BACKGROUND
[0001] In well cementing, such as well construction and remedial cementing, cement
compositions are commonly utilized. Cement slurries may be used in a variety of subterranean
applications. For example, in subterranean well construction, a pipe string (e.g., casing, liners,
expandable tubulars, etc.) may be run into a well bore and cemented in place. The process of
cementing the pipe string in place is commonly referred to as "primary cementing." In a typical
primary cementing method, a cement slurry may be pumped into an annulus between the walls of
the well bore and the exterior surface of the pipe string disposed therein. The cement slurry may
set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable
cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and
may bond the exterior surface of the pipe string to the subterranean formation. Among other
things, the cement sheath surrounding the pipe string functions to prevent the migration of fluids
in the annulus, as well as protecting the pipe string from corrosion. Cement slurries also may be
used in remedial cementing methods, for example, to seal cracks or holes in pipe strings or cement
sheaths, to seal highly permeable formation zones or fractures, to place a cement plug, and the
like.
[0002] A challenge in well cementing is the development of satisfactory properties of the
cement during placement. Oftentimes several cement slurries with varying additives are tested to
see if they meet the material engineering requirements for a particular well. The process of
selecting the components of the cement slurry are usually done by a best guess approach by
utilizing previous slurries and modifying them until a satisfactory solution is reached. The process
may be time consuming and the resulting slurry may be complex. Furthermore, the cement
components available in any one particular region may vary in slurry from those of another region
thereby further complicating the process of selecting a correct slurry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] These drawings illustrate certain aspects of some of the embodiments of the present
disclosure and should not be used to limit or define the disclosure.
[0004] FIG. 1 illustrates a method to design for thickening time.
[0005] FIG. 2 illustrates introduction of a cement slurry into a well bore.
[0006] FIG. 3 is a parity plot showing results of a thickening time test.
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DETAILED DESCRIPTION
[0007] The present disclosure may generally relate to cementing methods and systems.
More particularly, embodiments may be directed to designing cement slurries based at least
partially on a thickening time model.
[0008] Cement slurries may contain cement, supplementary cementitious additives, inert
materials, and chemical additives. A cement slurry for use in cementing wellbores is typically
mixed at a well bore pad site using cement mixing equipment and pumped into the well bore using
cement pumps. After the cement slurry is mixed, there is a time lag between when the cement is
in a liquid state and when the cement begins to set. As the cement slurry begins to set, the slurry
gradually becomes more viscous until fully set. There may be an upper limit of viscosity beyond
which the cement slurry becomes too viscous to pump. In general, the upper limit of viscosity is
typically defined to be when the fluid has a consistency of greater than 70 Bearden units of
consistency ("Be"). However, there may be other considerations where the cement slurry would
be considered unpumpable and thus a Be value of 30, 50, 70, 100, or any other value may be
selected as being "unpumpable." To determine the consistency or Be value of a cement slurry, an
atmospheric or a pressurized consistometer may be used in accordance with the procedure for
determining cement thickening times set forth in API RP Practice 1 OB-2, Recommended Practice
for Testing Well Cements, First Edition, July 2005. The time to reach the selected Bearden units
of consistency is reported as thickening time. It is often a design criteria for a cement slurry to
have a long enough thickening time such that there is enough time to pump the required volume
of cement into the wellbore while also not having too long of a thickening time where there is
excessive downtime from waiting on the cement to set. The thickening time for a cement slurry
may be a function of pressure, temperature, temperature ramp rate, density of the cement slurry,
and composition of the cement slurry.
[0009] Designing a cement slurry to have a desired thickening time is an inefficient trial
and error process often requiring multiple iterations of selecting slurry components and mass
fractions thereof and testing a thickening time for a slurry formed from the slurry components.
Small changes in composition may result in widely varying thickening times which is further
compounded by cementitious materials varying across different geographical areas. As such, a
cement recipe that is prepared in one region may have a different thickening time than the same
recipe prepared in a different region with same class of materials, due to the differences in
mineralogy and manufacturing processes of the cement components. The differences in thickening
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times may be difficult to predict as the thickening time of a cement slurry is a complex function
of various interacting factors.
[001 0] Cement slurries are typically blended with chemical additives such as accelerators,
retarders, fluid loss control additives, lost circulation control additives, rheological modifiers, and
other chemical additives to impart desirable properties on the cement slurry such as fluid loss
control, rheology, stability, and thickening time. The additive package that can satisfy all of these
properties is typically determined through an iterative process. This is because one additive used
to satisfy one property may affect another property. For example, a fluid loss control additive may
retard the cement slurry. Thus, when designing for thickening time, the effects of each additive
on thickening time must be accounted for.
[0011] At a given temperature, a thickening time model may include two mam
components, a component that models thickening time of the blend of cementitious components
along with inerts, and a component that models thickening time of cement additives. Equation 1
is a general model equation for thickening time where TT is the thickening time which is a function
of TTb which is a component that models thickening time of the blend of cementitious
components, and ofTTa which is a component that models thickening time of cement additives.
