PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, February 11-13, 2013
SGP-TR-198
UTILIZATION OF SUPPLEMENTARY CEMENTITIOUS MATERIALS IN
GEOTHERMAL WELL CEMENTING
Baris Alp1, Serhat Akin
2
1Turkish Petroleum Corporation, Research Center,
2180 Street No: 84 06100, Cankaya, Ankara, Turkey 2Middle East Technical University, Petroleum & Natural Gas Eng Dept,
Dumlupinar Blvd No: 1 06800 Cankaya, Ankara, Turkey
e-mail: [email protected], [email protected]
ABSTRACT
In high temperature geothermal wells commonly
conventional cement slurries based on silica blended
mixes, are prepared to catch up with the required
mechanical properties (compressive strength,
thickening time, fluid loss, etc.) for the fresh and
hardened cement slurry. Typically, 35 to 40 percent
silica is added to blends to decrease the Ca/Si ratio of
cement slurries to 1 in order to prevent retrogression
in the physical and chemical properties above
temperatures of 230 ºF (110 ºC). Ground granulated
blast furnace slag (GGBFS) has a Ca/Si ratio lower
than 1 and thus silica does not need to be added. The
hydration of GGBFS blended cement slurries are
improved at elevated temperatures that has vital
importance when drilling wells in high temperature
conditions.
This study presents an experimental study to
investigate the applicability of GGBFS in cementing
of geothermal wells. The materials used in the
analysis are API Class G cement, silica flour,
GGBFS and sodium silicate (water glass). In addition
to these materials, some chemical additives are used
to provide fluid loss control (as fluid loss control
agent), to arrange setting time (as retarder) and flow
properties (as dispersant). Compressive strength by
ultrasonic cement analyzer, HPHT (high pressure
high temperature) static fluid loss, and thickening
time analyses are conducted. The temperature of the
analyses and/or the curing temperature of cement
slurries conducted are 194 F (90 ºC), 248 F (120
ºC) and 374 F (190 ºC) which correspond to typical
low to high temperature geothermal wells. It has
been found that GGBFS improves compressive
strength of the set cement at high temperatures.
Presence of GGBFS in the slurry decreases fluid loss
amount and increases setting time when compared to
conventional silica blended cement slurries. GGBFS
is a byproduct of iron industry and its cost is
generally quite lower than Class G cement.
Utilization of GGBFS in geothermal wells is not only
economical but also environmentally appropriate.
INTRODUCTION
Hydrothermal Hydration of Portland Cement
Restricting movement of fluids between formations,
keeping casing in place and preventing corrosion e.g.
from saline and sulfated underground water are
important tasks accomplished by cementing. It is
typical to use API Class G cement with additives to
control properties of fresh or hardened cement paste
(also called as cement slurry in oil well cementing)
such as compressive strength, fluid loss control,
consistency and thickening time. Silica flour is added
to prevent strength retrogression. Bottom hole
temperature in a geothermal well can be as high as
700°F (370 ºC), and the formation brines are often
extremely saline and corrosive. As a result,
geothermal well cement should withstand higher
temperatures and tackle more aggressive
environments than those encountered in oil and gas
wells.
Hydrothermal Hydration of Portland Cement
In the hydration of Portland cement (PC) at elevated
temperatures, hydration rate of C3S increases at early
ages, on the other hand, hydration rate of C2S
increases both at early and later ages (i.e. months
later) especially at high temperatures (Odler, 2004).
The overall hydration rate of Portland cement
increases at elevated temperatures. The hydration
product of Portland cement, C-S-H gel, is
thermodynamically stable up to 110 °C; at higher
temperatures C-S-H gel metamorphose to more stable
structures. Hydration at high temperatures leads to
the formation of highly crystalline silicate hydrates
with more Ca/Si ratio. It takes free lime (CH), which
is already available due to C3S and C2S hydration,
and converts to the phases called mainly “alpha
dicalcium silicate hydrate” (α-C2SH) and / or Jaffeite
(C3SH1.5) (Andrew et al, 2008). α-C2SH is highly
crystalline and much denser than C-S-H gel.
Conversion of C-S-H to α-C2SH occurs with an
associated volume reduction and therefore, is
deleterious to the hardened cement. As a result,
compressive strength and permeability of the
hardened cement is adversely affected by the
formation of α-C2SH (Taylor, 1997; Nelson, 1990).
