Chilingarian G.V. et al. Surface Operations in Petroleum Production, II
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65
Fly ash, which is a combustion by-product of coal, is widely used as a pozzolan.
It is covered by the API and ASTM specifications.
Calcium hydroxide, wluch is liberated upon hydration of portland cement, does
not contribute to strength or water-tightness and can be removed by leachng. In the
presence of fly ash, however, it combines with the calcium hydroxide, contributing
to both strength and water tightness (Smith, 1976). Depending upon the source,
specific gravity of fly ash varies from 2.3 to 2.7, compared with the specific gravity
of 3.1-3.2 for portland cements. This explains the lower specific gravity of a
pozzolan cement slurry than that of slurries of similar consistency prepared with
portland cement.
Pozzolun-lime cements
Pozzolan-lime or silica-lime cements are mixtures of fly ash (silica), hydrated
lime, and small quantities of calcium chloride (Smith, 1956). Various forms
of
calcium silicate form upon hydration with water. Inasmuch as at low temperatures
the reactions are slower than similar reactions in portland cements, pozzolan-lime
cements are generally recommended for primary cementing at temperatures above
140
F.
According to Smith (1956), the merits of this type of cement include (1) ease
of retardation, (2) light weight, (3) economy, and
(4)
strength stability at high
temperatures.
Resin
or
plastic cements
Resin and plastic cements are used for squeezing perforations, selectively plug-
ging open holes, and cementing waste-disposal wells. They are usually mixtures
of
water, liquid resins, and a catalyst blended with an API Class A,
B,
G,
or
H
cement.
When pressure is applied to the slurry, the resin phase may be squeezed into a
permeable zone, forming a seal within the formation. These cements, which are used
in wells in relatively small volumes, are effective at temperatures ranging from 60
O
to
200°F
(Smith, 1976, p.
11).
Diesel
oil
cements
Diesel oil cement slurries are used when it is necessary to control water in drilling
or producing wells (Hower and Montgomery, 1953a). Diesel oil cements, which are
composed of API Class A,
B,
G,
or
H
cement mixed in diesel oil or kerosene with a
surface-active agent, have unlimited pumping times and will not set unless placed in
a water-bearing zone. In the latter case, the slurry absorbs water and sets to a hard,
dense cement (Smith, 1976, p. 12). Surfactant reduces the amount of the oil which is
needed to wet the cement particles. The reaction or thickening time is extended on
using anionic surfactant. This enables additional penetration into the formation.
Diesel oil cement can be used to
(1)
combat certain lost-circulation problems, (2)
repair casing leaks, (3) plug channels behind the pipe, and
(4)
control slurry
66
penetration (Hower and Montgomery, 195313). Its primary use, however, is that of
shutting off water.
Gypsum
cements
According to Smith (1976, p. 12), gypsum cements are often used for remedial
cementing work. They are available as (1) a hemihydrate form of gypsum (CaSO,
.
H,O),
and (2) gypsum (hemihydrate) containing a pondered resin additive (Smith,
1976, p. 12). Gypsum cement has
(1)
capacity to set rapidly, (2)
high
early strength,
and (3) positive expansion
(=
0.3%). In order to produce thixotropic properties,
gypsum cements are blended with API Class A,
G
or
H
cement in 8-10%
concentration. This combination minimizes fall-back after placement and is particu-
larly useful in shallow wells (Smith, 1976, p. 12). Gypsum cements are sometimes
mixed with equal volumes
of
portland cements to form a permanent plugging
material in the case of lost circulation. Inasmuch as such blends have very rapid
setting properties and could set prematurely during placement, they should be used
cautiously (Smith, 1976). Gypsum is usually considered a temporary plugging
material unless it is placed downhole where there is no moving water, because
of
its
solubility.
Expanding cements
In certain cases, it is desirable to use a cement that expands against the filter cake
and pipe (see Lafuma, 1952; Hansen, 1963). This expansion is caused by the
reactions which are similar to the process described in the cementing literature as
Ettringite,
which is the crystal-forming process that takes place between the tri-
calcium aluminate component in portland cement and sulfates (Lafuma, 1952).
According to Smith (1976, p. 13), the formula of commercial expanding cements is:
3CaO.Al20,.3CaS0,.32H,O.