Equation 1
TT = f(TTw TTb)
[0012] A thickening time model of the blend of cementitious components may account for
pressure, temperature, ramp rate, density, and chemical composition of a cement slurry. The
models thickening time of the blend of cementitious components may include two main
components, the first being effects of water on thickening time and the second being effects of
composition on thickening time. The first component is generally a function of the density of the
cement slurry which may be controlled by varying the amount the amount of water that is added
to a dry cement blend to produce the cement slurry. Further the type or source of water may affect
the thickening time as dissolved ions in the water may interact with the cement components and
additives. For example, a cement composition prepared with sea water may be expected to have a
different thickening time than a cement composition prepared with fresh water. The second
component is generally a function of the chemical identity of the components that make up the
cement slurry and their corresponding mass or volume fractions in the cement slurry.
[0013] A relationship between water and thickening time may be expressed as a power
law function as in equations 2 and 3. Equation 2 shows that the thickening time is proportional to
the amount of water used in the preparing of the cement slurry. In equation 2, water is a mass or
blend
volume ratio of water to the other components in the cement slurry such as Portland cement,
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supplementary cementitious materials and inert materials, and n is a measurement of sensitivity
to change in water of the blend where n may be a constant or a function of the blend materials. In
some instances, n may also be a function of the type of water. To determine n, two cement slurries
at different densities may be mixed and the thickening time may be analyzed using laboratory
methods. Thereafter, equation 2 or 3 may be used to calculate the value of n for the water. Equation
3 shows an alternate form of the relationship between water and thickening time as a function f
comprising a polynomial. Other forms of function f may be log, exponential, power law,
trigonometric, integral, differential, or combinations thereof.
(
water)n
TT a blend
(
water (water) 2
)
TT a f blend + blend + ...
Equation 2
Equation 3
[0014] A relationship between the effects of water on thickening time may be an
exponential relationship as shown in equation 4. While only two forms for the effect of water on
thickening time are described herein, the effect of water on thickening time may be expressed in
any suitable manner such as a logarithmic model, integral model, derivative model, or any other
suitable model.
(
water)
TT a e nblend
Equation 4
[0015] A relationship describing effects of composition on thickening time may be a linear
combination of individual contributions from each cement component as shown in equation 5. In
equation 5, xi and x1 are the mass fraction of component i andj, respectively, in the cement blend
and f31b [31i1, f32b etc are model parameters which characterize reactivity of component i and j, or
interaction between component i and j. For some components, f3 may be constant, whereas for
other components f3 may be depended upon temperature and pressure, for example. f3 for any
component may be experimentally determined. One method of obtaining f3i for a cement
component may be to select a Portland cement with a known or measured thickening time
measured at a reference temperature, pressure, and density. Thereafter a volume of the cement
component whose f3i is unknown may be mixed with the selected Portland cement and water to
the reference density. A thickening time test may be performed at the reference temperature and
pressure and equation 4 may be used to determine the f3i value for the cement component.
Equation 5
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TT a Lixd11i + LiLJ xixJf31iJ + Lixi2f3zi ...
[0016] An alternate form of a relationship describing effects of composition and additives
on thickening time may be a relationship as shown in equation 6 where TTb is contribution of
thickening time of blend which may be a function of the bulk bend composition, mass fraction of
water in the cement slurry, as well as temperature and pressure. In examples where the bulk blend
composition is pure cement without additional supplemental cementitious materials, or inert
materials, then TTb is the contribution of compressive strength from the cement alone. TTa is
contribution of thickening time of additives which may be a function of mass fraction of additives
as well as temperature and pressure.
Equation 6
TTa TTb * TTa
[0017] A thickening time model of cement additives (TTa) may account for whether the
additive has a tendency to accelerate or retard the cement hydration or both, whether the effect of
the additive on thickening time is temperature dependent, whether are interactions between
additives, and interactions between additives and the cement blend materials. A generalized
relationship between the effects of additives on thickening time is shown in equation 7 where fc is
a function and C is a concentration of an additive. In some examples, fc may comprise a
polynomial, log, exponential, power law, trigonometric, integral, differential, or combinations
thereof A thickening time model with additives and blend is shown in equation 8.
Equation 7
TTa a fc(C)
Equation 8
TT = TTb fc(C)
[0018] One form offc may be expressed as an exponential as in equation 9. In equation 9,
y is a measure of potency of an additive to accelerate or retard the cement hydration process and
cis the concentration of the additive, typically expressed with reference to a base material such as
cement such as by weight of cement (bwoc ), or as a mass fraction or volume fraction. The potency
of the additive y, is typically a function of temperature, pressure as well as the concentration of
the additive. For an additive that retards, y will be positive and for an additive that accelerates y
will be negative.
Equation 9
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[0019] A relationship between the effects of additives on thickening time may be
expressed as power law as in equation 10. In equation 10, c is the concentration of the additive
and a is a constant where a is positive for retarders and a is negative for accelerators.