Hydrothermal Hydration of Portland Cement in
the Presence of Silica
The strength retrogression of cement at high
temperatures can be prevented by reducing Ca/Si
ratio in the cement slurry. It can be reduced to 1.0
with addition of 35 to 40 percent silica by weight of
cement (Nelson, 1990). In the presence of finely
ground silica, pozzolanic reaction takes place and C-
S-H gel tend to form 1.1 nm tobermorite, (C5S6H5)
(Odler, 2004). At temperatures above 150 °C,
tobermorite converts to mainly xonotlite (C6S6H) and
gyrolite (C6S3H2). At 250 °C truscotite begins to
appear and both xonotlite and truscotite are stable up
to 400 °C (Nelson, 1990). Among pozzolans α-quartz
is the most effective pozzolanic material due to its
high silica content and is frequently used in thermal
wells to prevent strength retrogression (Nelson,
1990).
Supplemantary Cementitious Materials
The use of supplementary cementitious materials
(SCM) dates back to the ancient Greeks who
incorporated volcanic ash with hydraulic lime to
create a cementitious mortar. Most concrete mixture
contains supplementary cementitious material that
forms part of the cementitious component. These
materials are majority byproducts from other
processes or natural materials. The major benefit of
SCM is its ability to replace certain amount of
Portland cement and still be able to display
cementitious property, thus reducing the cost of using
Portland cement. Ground granulated blast furnace
slag (GGBFS) is such a material that can be used as a
substitute to Portland cement. Replacement of
GGBFS to Portland cement not only contributes to
waste management but also improves the properties
of fresh and hardened cement slurry.
Pozzolanic Reaction
During cement hydration, CH is liberated as a result
of hydration of calcium silicates. CH does not
contribute to the strength of hardened cement slurries
but decrease chemical resistance of the cement
slurries. In the presence of a pozzolan, silica reacts
with free CH to form more stable cementitious
compounds (called secondary C-S-H). Figure 1
shows the effect of slag content on the CH content of
the hydrated cements by time. CH content can go
down to zero percent with increasing content of
GGBFS in the cement paste due to the pozzolanic
reaction.
Figure 1: Effects of curing age and proportion of
slag on the calcium hydroxide content of
the Portland-slag cement paste (Lea,
1971)
Ground Granulated Blast Furnace Slag
GGBFS has hydraulic setting property and can be
utilized as a substitute to PC to produce slag blended
PC. However, its hydration rate is much slower than
PC at ambient temperatures According to ASTM
C595, slag content in the slag blended cement can be
up to 70 percent (by mass), whereas, EN 197-1
makes limitation of GGBFS in slag-cement blend up
to 95 percent by mass (CEM III/C). GGBFS is the
maximum amount of mineral additive that is allowed
to be used in the cement blends according to EN 197-
1.
The formation of the secondary C-S-H gel in the
cement reduces porosity because of pozzolanic
reaction between cement and GGBFS. Also increased
hydration rate of GGBFS at elevated temperatures
decreases porosity of hardened cement with the
contribution of pozzolanic reaction. The porosity
reduction can be less than five folds when compared
to hardened cement slurries prepared with neat
cements (Figure 2). All GGBFS blended cements
show lower porosity than neat Class G cement paste
at all ages. 60 percent of GGBFS substitution in the
cement paste distinctively decreases porosity. It is
also stated that the pores in the hydrated GGBFS
blended cements are finer than that of the hydrated
neat cements. (Uchikawa, 1986).
Figure 2: Porosity of hardened cement pastes at 80
ºC with w/c ratio of 0.44, G is prepared
with neat Class G cement and S20, S40,
S60 and S80 are PC blended cement
pastes with ratios by mass (80:20),
(60:40), (40:60) and (20:80) respectively,
(Alp, 2012)
The presence of GBBFS in the blend not only
decreases porosity but also increases the compressive
strength of hardened cement pastes at high
temperatures. Figure 3 shows the effect of GGBFS on
the compressive strength of hardened cement pastes.
GGBFS blended cement pastes show higher
compressive strength than neat cement pastes.
Mueller (1995) observed similar results at 24-hours
with GGBFS-PC ratio of 40:60. The compressive
strength of GGBFS blended cement pastes were
higher than reference cement paste at temperatures of
77 ˚C and 93 ˚C at 24-hours. The differences were
even higher at 72 hours.
