Drilling Company LTD)
Co-Authors: Mr. THORHALLSSON, Sverrir (ISOR)
Presenters: Mr. BIRKISSON, Sturla
Material: slides Slides

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