Equation 10
TTaa(C)a
[0020] A relationship between the effects of additives on thickening time may be
expressed as an exponential in equation 11 and 12. In equations 11 and 12, Tis temperature, E is
activation energy, c is the concentration of the additive, and y0 is a potency at a reference
temperature. The activation energy E captures the effect of temperature on additive. In equation
13, the term S(T) is a sigmoid function with temperature and equation 13 is one form of a sigmoid
function and equation 14 shows one form of a sigmoid function.
TT (ro+f)xc aae
TT a e(y0 +ET)xc
a
S(T)- 1
- l+e(T-To)xA.
Equation 11
Equation 12
Equation 13
Equation 14
[0021] A relationship between the effects of additives on thickening time may be
expressed as polynomial as in equation 15. In equation 15, Cis the concentration ofthe additive
and a and b are polynomial coefficients.
Equation 15
TTaa aC + bC2 + ···
[0022] A relationship between the effects of additives on thickening time may be
expressed as polynomial as in equation 16. In equation 16, y0 is a potency below a threshold
temperature beyond which the additive can undergo changes in orientation, shape, dissolution
kinetics, dissociation tendency etc. and thus increase/decrease its potency as a function of
temperature, E1 and E2 are polynomial coefficients, C is the concentration of the additive, and T
is temperature.
Equation 16
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[0023] A relationship between the additives and thickening time may be expressed as a
function of concentration of additives as in equation 17.
TT 0( eCYo+Yl*S(C))xC
a
Equation 17
In Equation 17, S(C) may be a sigmoid function in concentration and C is concentration of
additive expressed as by weight of water.
[0024] A relationship between the additives potency and concentration may be expressed
as equation 18.
Equation 18
_ + Y1
Yeff- Yo 1+e(c-c0 )xA.
In equation 18, II. is a measure of how rapidly the potency changes due to changes in concentration.
C0 is the threshold concentration around which the potency changes. Yo and y1 determine the limits
for potency.
[0025] When two or more additives are used together in the same cement slurry, there may
be interactions between the additives. A relationship between the effects of additives on thickening
time with interactions may be expressed as in equation 19. In equation 19, y1 is potency ofthe
first additive, Cl is the concentration ofthe first additive, y2 is potency ofthe second additive, c2
is the concentration of the second additive, and Yint is potency of the interaction.
Equation 19
[0026] Another form of a relationship between the effects of additives on thickening time
with interactions may be expressed as in equation 20.
Equation 20
[0027] Using any equations 2-5 for TTb, or any other thickening time model of blend TTb,
and any of equations 6-20 for TTa a number of models of thickening time (TT) may be derived.
Some forms of the thickening time model may be described by equation 21-24.
Equation 21
Equation 22
Equation 23
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Equation 24
TT = TTb x eY1Cf(c)*g(T)*P(P)) x eYzCz
[0028] FIG. 1 illustrates a method 100 of using the models of thickening time discussed
above. Method 100 may begin at step 102 where bulk material availability such as cement,
supplementary cementitious materials, and cement additives available may be defined. Bulk
material availability is typically location dependent whereby some geographic locations may have
access to bulk materials that other geographic locations do not. Further, bulk materials such as
mined materials and cements may vary across geographic locations due to differences in raw
materials for manufacturing and manufacturing methods, as well as natural variations among
deposits of mineable minerals across geographic locations. In step 102, engineering parameters
such as fluid loss control requirements, rheology requirements, stability requirements, and
thickening time requirement, as well as density and temperature. After defining materials available
and engineering parameters, method 100 may proceed to step 104. In step 104, a proposed cement
composition may be selected which may include cement components and mass fractions thereof,
water and mass fraction thereof, and chemical additives and mass fractions thereof. The selecting
of chemical additives and mass fractions thereof may be at least partially based on the fluid loss
control, rheology, and stability. The cement components may include any one of a cement, a
supplementary cementitious additive, an inert material, and/or a chemical additive that is available
as defined in step 102. In step 106 the thickening time of the proposed cement composition may
be calculated using any of the above thickening time models. For example, equations 22-24 may
be used or any other models derived from the equations disclosed herein. In examples where
cement components are selected in step 104 for which a potency or other model variable is not
known, the unknown value may be calculated in step 108 using any of the above-mentioned
methods. From step 106, method 100 may proceed to step 10 where the calculated thickening time
from step 106 may be compared to the required thickening time defined in step 102. If the
calculated thickening time is not within tolerance of the required thickening time, method 100
may proceed back to step 104 where a second proposed cement composition may be selected
which may include disparate cement components and/or disparate mass fractions thereof and or
chemical additives and components thereof. If the calculated thickening time is within tolerance
of the required thickening time, method 100 may proceed to step 112. In step 112, the proposed
cement composition may be prepared, and the thickening time measured to verify that the cement
composition has the required thickening time.
