Table 7-9. Rheology and Fluid-Loss Performance of Proprietary Salt Cement Systems
NaCl
(%BWOW)
Cellulose/Organic Acid
(%BWOC)
BHCT
(°F)
Fluid Loss
(mL/30 min)
30
0.8/0.1
120
178
30
–†
120
64
0.2
30
0.8/0.1
160
285
31.3
30
–
160
52
0.1
† Not
Rheological Properties at BCHT
Yield Value
(lbf/100 ft2)
Plastic Viscosity
(cp)
22
244
33.1
105
17.5
applicable
Today, selecting the appropriate salt concentration in a
cement slurry is largely a function of the local formation
characteristics, operational limitations, and success
ratios. There is no official industry consensus regarding
slurry design for salt-zone cementing. At present, anecdotal evidence shows that most salt zones are cemented
with systems containing between 8 to 18% NaCl (BWOW).
In addition, scrupulous attention is given to proper hole
conditioning and good cementing practices.
Blast-furnace slag (BFS) combined with Portland
cement or drilling fluids has also been used recently
to cement salt zones. This application is discussed in
Section 7-8.2.2.
As discussed in Chapter 2, absolute-volume shrinkage
and internal volume reduction are a result of Portland
cement hydration. Upon cement setting, stresses build
within the matrix, resulting in the formation of microcracks (Fig. 7-9). The propagation of the cracks lowers the
tensile capacity of the set cement and increases its permeability. In latex-modified systems (Fig. 7-10), the latex
particles coalesce to form a plastic film that surrounds
and coats the C-S-H phase. Because of its elasticity and
high bonding strength, the latex bridges the microcracks
and limits their propagation; as a result, the tensile
strength of the set cement increases and the permeability
decreases.
7-6 Latex-modified cement systems
Latex is a general term describing an emulsion polymer.
The material is usually supplied as a milky suspension
of very small spherical polymer particles (200 to 500 nm
in diameter), often stabilized by surfactants to improve
freeze/thaw resistance and prevent coagulation when
added to Portland cement. Most latex dispersions contain
about 50% solids. A wide variety of monomers, including
vinyl acetate, vinyl chloride, acrylics, acrylonitrile, ethylene, styrene, and butadiene, is emulsion-polymerized to
prepare commercial latexes.
The first use of latexes in Portland cements occurred
in the 1920s, when natural rubber latex was added to
mortars and concretes. Since then, latex-modified concretes have become commonplace because of the following improvements in performance (Ohama, 1987):
■ improved pumpability
■ decreased permeability
■ increased tensile strength
■ reduced shrinkage
■ increased elasticity
■ improved bonding between cement/steel and
cement/formation interfaces.
242
Fig. 7-9. Photograph of microcracks in set Portland cement (from
Kuhlmann, 1985). Reprinted with permission from Elsevier.
Well Cementing
7-6.2 Early latex-modified well cement systems
In 1958, Eberhard and Park patented the use of vinylidene chloride latex in well cements. Improved performance was observed for systems containing up to 35%
latex solids BWOC. Later, polyvinyl acetate latex was
identified as a suitable material (Woodard and Merkle,
1964). The preferred concentration of latex solids varied
from 2.5% to 25% BWOC. The polyvinyl acetate system
has been used successfully for many years; however, it is
limited to applications at temperatures less than 122°F
[50°C] bottomhole static temperature (BHST).
7-6.3 Styrene butadiene latex systems
Fig. 7-10. Photograph of latex-modified Portland cement, magnified
1,200 times (from Kuhlmann, 1985). Reprinted with permission from
Elsevier.
7-6.1 Behavior of latexes in well cement slurries
The use of latex in well cements occurred much later.
In 1957, Rollins and Davidson reported improved performance when latex was added to the mix water. In
addition to the attributes mentioned above, adding
latex provided the following additional benefits:
■ better bonding to oil-wet and water-wet surfaces
■ less shattering when perforated
■ increased resistance to contamination by well fluids
■ lowered fluid-loss rate
■ improved durability.
When latex is added as part of the liquid phase of a
Portland cement system, the resulting slurry has a normal
color and consistency; however, because of the solids content of the latex, such slurries contain 20% to 35% less water.
After curing, the set product consists of hydrated cement
particles connected by a film of latex particles (Kuhlmann,
1985). The film of latex particles imparts the physical
and chemical properties described above (Parcevaux
and Sault, 1984; Drecq and Parcevaux, 1988). While the
slurry is still liquid, the latex particles impart excellent
rheological properties because of a lubricating action. In
addition, the latex particles provide excellent fluid-loss
control by physically plugging small pores in the cement
filtercake (Drecq and Parcevaux, 1988) (Chapter 3).
An improvement in latex cement technology occurred
when Parcevaux et al. (1985) identified styrene butadiene
latex as an effective additive for the prevention of annular
gas migration (Chapter 9). Additional refinements were
made by Sault et al. (1986).