They are prepared by adding an anhydrous calcium
sulfoaluminate (4Ca0.3Al
,O,.SO,),
lime (CaO), and calcium sulfate (CaSO,). Jour-
nal of American Concrete Institute has described three types of commercial expan-
ding cement: (1) Type K-the calcium sulfoaluminate component is blended with a
portland cement. The linear expansion due to the hydration reaction is approxi-
mately 0.05-0.20% (Klein and Troxell, 1958). (2) Type S-a high C,A cement,
which is similar to API Class A cement, with approximately 10-15% gypsum,
expansion characteristics of which are similar to those of Type K cement (Portland
Cement Association). (3) Type M-small quantities
of
refractory cement are added
to portland cement to produce expansive forces.
Smith (1976) listed several other formulations of expanding cement: (1) API
Class A portland cement containing a 5-10% of the hemihydrate forms of gypsum
(Newman, 1960), (2) API Class A,
G,
or
H
cement containing from 5% to saturation
of NaC1, with expansion being caused by the chlorosilicate reactions, and (3)
pozzolan cements-expansive forces are created due to the formation
of
sulfo-
aluminate crystals when the alkali reacts with the Class
A,
G, or H cement.
61
The expansive force must be measured shortly after the cement sets (base
reference) and then at various time intervals until the maximum expansion is
reached. Hydraulic bonding tests are also used to evaluate the crystal growth of
these cements (Smith, 1976, p.
13).
Latex cement
Smith (1976) pointed out that latex cement
is
a blend of API Class
A,
G,
or H
cement with either a liquid or a powdered latex, which may be polyvinyl acetate,
polyvinyl chloride, or butadiene styrene emulsion. Approximately
1
gal of liquid
latex is added per sack of cement. Latex reduces the permeability
of
cement and
improves the bonding strength of cement. Inasmuch as latex in powdered form does
not freeze, it can be dry blended with cement before transportation to the wellsite
(Smith, 1976).
Calcium aluminate cements
Calcium aluminate cements (refractory) are high-alumina cements prepared by
blending limestone with bauxite (aluminum ore) and then heating the mixture in a
reverberatory open hearth furnace until it is liquefied (Newman, 1960). Smith (1976)
pointed out that the composition of these cements differs from those of portland
cements, because bauxite is used instead of clay or shale. These refractory cements
contain approximately 40% lime (CaO) and small amounts
of
silica and iron. The
high early strength and greater resistance
to
high temperatures and to attack by
corrosive chemicals is due to the formation of calcium aluminate (Smith, 1976, p.
14).
In the in-situ combustion wells, where temperatures may range from 750" to
2000
OF
during the burning process, one can use these cements. These high-alumina
cements can be either accelerated or retarded. The retardation characteristics,
however, differ from those of portland cements. Inasmuch as the addition of
portland cement to a refractory cement causes a flash set, they should be stored
separately (Smith, 1976,
p.
14).
Low-density cement (Spherelite
TM)
Spherelite cement additive (Halliburton Services, 1979) provides a means of
preparing 9-12 lb/gal light-weight cement slurries. Adequate compressive strength
is achieved in a minimum period of time, which is true even if the cement is cured
under relatively low temperatures. The spherelite additive consist
of
hollow, in-
organic spheres, which are competent up
to 6000 psi total exposure pressure
(Halliburton Services, 1979).
Ultra-low density grouting mixtures (7.8-8.8 lb/gal) can be prepared with
high-alumina cement that will yield
=
200 psi compressive strength in 24 hr at
65
OF
(Halliburton Services, 1979).
68
Permafrost cement
There are ice-bearing formations in the Arctic areas that extend to depths of up
to
3000
ft. In some areas they can be described as
frozen earth,
whereas in others as
glacier-like ice blocks (Maier et al., 1971). As a result, cement of conductor and
surface casing in permafrost areas presents many problems. Usually one should use
a quick-setting, low-heat-of-hydration cement that will not generate heat and melt
the permafrost (Smith, 1976).
Gypsum-cement blends and refractory cement blends have been used very
successfully for such low-temperature conditions (see Morris, 1970). In cementing
surface pipe, gypsum-cement slurries are usually designed for two to four hours of
pumpability; however, their strength development is quite rapid and varies little at
temperatures between 20
O
and 80
OF
(Smith, 1976, p. 14). These cement blends can
be retarded or accelerated, setting at
15
OF
below freezing.