Figure 3: Compressive strength of hardened cement
pastes by UCA at 80 ºC with w/c ratio of
0.44, G is prepared with neat Class G
cement and S20, S40, S60 and S80 are PC
blended cement pastes with ratios by mass
(80:20), (60:40), (40:60) and (20:80)
respectively, (Alp, 2012)
The hydraulic property of GGBFS can be improved
by activators. Alkali hydroxides and alkali salts are
generally activators, but the most popular ones are
sodium hydroxide, sodium silicate, sodium sulfate,
calcium sulfate and calcium hydroxide. Even
Portland cement can be used as a GGBFS activator.
Alkalis increase the pH of the aqueous solution
which contributes to the dissolution of slag particles.
These activators break of the bonds in the three-
dimensional network of the glass phase of GGBFS
and release the ions of calcium, silica, aluminum and
magnesium. Conventional silica blended cements
can withstand up to 400 ºC (Taylor, 1997; Nelson,
1990), however, alkali activated slag can be used up
to 800-1000 ºC, (Odler, 2000). The chemical
corrosion resistance of alkali activated slag is very
high. It is completely resistant to sodium sulfate and
has high resistance to magnesium chloride and nitrate
attack (Talling and Brandsetr, 1993)
EXPERIMENTAL STUDY
The materials used in this study are API Class G
cement, GGBFS, silica flour and liquid sodium
silicate (water glass). API Class G cement and
GGBFS are obtained from Bolu Cement plant.
Chemical analysis of these materials and
mineralogical composition of Class G cement are
presented in Table 1 and Table 2 respectively;.
Table 1: Chemical composition (%) of materials
Materials
Components
Class G
cement GGBFS
Sodium
silicate
CaO 63.52 32.46 -
SiO2 18.69 39.42 27.61
Al2O3 4.35 13.84 -
Fe2O3 5.19 1.62 -
MgO 1.43 8.34 -
SO3 2.94 0.15 -
Na2O 0.31 0.66 8.95
K2O 0.54 0.92 -
Cl- 0.02 0.01 -
TiO2 - 1.02 -
Mn2O3 - 0.77 -
LOI 2.60 0.07 -
Free CaO 1.79 - -
0
10
20
30
40
0.00 0.50 1.00Co
mp
ress
ive
str
en
gth
as
est
imat
ed
by
UC
A (
MP
a)
Time, day
G S20 S40 S60 S80
Table 2: Mineralogical composition (%) of Class G
cement clinker
C3S C3A 2C3A+ C4AF
Percentages 50.6 1.94 2.75
The specific surface area (Blaine’s fineness) of
GGBFS is 5092 cm2/g which is quite higher than that
of API Class G cement (3220 cm2/g). It is stated that,
an increase in the fineness of slag two to three times
that of normal Portland cement contributes in a
variety of engineering properties such as segregation,
time of setting, heat evolution, better strength
development and excellent durability (Swamy, 1998).
The specific gravity of GGBFS used in the study is
2.86. It is highly vitreous and glassy in structure that
also improves the slag reactivity.
The SiO2/Na2O molar ratio of sodium silicate used in
the study is 3.2. It is stated that SiO2/Na2O is one of
the most important factor that influences hydration of
GGBFS and strength development of slurries at
hydrothermal conditions (Sugama, 2006).
Six cement slurry compositions are prepared. First
composition is the conventional silica flour blended
Class G cement (G-SI). The second and third one is
the blends of GGBFS and Class G cement in different
proportions (G-S1 and G-S2). The forth composition
is the ternary mix of GGBFS, Class G cement and
silica flour (G-S-SI). The fifth composition is
prepared with neat GGBFS (S) and the last one is the
alkali activated GGBFS (AA-S). Silica is added to
slurry BWOC (by weight of cementitious materials;
total of Class G cement and GBBFS). Ratio of water
to solid constituents of the cement compositions are
taken as 0.44 and the compositions are illustrated in
Table 3.
Table 3: Composition of cement slurries, (Alp, 2012)
Cement Slurries
Constituents G-SI G-S1 G-S2 G-S-SI S AA-S
Class G, % 100 50 25 75 - -
GGBFS, % - 50 75 25 100 100
Silica Flour,
% BWOC 35 - - 35 - -
Na2SiO3,
ml/100 gr - - - - - 10
Water, % 44 44 44 44 44 39*
* Less water is added due to presence of water in sodium
silicate
The compressive strength of the cement slurries are
investigated by Ultrasonic Cement Analyzer (UCA).