[0029] Cement compositions described herein may generally include a hydraulic cement
and water. A variety of hydraulic cements may be utilized in accordance with the present
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disclosure, including, but not limited to, those comprising calcium, aluminum, silicon, oxygen,
iron, and/or sulfur, which set and harden by reaction with water. Suitable hydraulic cements may
include, but are not limited to, Portland cements, pozzolana cements, gypsum cements, high
alumina content cements, silica cements, and any combination thereof. In certain examples, the
hydraulic cement may include a Portland cement. In some examples, the Portland cements may
include Portland cements that are classified as Classes A, C, H, and G cements according to
American Petroleum Institute, API Specification for Materials and Testingfor Well Cements, API
Specification 10, Fifth Ed., July 1, 1990. In addition, hydraulic cements may include cements
classified by American Society for Testing and Materials (ASTM) in C150 (Standard
Specification for Portland Cement), C595 (Standard Specification for Blended Hydraulic Cement)
or C1157 (Performance Specification for Hydraulic Cements) such as those cements classified as
ASTM Type I, II, or III. The hydraulic cement may be included in the cement composition in any
amount suitable for a particular composition. Without limitation, the hydraulic cement may be
included in the cement compositions in an amount in the range of from about 10% to about 80%
by weight of dry blend in the cement composition. For example, the hydraulic cement may be
present in an amount ranging between any of and/or including any of about 10%, about 15%,
about 20%, about 25%, about 30%, about 35%, about 40%, about 45%, about 50%, about 55%,
about 60%, about 65%, about 70%, about 75%, or about 80% by weight of the cement
compositions.
[0030] The water may be from any source provided that it does not contain an excess of
compounds that may undesirably affect other components in the cement compositions. For
example, a cement composition may include fresh water or saltwater. Saltwater generally may
include one or more dissolved salts therein and may be saturated or unsaturated as desired for a
particular application. Seawater or brines may be suitable for use in some examples. Further, the
water may be present in an amount sufficient to form a pumpable slurry. In certain examples, the
water may be present in the cement composition in an amount in the range of from about 33% to
about 200% by weight of the cementitious materials. For example, the water cement may be
present in an amount ranging between any of and/or including any of about 33%, about 50%,
about 75%, about 100%, about 125%, about 150%, about 175%, or about 200% by weight of the
cementitious materials. The cementitious materials referenced may include all components which
contribute to the compressive strength of the cement composition such as the hydraulic cement
and supplementary cementitious materials, for example.
[0031] As mentioned above, the cement composition may include supplementary
cementitious materials. The supplementary cementitious material may be any material that
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contributes to the desired properties of the cement composition. Some supplementary cementitious
materials may include, without limitation, fly ash, blast furnace slag, silica fume, pozzolans, kiln
dust, and clays, for example.
[0032] The cement composition may include kiln dust as a supplementary cementitious
material. "Kiln dust," as that term is used herein, refers to a solid material generated as a byproduct
of the heating of certain materials in kilns. The term "kiln dust" as used herein is intended
to include kiln dust made as described herein and equivalent forms of kiln dust. Depending on its
source, kiln dust may exhibit cementitious properties in that it can set and harden in the presence
of water. Examples of suitable kiln dusts include cement kiln dust, lime kiln dust, and
combinations thereof. Cement kiln dust may be generated as a by-product of cement production
that is removed from the gas stream and collected, for example, in a dust collector. Usually, large
quantities of cement kiln dust are collected in the production of cement that are commonly
disposed of as waste. The chemical analysis of the cement kiln dust from various cement
manufactures varies depending on a number of factors, including the particular kiln feed, the
efficiencies of the cement production operation, and the associated dust collection systems.
Cement kiln dust generally may include a variety of oxides, such as Si02, Ab03, Fe203, CaO,
MgO, S03, Na20, and K20. The chemical analysis of lime kiln dust from various lime
manufacturers varies depending on several factors, including the particular limestone or dolomitic
limestone feed, the type of kiln, the mode of operation of the kiln, the efficiencies of the lime
production operation, and the associated dust collection systems. Lime kiln dust generally may
include varying amounts of free lime and free magnesium, lime stone, and/or dolomitic limestone
and a variety of oxides, such as Si02, Ab03, Fe203, CaO, MgO, S03, Na20, and K20, and other
components, such as chlorides. A cement kiln dust may be added to the cement composition prior
to, concurrently with, or after activation. Cement kiln dust may include a partially calcined kiln
feed which is removed from the gas stream and collected in a dust collector during the manufacture
of cement. The chemical analysis of CKD from various cement manufactures varies depending
on a number of factors, including the particular kiln feed, the efficiencies of the cement production
operation, and the associated dust collection systems. CKD generally may comprise a variety of
oxides, such as Si02, Ah03, Fe203, CaO, MgO, S03, Na20, and K20. The CKD and/or lime kiln
dust may be included in examples of the cement composition in an amount suitable for a particular
application.