ADDITIVES
FOR
OILWELL
CEMENTS
Smith (1976, p. 16) pointed out that there are over 40 additives which are used
with various API Classes of cement to provide optimum slurry characteristics for
most downhole blends
of
cement.
The cement additives may be classified into the following basic groupings:
(1)
accelerators,
(2)
heavy-weight additives,
(3)
light-weight additives,
(4)
retarders,
(5)
filtration-control agents, (6) lost-circulation control agents, (7) friction reducers, and
(8)
specialty materials.
Accelerators
Accelerators are used to shorten the “set” time (Farris, 1946; Bearden and Lane,
1961a, b). Cement slurries in shallow, low-temperature wells often use an accelerator
to shorten the thickening time and increase the early bonding strength of the
cement. Accelerators can reduce the cost by decreasing the period of “stand-by”
time required while the cement sets-up and gains the strength required to hold the
casing in primary cementation work. They are also used in the case of remedial
work where a short set time is desired to shorten the time the cement is required to
be held in position prior to setting up. Common additives include 2-4% (by weight
of cement) CaCl,,
1.5-5.0%
NaCl (or 3-10% by weight
of
water), and
20-100%
of
hemihydrate forms of calcium sulfate (plaster of Paris) (Smith, 1976). Other
accelerators discussed by Smith (1976, pp. 17-18) are sodium silicate (Na,SiO,)
and sea water, which contains up to
23,000
ppm of chlorides.
Light
-
weigh
t
additives
When prepared from
API
Class cement, neat cement slurries typically have a
weight of about
15
lb/gal or higher. Inasmuch as many formations can not support
69
long cement columns having such a high density, materials are often added to the
cement
to
lighten the density
of
the cement slurry (Coffer et al., 1954; and
Dumbauld et
al.,
1956). The side benefits include
(1)
the reduction of the overall
cost of the slurry, (2) increase in the yield, and (3) reduction of the filter loss.
Inasmuch as fresh water has a specific weight
of
about 8.33 lb/gal, the weight
of
a
cement slurry can be reduced by adding water. The specific weight of cement
slurries can also be reduced by adding solids
of
a low specific gravity or by the
addition
of
both water and these solids (Smith, 1976, p. 19). Solids commonly
utilized include:
(1)
bentonite; 2-16% by weight of the cement, (2) diatomaceous
earth, 10-40% by weight of the cement,
(3)
gilsonite, about
10-50
lb/sack of
cement,
(4)
expanded perlite,
5-20
lb/sack
of
cement,
(5)
artificial pozzolan (fly
ash) up to 74 lb/sack of cement, and (6) sodium silicate, 1-7.5 lb/sack of cement.
In general, the higher concentrations
of
additive reduce the compressive strength
and thickening time of the slurry. Water and bentonite also lower the resistance to
chemical attack by formation waters (Smith, 1976, p. 20).
Heauy-weight additives
Cement slurries of high density are prepared when it is necessary to offset
abnormally high formation pressures. Generally, the additives added to the slurry in
order to increase its specific weight have a specific gravity
of
4.5-5.0. Hematite is
the most widely used heavy additive, because of having a specific gravity of 5.02 and
the best physical requirements. In general, 4-104% by weight of hematite (sp.gr.
=
5.02),
10-100%
by weight of barite (sp.gr.
=
4.23), 5-25% by weight of sand
(sp.gr.
=
2.65), 5-16% of salt (sp.gr.
=
2.16-2.17), or
5-100%
of illmenite
(iron-titanium oxide), can be used to increase the slurry density (Smith, 1976, p.
23).
Cement retarders
When
wells are drilled to a depth
of
6000 ft or deeper, bottomhole temperatures
encountered are in excess
of
170
O
F.
Inasmuch as at these high temperatures cement
sets up quickly, retarders are added to slow down the setting time.
As
a general rule,
retarders are required below a depth of 8000 ft. The retarder must be compatible
with the cement and the additives in cement. Commonly utilized retarders include
lignins, gums, starches, weak organic acids, and cellulose derivatives (carboxymethyl
hydroxyethyl cellulose-CMHEC). The percentage
of
these chemicals that must be
added ranges from
0.1
to
2.5% by weight
of
cement. Salt
is
also
utilized in
concentrations ranging from 14 to 16 lb/sack of cement (Smith, 1976, p. 23).