Styrene butadiene latexes impart the same beneficial
effects described above; however, they are effective at
temperatures as high as 350°F [176°C]. Fig. 7-11 is a plot
of API/ISO fluid-loss rate versus latex concentration for
various well cement slurries. The results illustrate that
normal-density neat slurries require less latex to
achieve a given fluid-loss rate. More latex is required for
slurries containing extenders or weighting agents, especially those with a lower solids content (extended with
sodium silicate). Figure 7-12 illustrates the decreased
volumetric shrinkage observed with a latex-modified
Portland cement system cured at 158°F [70°C].
3.0
Neat: 15.8 lbm/gal
Barite: 18 lbm/gal
Bentonite: 13.3 lbm/gal
Fly ash: 13.6 lbm/gal
Sodium silicate: 13 lbm/gal
2.5
Styrene
butadiene
latex
(gal/sk)
2.0
1.5
1.0
0.5
0
50
100
150
200
250
300
API/ISO fluid loss (mL/30 min)
Fig. 7-11. Fluid-loss rates of latex-modified cement slurries (185°F
[85°C]).
Chapter 7 Special Cement Systems
243
7
Neat cement
6
5
Absolute
volume
shrinkage
(%)
4
3
Latex-modified cement
2
1
0
0
6
12
18
24
Time (hr)
Fig. 7-12. Absolute volume shrinkage of normal-density Portland
cements (from Parcevaux and Sault, 1984). Reprinted with permission of SPE.
Portland cement systems are not compatible with
strong inorganic acids such as sulfuric, hydrochloric,
and nitric acid. In such environments, organic polymer
cements, usually epoxy-based, must be used to provide
sufficient chemical resistance (Cole, 1979). Such systems are also known as “synthetic cements.”
Epoxy cements are prepared by mixing an epoxy resin
such as bisphenol-A (Fig. 7-13) with a hardening agent.
Depending upon the desired end properties, the hardening agent can be an anhydride, aliphatic amine, or
polyamide (Sherman et al., 1980). A solid filler such as
silica flour is often used to build density, reduce cost,
and act as a heat sink for the exothermic reaction that
occurs during the curing period. Depending upon the
circulating and static well temperatures, various catalysts and accelerators can also be added to control the
placement and setting times.
7-7 Cements for corrosive environments
Set Portland cement is a remarkably durable and forgiving
material, but it has its limits. In a wellbore environment,
Portland cement is subject to chemical attack by certain
formations, migrating fluids, and substances injected from
the surface. As discussed in Chapter 10, saline geothermal
brines containing CO2 are particularly deleterious to the
integrity of the set cement. Cement durability is also a
key consideration in wells for chemical waste disposal
and for enhanced oil recovery by CO2 flooding.
7-7.1 Cements for chemical waste disposal wells
Zonal isolation is of paramount importance in chemical
waste disposal wells. If not properly confined, injected
waste fluids could contaminate freshwater aquifers and
corrode the exterior of the casing. To ensure zonal isolation throughout the life of such wells, the cement and
the tubular hardware in the well must be chemically
resistant to the waste fluids (Runyan, 1974).
The chemically resistant casings used in waste disposal
wells include modified polyester and epoxy fibercast,
or metal alloys such as Carpenter 20,† Incoly 825,‡ and
Hastalloy G.§ The cement systems are chosen for compatibility with the injected waste material.
Modified Portland cements are generally appropriate
for disposal wells involving weak organic acids, sewage
waters, or solutions having a pH of 6 or above. The durability of the set cement is improved by adding pozzolans,
increasing the density by addition of dispersants,
employing controlled particle-size technology, or adding
liquid latexes to the slurry. These methods substantially
reduce the permeability of the set cement.
O
CH2 CHCH2 O
CH3
C
CH3
OH
OCH2CHCH2 O
n
CH3
C
O
OCH2CH CH2
CH3
Fig. 7-13. Chemical structure of bisphenol-A.
Epoxy resin cement systems are characterized by
their corrosion resistance and high compressive and
shear-bond strength. They are compatible with strong
acids and bases (up to 37% HCl, 60% H2SO4, and 50%
NaOH) at temperatures up to 200°F [93°C] during
extended exposure periods. Epoxies are also resistant
to hydrocarbons and alcohols, but not to chlorinated
organics or acetone. Typically, the compressive strengths
range between 8,000 and 10,000 psi [56 to 70 MPa], and
shear-bond strengths can be as much as 9 times higher
than those of Portland cement (R.A. Bruckdorfer,
unpublished data, 1985).
Until recently, to prevent premature setting and to
enhance cement bonding, nonaqueous spacers such as
gelled oil, diesel, or alcohol were required on all epoxy
cement jobs. The use of such preflushes is costly and
time consuming. In 2001, Eoff et al. introduced a watercompatible epoxy cement system. Instead of bisphenol-A,
the epoxy sealant is the diglycidyl ether of cyclohexane
dimethanol. The hardening agent is a mixture of diethyltoluenediamine and tris(dimethylaminomethylphenol).
The performance of this system is unaffected by water
contamination, up to 25% by volume.
† Trademark
of CRS Holdings, Inc.
of International Nickel Co.
§ Trademark of Haynes International, Inc.
‡ Trademark
244
Well Cementing
0
