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: baralp@tpao.gov.tr, serhat@metu.edu.tr

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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