Lost circulation additives
Smith (1976, p. 24) pointed out that there are two basic steps to combat lost
circulation problems:
(1)
reduction of the slurry density, and (2) addition
of
a
bridging or plugging material (see Wieland et al., 1969).
70
Perlite, which is a bridging agent, has been used successfully to prevent lost
circulation. Perlites weigh 8-10 lb/ft3 (dry), occur in particle sizes
of
4-40 mesh
(U.S.
Series), and are cellular, which allows them to absorb water under pressure
(Shryock, 1982, p. 314). In order to keep perlite particles evenly dispersed thoughout
the slurry, it is necessary to use 2-4% bentonite (by weight of the total cement).
According to Shryock (1982, p. 314), the most commonly used composition is:
1
ft3
(94 lb) of cement plus
1
ft3 (8 lb) of perlite, with 40% silica flour and 2% bentonite
(by weight of cement). The normal amount of mixing water with
1
ft3 of perlite per
sack of cement is 1.46 ft3. This results in an inhole slurry specific weight of 104
lb/ft3 and a slurry volume of 2.28 ft3/sack of cement.
Friction reducers or cement dispersants
Additives are often required to improve the flow properties of slurries. A
dispersed slurry generally has a lower viscosity and can be pumped in turbulent
conditions with a lower pump pressure, thereby reducing the horsepower required to
displace the cement (see Brice and Holmes, 1964). Smith (1976, p. 25) pointed out
that dispersants also lower the yield point and gel strength of the slurry. Common
dispersants are polymers and NaC1, which are used at lower reservoir temperatures.
Calcium lignosulfonates (organic acid blends) are used at higher reservoir temper-
atures.
Filtration-control additives
In controlling the filter loss of cement slurries, Smith (1976, p. 24) pointed out
that
it
must be lowered by use of additives to
(1)
prevent premature dehydration or
loss
of water to the porous zones adjacent to the displaced cement, (2) protection
of
water-sensitive formations that can be plugged or damaged by the filtrate solutions,
and
(3)
improvement of squeeze-type cementing. The 30-min API filter loss for a
neat slurry of API Class
G
or H cement is in excess of 1000 cm3.
The two most common additives are organic polymers (cellulose) and friction
reducers (Smith, 1976, p. 25). The high-molecular-weight cellulose compounds are
used in concentrations of 0.5-1.5% by weight of cement, sometimes requiring higher
volumes of water to maintain the desired slurry viscosity. In order to control filter
loss,
dispersants (friction reducers) are commonly added to cement slurries. They
disperse and pack the cement particles, thus making the cement slurry more dense
(Smith, 1976, p. 25).
OILWELL CEMENT STANDARDS OUTSIDE
OF
THE UNITED STATES
The American Petroleum Institute, Halliburton Co., and CEMBUREAU (Paris)
presented tables (Tables 3-1 and
3-11)
as an aid in the selection of locally available
71
TABLE
3-1
Properties of various types of cement used in U.S.A. (Courtesy of the Halliburton Services)
Properties
of
API classes of cement
Class A Class C Classes Classes
G
and
H
D and E
Specific gravity (average)
3.14
3.14
3.15 3.16
Surface area (range); (cm2/g)
1500-1900
2000-2800 1400-1700 1200-1600
Weight per sack (Ib)
94 94 94 94
Bulk volume (cu ft/sack)
1
1
1 1
Absolute volume (gal/sack)
3.6
3.6
3.58 3.57
Properties
of
neat slurries
Portland High early API API Retarded
strength Class
G
Class
H
Water (gal/API)
5.19 6.32 4.97 4.29 4.29
Slurry weight (lb/gal)
15.6 14.8
15.8 16.5 16.5
Slurry volume (cu ft/sack)
1.18 1.33 1.14
1.05 1.05
Temperature Pressure
(OF)
(Psi)
60
80
95
110
140
170
200
0
0
800
1600
3000
3000
3000
Typical compressive strength (psi) at
24
hr
615
780
440
325
1470 1870 1185 1065
2085 2015 2540 2110
2925 2705 2915 2525
5050
3560 4200 3160 3045
5920 3710 4830
4485
4150
5110 4575 4775
a
a
a a
Typical compressive strength (psi) at
72
hr
60
80
95
110
140
170
200
0
2870 2535
- -
4130 3935
- -
0
800
4670
4105
-
-
1600
5840 4780
-
-
3000 6550
4960
-
7125
4000
3000 6210
4460
5685 7310 5425
3000 7360 9900 5920
a
a
a a
Depth Temperature
(
F)
High-pressure thickening time
(hr
:
min.)