[0033] In some examples, the cement composition may further include one or more of
slag, natural glass, shale, amorphous silica, or metakaolin as a supplementary cementitious
material. Slag is generally a granulated, blast furnace by-product from the production of cast iron
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including the oxidized impurities found in iron ore. The cement may further include shale. A
variety of shales may be suitable, including those including silicon, aluminum, calcium, and/or
magnesium. Examples of suitable shales include vitrified shale and/or calcined shale. In some
examples, the cement composition may further include amorphous silica as a supplementary
cementitious material. Amorphous silica is a powder that may be included in embodiments to
increase cement compressive strength. Amorphous silica is generally a byproduct of a ferrosilicon
production process, wherein the amorphous silica may be formed by oxidation and condensation
of gaseous silicon suboxide, SiO, which is formed as an intermediate during the process
[0034] In some examples, the cement composition may further include a variety of fly
ashes as a supplementary cementitious material which may include fly ash classified as Class C,
Class F, or Class N fly ash according to American Petroleum Institute, API Specification for
Materials and Testing for Well Cements, API Specification 10, Fifth Ed., July 1, 1990. In some
examples, the cement composition may further include zeolites as supplementary cementitious
materials. Zeolites are generally porous alumino-silicate minerals that may be either natural or
synthetic. Synthetic zeolites are based on the same type of structural cell as natural zeolites and
may comprise aluminosilicate hydrates. As used herein, the term "zeolite" refers to all natural and
synthetic forms of zeolite.
[0035] Where used, one or more of the aforementioned supplementary cementitious
materials may be present in the cement composition. For example, without limitation, one or more
supplementary cementitious materials may be present in an amount of about 0.1% to about 80%
by weight of the cement composition. For example, the supplementary cementitious materials may
be present in an amount ranging between any of and/ or including any of about 0. 1 %, about 1 0%,
about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, or about 80% by weight
of the cement.
[0036] In some examples, the cement composition may further include hydrated lime. As
used herein, the term "hydrated lime" will be understood to mean calcium hydroxide. In some
embodiments, the hydrated lime may be provided as quicklime (calcium oxide) which hydrates
when mixed with water to form the hydrated lime. The hydrated lime may be included in examples
of the cement composition, for example, to form a hydraulic composition with the supplementary
cementitious components. For example, the hydrated lime may be included in a supplementary
cementitious material-to-hydrated-lime weight ratio of about 10:1 to about 1:1 or 3:1 to about 5:1.
Where present, the hydrated lime may be included in the set cement composition in an amount in
the range of from about 10% to about 100% by weight of the cement composition, for example.
In some examples, the hydrated lime may be present in an amount ranging between any of and/or
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including any of about 10%, about 20%, about 40%, about 60%, about 80%, or about 100% by
weight of the cement composition. In some examples, the cementitious components present in the
cement composition may consist essentially of one or more supplementary cementitious materials
and the hydrated lime. For example, the cementitious components may primarily comprise the
supplementary cementitious materials and the hydrated lime without any additional components
(e.g., Portland cement, fly ash, slag cement) that hydraulically set in the presence of water.
[0037] Lime may be present in the cement composition in several; forms, including as
calcium oxide and or calcium hydroxide or as a reaction product such as when Portland cement
reacts with water. Alternatively, lime may be included in the cement composition by amount of
silica in the cement composition. A cement composition may be designed to have a target lime to
silica weight ratio. The target lime to silica ratio may be a molar ratio, molal ratio, or any other
equivalent way of expressing a relative amount of silica to lime. Any suitable target time to silica
weight ratio may be selected including from about 10/90 lime to silica by weight to about 40/60
lime to silica by weight. Alternatively, about 10/90 lime to silica by weight to about 20/80 lime to
silica by weight, about 20/80 lime to silica by weight to about 30/70 lime to silica by weight, or
about 30/70 lime to silica by weight to about 40/63 lime to silica by weight.
[0038] Other additives suitable for use in subterranean cementing operations also may be
included in embodiments of the cement composition. Examples of such additives include, but are
not limited to: weighting agents, lightweight additives, gas-generating additives, mechanicalproperty-
enhancing additives, lost-circulation materials, filtration-control additives, fluid-losscontrol
additives, defoaming agents, foaming agents, thixotropic additives, and combinations
thereof. In embodiments, one or more of these additives may be added to the cement composition
after storing but prior to the placement of a cement composition into a subterranean formation. In
some examples, the cement composition may further include a dispersant. Examples of suitable
dispersants include, without limitation, sulfonated-formaldehyde-based dispersants (e.g.,
sulfonated acetone formaldehyde condensate) or polycarboxylated ether dispersants. In some
examples, the dispersant may be included in the cement composition in an amount in the range of
from about 0.01% to about 5% by weight of the cementitious materials. In specific examples, the
dispersant may be present in an amount ranging between any of and/or including any of about
0.01 %, about 0.1 %, about 0.5%, about 1%, about 2%, about 3%, about 4%, or about 5% by weight
ofthe cementitious materials.