UCA measures the transit time (second/meter) of
ultrasonic waves through the cement slurry. It is a
non-destructive test method and simulates the
wellbore conditions of temperature and pressure. The
freshly mixed cement slurries are put into slurry cup
and investigated for 24 hours at a constant pressure of
3000 psi (20.7 MPa). The temperature gradually
increases up to 190 ºC (374 ºF) at 240 minutes and
this temperature continues to the end of 24 hours.
In the HPHT static fluid loss analysis, cement slurries
are first conditioned at 100 ºC (212 ºF) in the
atmospheric consistometer for 20 minutes. Then,
recently conditioned cement slurry is put into HPHT
filter cup and a differential pressure of 500 psi is
applied at a static temperature of 150 ºC (302 ºF).
The aqueous phase of cement slurry is forced to filter
out for 30 minutes and the amount of filtrate is noted.
The amount of fluid loss is proportional to the square
root of time. If “blowing out” occurs within 30
minutes then the API fluid loss is calculated
according to Equation 1. (API Spec 10B)
Calculated API Fluid Loss = 2 Qt
√ (1)
Pressurized consistometer is used to measure the
consistency and thickening time (pumpability time)
of the cement slurries under the pressure of 5000 psi
and at a temperature of 248 ºF (120 ºC).
Chemical additives are needed to be used in the
slurries to control fluid loss, consistency and setting
time. Therefore, cement slurries are mixed with fluid
loss additive (Halad-23), dispersant (CFR-3) and
retarder (HR-12). On the other hand, no chemical
additives are used in the compressive strength
analysis. The amounts of chemical additives (Table
4) are calculated by weight of total solid constituents
in the slurry (BWOS).
Table 4: Chemical additives of cement slurries
Chemical Additives, % (BWOS)
Components Halad-23 CFR-3 HR-12
%, (BWOS) 0.7 1.0 0.3
RESULTS AND DISCUSSION
Compressive Strength
Compressive strength of the set cements is important
as it commonly represents the overall quality of
cements. Higher compressive strength generally
means lower porosity and increased durability. The
UCA actually measures the compressibility of
samples, but a previously developed correlation with
compressive strength (Nelson, 1990) is used. Figure 4
shows the time dependant compressive strength of
hardened cements measured by UCA at 374 ºF.
Figure 4: Compressive strength of the hardened
cements measured by UCA
According to Figure 4, conventional silica blended
Class G cement slurry (G-SI) shows moderate
compressive strength. The strength retrogression is
prevented as mentioned in the literature. GGBFS
blended cement slurry (G-S1) showed lowest
compressive strength. The strength increases up to a
threshold point. Then retrogression occurs within the
cement because of exposure to high temperatures. G-
S2 with 75 percent of GGBFS in the slurry shows
comparable results with G-SI. Ternary mix prepared
with Class G cement, GGBFS and silica flour (G-S-
SI) in the 2nd
place with a compressive strength of
nearly 2500 psi in the middle period. However, after
1-day its compressive strength is lower than that of
G-S2 and G-SI. Cement slurries prepared with neat
GGBFS has lowest compressive strength at early
periods however; it is in 2nd
place with a compressive
strength of more than 3000 psi after 1-day. Alkali
activated GGBFS has a compressive strength of
nearly 4000 psi and shows the highest compressive
strength among slurries. Alkali activation clearly
increases both initial and final compressive strength
(after 24 hours) of hardened GGBFS. No strength
retrogression is observed both in S and AA-S, and
also negligible strength retrogression is observed in
GS-2. The compressive strength of S and AA-S tends
to increase gradually after 1-day while the other
slurries go more asymptotic to x axis.
The compressive strength data contained in Table 5
shows time to reach (TTR) compressive strength of
hardened cements to 50 and 500 psi (0.34 and 3.44
MPa), maximum achieved compressive strength
within 24 hours and final compressive strength at 1
day. Despite the high compressive strength of neat
GGBFS after 1-day, it has the highest period to reach
compressive strength of 50 psi and 500 psi. However,
sodium silicate activation clearly decreases these
periods. GGBFS blended cements; G-S1 and G-S2
have the lowest time to reach 50 psi and 500 psi.
Table 5: Parameters of compressive strength analysis
of hardened cement slurries
Cement Slurries
G-SI G-S1 G-S2 G-S-SI S AA-S
TTR 50 psi,
hh:mm 01:56 01:15 01:31 01:16 04:54 03:14
TTR 500 psi,
hh:mm 02:54 02:00 02:16 02:04 07:49 03:33
Max. comp.
strength, psi 2301 1926 2303 2483 3128 3965
Final comp.
strength, psi 2217 766 2265 1751 3128 3965
Thickening Time
The results of the laboratory thickening time tests
provide an indication of the length of time that
cement slurry remain pumpable. Consistency of
cement slurry is expressed in Bearden units (Bc).