(ft) Static Circulating
2000 110 91
4:00+
4:00+ 3:00+ 3:57
4000
140
103 3
:
36 3:lO 2:30 3:20
4:00+
6000
170
113 2:25 2:06 2:
10
1
:
57
4:00+
8000
200
125
1:40a
1:37a
1
:44
1:40
4:00+
a
a
Not
generally recommended
at
this temperature.
v
N
TABLE 3-11
Equivalent cement classifications
in
various
countries (Courtesy of CEMBUREAU)
International Australia Canada France Japan United Kingdom West Germany
designation
oc
Type A, Normal CPA-250 Ordinary Ordinary Portland 2375
RHC Type B, High High Early
CPA-400 Rapid Hardening Rapid Hardening
-
HSC
- -
-
-
-
2450
LHC Type C, Low
-
-
Medium Low
Low Heat Portland
-
SRC
-
Sulfate
-
-
Sulfate Resisting 2275
Ordinary
Portland CPA-325 Portland (B.S. 12: 1958)
Early Strength Strength CPA-500 Portland
Heat of Heat Portland (B.S. 1370 1958)
Hydration
Resisting Portland
(B.S. 4027: 1955)
AEC
-
-
-
-
- -
Designation
of Standards AS A2 CSA A5 NF P15-302 JIS R5210 (B.S. 12; 1370; 4027) DIN 1164
Year Published 1963 1961 1964 1964 1958and1966 1969
73
cements in countries outside the United States. For several cements, additional
classifications were required, which have been included in the table (see Smith,
1976, for additional details).
CEMENTING OPERATIONS
Oilwell cementing practice
Regulatory bodies, which require drilling and completion of oil and gas wells in
such a manner as to prevent the escape of oil, gas, or formation waters from one
geologic strata to another, have been established in practically every oil-producing
country. The pollution of fresh-water sands is of prime concern. There are also
regulations, particularly in the U.S.A., on:
(1)
establishing the minimum quantities
of cement to be placed in the wellbore above the producing zone,
(2)
methods of
segregating producing zones,
(3)
protection of surface areas,
(4)
testing of cement
jobs,
(5)
squeezing, plugging, and testing cementing plugs, (6) determining waiting-
on-cement job (WOC), and (7) methods of setting casing.
Most cementation operations can be defined as either primary or secondary.
Primary cementing operations are those that pertain to the cementing job after
installation of casing in the wellbore and laying of bottomhole plugs. Secondary
operations include squeeze cementing and other similar remedial cementing oper-
ations. Squeeze cementing can be defined as follows: squeezing (forcing) of a
cement slurry into pores,
vugs,
fractures, etc. of a formation, to acheve a seal
between the formation and casing.
Primary oilwell cementing
The important functions of primary cementing are: (1) to allow segregation
of
formations behind the well casing (to prevent movement
of
fluids between different
zones),
(2)
to give additional support to the casing in the wellbore to protect it from
shock loads,
(3)
to retard corrosion of the casing in the wellbore by minimizing
contact between formation fluids and outside surface
of
the pipe,
(4)
to prevent
blowouts, and
(5)
to seal off lost-circulation (thef) zones.
Primary cement jobs are usually performed by pumping the slurry down through
the tubing or casing and up around the outside
of
the casing in the wellbore-casing
annulus. Figure
3-2
illustrates the typical procedure for a single-stage primary
cement job. Gatlin (1960) showed a normal displacement cement operation: Neat
cement is introduced in a dry form into a hopper, where it is mixed with a turbulent
water stream by a high-velocity jet mixer. Cement slurry is then pumped down
in-between two plugs which have wiping fins. Plugs are placed in the system
immediately after and before the cement slurry while it is being pumped down the
tubing. The bottom plug stops upon reachng the float collar. The pressure increases
behind the plug until the hollow rubber plug ruptures. Ths allows cement to pass
74
Fig.
3-2.
Typical surface and subsurface primary cementing operations. (Courtesy
of
the Halliburton
Services.)