[0039] In some examples, the cement composition may further include a set retarder. A
broad variety of set retarders may be suitable for use in the cement compositions. For example,
the set retarder may comprise phosphonic acids, such as ethylenediamine tetra(methylene
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phosphonic acid), diethylenetriamine penta(methylene phosphonic acid), etc.; lignosulfonates,
such as sodium lignosulfonate, calcium lignosulfonate, etc.; salts such as stannous sulfate, lead
acetate, monobasic calcium phosphate, organic acids, such as citric acid, tartaric acid, etc.;
cellulose derivatives such as hydroxyl ethyl cellulose (HEC) and carboxymethyl hydroxyethyl
cellulose (CMHEC); synthetic co- or ter-polymers comprising sulfonate and carboxylic acid
groups such as sulfonate-functionalized acrylamide-acrylic acid co-polymers; borate compounds
such as alkali borates, sodium metaborate, sodium tetraborate, potassium pentaborate; derivatives
thereof, or mixtures thereof. Examples of suitable set retarders include, among others, phosphonic
acid derivatives. Generally, the set retarder may be present in the cement composition in an
amount sufficient to delay the setting for a desired time. In some examples, the set retarder may
be present in the cement composition in an amount in the range of from about 0. 01% to about 10%
by weight of the cementitious materials. In specific examples, the set retarder may be present in
an amount ranging between any of and/or including any of about 0.01 %, about 0.1 %, about 1%,
about 2%, about 4%, about 6%, about 8%, or about 10% by weight of the cementitious materials.
[0040] In some examples, the cement composition may further include an accelerator. A
broad variety of accelerators may be suitable for use in the cement compositions. For example,
the accelerator may include, but are not limited to, aluminum sulfate, alums, calcium chloride,
calcium nitrate, calcium nitrite, calcium formate, calcium sulphoaluminate, calcium sulfate,
gypsum-hemihydrate, sodium aluminate, sodium carbonate, sodium chloride, sodium silicate,
sodium sulfate, ferric chloride, or a combination thereof. In some examples, the accelerators may
be present in the cement composition in an amount in the range of from about 0. 01% to about 10%
by weight of the cementitious materials. In specific examples, the accelerators may be present in
an amount ranging between any of and/or including any of about 0.01 %, about 0.1 %, about 1%,
about 2%, about 4%, about 6%, about 8%, or about 10% by weight of the cementitious materials.
[0041] Cement compositions generally should have a density suitable for a particular
application. By way of example, the cement composition may have a density in the range of from
about 8 pounds per gallon ("ppg") (959 kg/m3
) to about 20 ppg (2397 kg/m3
), or about 8 ppg to
about 12 ppg (1437. kg/m3
), or about 12 ppg to about 16 ppg (1917.22 kg/m3
), or about 16 ppg to
about 20 ppg, or any ranges therebetween. Examples of the cement compositions may be foamed
or unfoamed or may comprise other means to reduce their densities, such as hollow microspheres,
low-density elastic beads, or other density-reducing additives known in the art.
[0042] The cement slurries disclosed herein may be used in a variety of subterranean
applications, including primary and remedial cementing. The cement slurries may be introduced
into a subterranean formation and allowed to set. In primary cementing applications, for example,
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the cement slurries may be introduced into the annular space between a conduit located in a
wellbore and the walls of the wellbore (and/or a larger conduit in the wellbore), wherein the
wellbore penetrates the subterranean formation. The cement slurry may be allowed to set in the
annular space to form an annular sheath of hardened cement. The cement slurry may form a barrier
that prevents the migration of fluids in the wellbore. The cement composition may also, for
example, support the conduit in the wellbore. In remedial cementing applications, the cement
compositions may be used, for example, in squeeze cementing operations or in the placement of
cement plugs. By way of example, the cement compositions may be placed in a well bore to plug
an opening (e.g., a void or crack) in the formation, in a gravel pack, in the conduit, in the cement
sheath, and /or between the cement sheath and the conduit (e.g., a micro annulus).
[0043] Reference is now made to FIG. 2, illustrating use of a cement slurry 200. Cement
slurry 200 may comprise any of the components described herein. Cement slurry 200 may be
designed, for example, using the thickening time models describe herein. Cement slurry 200 may
be placed into a subterranean formation 205 in accordance with example systems, methods and
cement slurries. As illustrated, a wellbore 210 may be drilled into the subterranean formation 205.
While wellbore 210 is shown extending generally vertically into the subterranean formation 205,
the principles described herein are also applicable to well bores that extend at an angle through the
subterranean formation 205, such as horizontal and slanted well bores. As illustrated, the wellbore
210 comprises walls 215. In the illustration, casing 230 may be cemented to the walls 215 ofthe
wellbore 210 by cement sheath 220. In the illustration, one or more additional conduits (e.g.,
intermediate casing, production casing, liners, etc.), shown here as casing 230 may also be
disposed in the wellbore 210. As illustrated, there is a wellbore annulus 235 formed between the
casing 230 and the walls 215 of the wellbore 210. One or more centralizers 240 may be attached
to the casing 230, for example, to centralize the casing 230 in the well bore 210 prior to and during
the cementing operation. The cement slurry 200 may be pumped down the interior of the casing
230. The cement slurry 200 may be allowed to flow down the interior of the casing 230 through
the casing shoe 245 at the bottom of the casing 230 and up around the casing 230 into the well bore
annulus 235. The cement slurry 200 may be allowed to set in the wellbore annulus 235, for
example, to form a cement sheath that supports and positions the casing 230 in the wellbore 210.