Consistency of 40 Bc indicates the maximum
pumpability while 70 Bc indicates the starting of
cement setting. Table 6 shows times to reach (TTR)
40 Bc and 70 Bc of cement slurries at 248 ºF and
under pressure of 5000 psi.
Table 6: Thickening time of cement slurries
Cement Slurries
G-SI G-S1 G-S2 G-S-SI S AA-S
TTR 40 Bc,
hh:mm 02:05 03:49 3.18* 03:40 2:22* NA**
TTR 70 Bc,
hh:mm 02:09 03:53 3.22* 03:43 3:10* NA**
* Without retarder
** Workable cement slurry cannot be achieved.
Lower amounts of Class G cement in the cement
slurry decreases setting time as seen in the Table 5.
Because, decreasing cement amount in the slurry also
decreases the amount of rapid hydrating C3S and C3A
in the blend. The hydration rate of GGBFS is much
slower than cement because it requires alkaline rich
0
1
2
3
4
0.0 0.2 0.4 0.6 0.8 1.0
Co
mp
ress
ive
str
en
gth
, x1
03 p
si
Time, day
G-SI GS-1 GS-2
G-S-SI S AA-S
environment. This alkaline environment can be
provided by releasing lime in the hydration of
cement. Similar to GGBFS, silica flour also needs
lime to form calcium silicate hydrates within set
cement. Therefore, increase of total amounts of
GGBFS and silica flour decreases setting time of
cement slurry. In the neat GGBFS slurry, setting
cannot be achieved within 8 hours but without
retarder its setting time is 3 hours and 10 minutes. On
the other hand, it is not possible to mix workable
alkali activated GGBFS slurry with specified
chemical additives that are given in Table 4.
Fluid Loss Control
A series of tests are conducted to determine fluid loss
efficiency of cement slurries and findings are
contained in Table 7. Fluid loss performance is
better in the GGBFS systems. The increased fineness
of GGBFS improves fluid loss control of the cement
slurry when compared to systems of Class G cement
and silica flour. Even small amounts of GGBFS
replacement in the cement blend contribute to fluid
loss control as seen in the ternary mix of G-S-SI.
However, it is not possible to mix a workable alkali
activated GGBFS with specified chemical additives
that are given in Table 4.
Table 7: HPHT fluid loss of cement slurries
Cement Slurries
G-SI G-S1 G-S2 G-S-SI S AA-S
API Fluid
Loss, cc 96* 31 26 36 22 NA**
* Blowing out at 25 min., calculated using to Eq. 1
**Workable cement slurry cannot be achieved.
Density of the cement slurries are shown in Table 8.
It is possible to make GGBFS blended slurries with
lower density than Class G cement systems. In
addition, water requirement of GGBFS is higher than
neat cement due to its high fineness. Therefore, water
to cement ratio of the GGBFS slurries can be
increased more than 0.44 and density can even be
lower than values in Table 8.
Table 8: Density of cement slurries
Cement Slurries
G-SI G-S1 G-S2 G-S-SI S AA-S
Denstity,
gal/ppcuft 15.5 15.5 15.4 15.4 15.2 15.2
CONCLUSION
Several laboratory tests were conducted to study high
temperature application of ground granulated blast
furnace slag. The results showed that:
It is possible to prepare GGBFS blended
cement slurries with higher compressive
strength than conventional silica blended
cement slurries.
Strength retrogression is not observed in the
neat GGBFS and sodium silicate activated
GGBFS.
GGBFS shows superior performance in
HPHT static fluid loss than Class G cement
and silica flour.
GGBFS and silica flour increases setting
time of cement decreasing the required
amount of retarder used in the cement slurry.
Chemical additives that are used in the silica
blended cement slurries can also be used in
the neat GGBFS slurry and GGBFS blended
cement slurries.
Sodium silicate activated GGBFS slurry
shows the highest compressive strength but
it is not possible to mix workable slurry with
fluid loss control additives.
It is possible to prepare GGBFS blended
cement slurries with lower density than
conventional silica blended cement slurries.
Utilization of GGBFS in geothermal well
cementing is both economical and
environmental friendly.
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