While not illustrated, other techniques may also be utilized for introduction of the cement slurry
200. By way of example, reverse circulation techniques may be used that include introducing the
cement slurry 200 into the subterranean formation 205 by way of the well bore annulus 235 instead
of through the casing 230. As it is introduced, the cement slurry 200 may displace other fluids
250, such as drilling fluids and/or spacer fluids that may be present in the interior of the casing
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230 and/or the well bore annulus 235. While not illustrated, at least a portion of the displaced fluids
250 may exit the wellbore annulus 235 via a flow line and be deposited, for example, in one or
more retention pits. A bottom plug 255 may be introduced into the wellbore 210 ahead of the
cement slurry 200, for example, to separate the cement slurry 200 from the fluids 250 that may be
inside the casing 230 prior to cementing. After the bottom plug 255 reaches the landing collar 280,
a diaphragm or other suitable device should rupture to allow the cement slurry 200 through the
bottom plug 255. The bottom plug 255 is shown on the landing collar 280. In the illustration, a
top plug 285 may be introduced into the wellbore 210 behind the cement slurry 200. The top plug
260 may separate the cement slurry 200 from a displacement fluid 265 and also push the cement
slurry 200 through the bottom plug 255.
[0044] The following statements may describe certain embodiments of the disclosure but
should be read to be limiting to any particular embodiment.
[0045] Statement 1. A method of designing a cement slurry comprising: (a) selecting at
least a cement and concentration thereof, a water and concentration thereof, and one or more
chemical additives and a concentration thereof such that a cement slurry formed from the cement,
water, and the one or more chemical additives meet a density requirement; (b) calculating a
thickening time of the cement slurry using a thickening time model; (c) comparing the thickening
time of the cement slurry to a thickening time requirement, wherein steps (a)-(c) are repeated if
the thickening time of the cement slurry does not meet or exceed the thickening time requirement,
wherein each repeated step of selecting comprises selecting different concentrations and/or
different chemical identities for the one or more chemical additives, cement, or water than
previously selected, or step (d) is performed if the thickening time of the cement slurry meets or
exceeds the thickening time requirement; and (d) preparing the cement slurry.

CLAIMS
What is claimed is:
1. A method of designing a cement slurry comprising:
(a) selecting at least a cement and concentration thereof, a water and concentration thereof,
and one or more chemical additives and a concentration thereof such that a cement slurry formed
from the cement, water, and the one or more chemical additives meet a density requirement;
(b) calculating a thickening time of the cement slurry using a thickening time model;
(c) comparing the thickening time of the cement slurry to a thickening time requirement,
wherein steps (a)-( c) are repeated if the thickening time of the cement slurry does not meet or
exceed the thickening time requirement, wherein each repeated step of selecting comprises
selecting different concentrations and/ or different chemical identities for the one or more chemical
additives, cement, or water than previously selected, or step (d) is performed if the thickening time
of the cement slurry meets or exceeds the thickening time requirement; and
(d) preparing the cement slurry.
2. The method of claim 1 wherein the one or more chemical additives is selected from the group
consisting of weighting agents, lightweight additives, gas-generating additives, mechanicalproperty-
enhancing additives, lost-circulation control materials, filtration-control additives, fluidloss-
control additives, defoaming agents, foaming agents, thixotropic additives, dispersants,
suspending aids, viscosifiers, transition time control additives and combinations thereof.
3. The method of claim 1 wherein the one or more chemical additives is selected from the group
consisting of cement set retarders, cement accelerators, and combinations thereof.
4. The method of claim 1 wherein the thickening time model comprises the following equation:
TT = TTb x ere
where TT is the thickening time, TTb is a thickening time model time of the cement, y is a potency
of the chemical additive, and cis the concentration of the chemical additive.
5. The method of claim 1 wherein the thickening time model comprises the following equation:
TT = TTb X e(ro-Y1 * l+ec/ To)x.<.)xc
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where TT is the thickening time, TTb is a thickening time model of the cement, Yo and y1 are
limits of potency, T0 is a transition threshold temperature, /lis a measure of potency change with
respect to temperature, Tis temperature, and cis a concentration of the chemical additive.
6. The method of claim 1 wherein the thickening time model comprises the following equation:
TT = TTb X eY1C1 X eYzCz X eYintC1Cz
where TT is the thickening time, TTb is a thickening time model of the cement, y1 is potency of a
first chemical additive, c1 is a concentration of the first chemical additive, y 2 is potency of a second
chemical additive, c2 is a concentration of the second chemical additive, and Yint is potency of an
interaction between the first chemical additive and the second chemical additive.
7. A method comprising:
providing a cement blend and a thickening time of the cement blend;
preparing a slurry comprising the cement blend, water, and a chemical additive;
measuring a thickening time of the slurry; and
calculating a potency (y) of the chemical additive using a thickening time model, the
measured thickening time of the slurry, and the thickening time of the cement blend.
8. The method of claim 7 wherein the one or more chemical additives is selected from the group
consisting of weighting agents, lightweight additives, gas-generating additives, mechanicalproperty-
enhancing additives, lost-circulation control materials, filtration-control additives, fluidloss-
control additives, defoaming agents, foaming agents, thixotropic additives, dispersants,
suspending aids, viscosifiers, transition time control additives and combinations thereof.
9. The method of claim 7 wherein the one or more chemical additives is selected from the group
consisting of cement set retarders, cement accelerators, and combinations thereof.
10. The method of claim 7 wherein the thickening time model comprises the following equation:
TT = TTb x ere
where TT is the thickening time, TTb is a thickening time model of the cement blend, y is a potency
of the chemical additive, and cis the concentration of the chemical additive.
11. The method of claim 7 wherein the thickening time model comprises the following equation:
TT = TTb X e(ro-Y1 * l+ec/ To)xA.)xc
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where TT is the thickening time, TTb is a thickening time model of the cement blend, Yo and y1
are limits of potency, T0 is a transition threshold temperature, /l is a measure of potency change
with respect to temperature, Tis temperature, and cis a concentration of the chemical additive.
12. The method of claim 7 wherein thickening time model comprises the following equation:
TT = TTb X eY1C1 X eYzCz X eYintC1Cz
where TT is the thickening time, TTb is a thickening time model of the cement blend, y1 is potency
of a first chemical additive, c1 is a concentration of the first chemical additive, y2 is potency of a
second chemical additive, c2 is a concentration of the second chemical additive, and Yint is potency
of an interaction between the first chemical additive and the second chemical additive.
13. The method of claim 7 further comprising:
(a) selecting a mass fraction of a the cement blend, a mass fraction of water, and a mass
fraction of the chemical additive such that a second cement slurry formed from the cement blend
and mass fraction thereof, the chemical additive and mass fraction thereof, and the water and mass
fraction thereof meet a density requirement;
(b) calculating a thickening time of the second cement slurry using the thickening time
model;
(c) comparing the thickening time of the second cement slurry to a thickening time
requirement, wherein steps (a)-( c) are repeated if the thickening time of the second cement slurry
does not meet or exceed the thickening time requirement, wherein each repeated step of selecting
comprises selecting different concentrations and/or different chemical identities for the one or
more chemical additives, cement, or water than previously selected, or step (d) is performed if the
thickening time of the second cement slurry meets or exceeds the thickening time requirement;
and
(d) preparing the cement slurry.
14. The method of claim 13 wherein the thickening time model comprises the following equation:
TT = TTb x ere
where TT is the thickening time, TTb is a thickening time model of the cement, y is a potency of
the chemical additive, and cis the concentration of the chemical additive in the second cement
slurry.
15. The method of claim 13 wherein the thickening time model comprises the following equation:
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TT = TTb X e(ro-Y1 * l+e(T~To)xA.)xc
where TT is the thickening time of the second cement slurry, TTb is a thickening time model of
the cement, Yo and y1 are limits of potency, T0 is a transition threshold temperature, /lis a measure
of potency change with respect to temperature, T is temperature, and c is a concentration of the
chemical additive in the second cement slurry.
16. The method of claim 13 wherein the thickening time model comprises the following equation:
TT = TTb X eY1C1 X eYzCz X eYintC1Cz
where TT is the thickening time of the second cement slurry, TTb is a thickening time model of
the cement, y1 is potency of a first chemical additive, Cl is a concentration of the first chemical
additive in the second cement slurry, y2 is potency of a second chemical additive, c2 is a
concentration of the second chemical additive in the second cement slurry, and Yint is potency of
an interaction between the first chemical additive and the second chemical additive.
17. A method comprising:
defining a thickening time requirement and a density requirement;
selecting a mass fraction of a cement blend, a mass fraction of water, and a mass fraction
of a chemical additive such that a cement slurry formed from the cement blend and mass fraction
thereof, the chemical additive and mass fraction thereof, and the water and mass fraction thereof
meet the density requirement, and wherein the selecting further comprises using a thickening time
model to select the mass fraction of the cement blend, the mass fraction of water, and the mass
fraction of the chemical additive meets or exceeds the thickening time requirement; and
preparing the cement slurry.
18. The method of claim 17 wherein the thickening time model comprises the following equation:
TT = TTb x ere
where TT is the thickening time, TTb is a thickening time model of the cement blend, y is a potency
of the chemical additive, and cis the concentration of the chemical additive.
19. The method of claim 17 wherein the thickening time model comprises the following equation:
TT = TTb X e(ro-Y1 * l+e(T~To)xA.)xc