Ministry of Higher Education and

Scientific Research
University of Technology
Petroleum Technology Department

Drilling Fluids

Cementing

Mohammad Ali Kareem Shukur

3th Stage (2019/2020)
Drilling fluid
Abstract:
wells are cemented using many of the same techniques as in the oil and gas industry. However, high temperatures,
corrosive brines and carbon dioxide severely challenge the long-term durability of well cements. Well cementing is a
critical part of well construction and requires a dedicated design and engineering process. Portland cement
manufactured to API specifications, classes A and G, is most commonly utilised in geothermal wells cementing. The
cements and cement additives selected and the cementing practices utilised are an integral part of sound well design,
construction and well integrity. Selected cements, additives and mixing fluids should be laboratory tested in advance to
ensure they meet the requirements of the well design. Silica flour is included in the design of cement slurry to prevent
strength retrogression that occurs as a result of elevated temperatures encountered in geothermal wells. Proper well
conditioning before cementing ensures a sound cement sheath. There are four common techniques utilised in the
primary cementing of geothermal wells. There are other techniques used to execute remedial cementing jobs when the
need arises. During cementing operations, various parameters are recorded as part of the monitoring of the cementing
execution job and for post-job analyses. In the post-job evaluation, it is important to carry out acoustic logs.

Introduction:
Well cementing technology is an amalgam of many interdependent scientific and engineering disciplines, including
chemistry, geology, physics, and petroleum, mechanical, and electrical engineering. Each is essential to achieve the
primary goal of well cementing-zonal rsolation. By preparing this textbook, the authors have aspired to produce a
comprehensive and up-to-date reference concerning the application of these disciplines toward cementing a well. Well
cementing exposes Portland cement to conditions far different from those anticipated by its inventor. Cement systems
must be designed to be pumped under conditions ranging from below freezing in permafrost zones to greater than
1,000” F (538°C) in some thermal recovery wells. After placement, the cement systems must preserve their integrity and
provide zonal isolation during the life of the well. It has only been possible to accommodate such a wide range of
conditions through the development of additives which modify the available Portland cements for individual well
requirements. The chemical nature of the various classes of additives is described, and typical performance data are
provided.

Primary cementing:
Primary cementing is the process of placing cement in the annulus between the casing and the formations exposed to
the wellbore. Since its inception in 1903, the major objective of primary cementing has always been to provide zonal
isolation in oil, gas, and water wells, i.e., to exclude fluids such as water or gas in one zone from oil in another zone in
the well. To achieve this objective, a hydraulic seal must be created between the casing and cement and between the
cement and the formations, while at the same time preventing fluid channels in the cement sheath (Fig. P-1). This
requirement makes primary cementing the most important operation performed on a well. Without complete isolation
in the wellbore, the well may never reach its full producing potential.
The basic process for accomplishing a primary cementing job uses the two-plug method for pumping and displacement.
This method was first used in 1910 in shallow wells in California. After the well reaches the desired depth, the drillpipe is
removed, and a largerdiameter string of casing is run to the bottom of the well. At this time, the drilling mud is still in
the wellbore. This mud must be removed and replaced with a cement slurry. The most common process to accomplish
this is the two-plug cementing method (Fig. P-2). To prevent contamination with mud, two plugs isolate the cement as it
is pumped down the casing. Sufficient cement slurry is pumped into the casing to fill the annular column from the
bottom to at least the top of the proWell Cementing Consultant Fig. P-1. Objectives of primary cementing. Oil or gas pay
zone Cement bonded to casing Cement bonded to formations Complete cement sheath with no mud or gas channels
ductive zones. Typically, cement slurry is brought to higher locations to exclude other undesirable fluids from the
wellbore, to protect freshwater zones, and to protect the casing from corrosion. The cementing process is completed
when a pressure increase at the surface indicates the top plug has reached the landing collar or float collar and
displacement with mud or water is terminated. The well is shut in for a time to allow the cement to harden before
completion work or drilling to a deeper horizon begins.

Portland cement:
Portland cement is by far the most important binding material in terms of quantity produced; indeed, it is possible that it
may be the most ubiquitous manufactured material. Portland cement is used in nearly all well cementing operations.
The conditions to which Portland cements are exposed in a well differ significantly from those encountered at ambient
conditions during construction operations: as a result, special Portland cements are manufactured for use as well
cements. Certain other cements, used to a far lesser extent for the solution of special well problems, are discussed in
Chapters 7 and 9. Portland cement is the most common example of a II-Ydmulic cement. Such cements set and develop
compressive strength as a result of hydration, which involves chemical reactions between water and the compounds
present in the cement, 1101 upon a drying-out process. The setting and hardening occur not only if the cement/water
mixture is left to stand in air, but also if it is placed in water. The development of strength is predictable, uniform and
relatively rapid. The set cement also has low permeability, and is nearly insoluble in water; therefore, exposure to water
does not destroy the hardened material. Such attributes are essential for a cement to achieve and maintain zonal
isolation. In this chapter, fundamental information is presented regarding the mamtfacture, hydration and classification
of Portland cements. In addition, the effects of various chemical and physical parameters upon performance are
discussed.

Classification of Portland cements:

Portland cements are manufactured to meet certain chemical and physical standards that depend upon their
application. To promote consistency of performance among cement manufacturers, classification systems and
specifications have been established by various user groups. The best known systems are those of ASTM International
(formerly the American Society for Testing and Materials) (ASTM C 150, Standard Specification for Portland Cement) and
API (API Spec 10A, Specification for Cements and Materials for Well Cementing). The API classification scheme has been
adopted by the ISO as Standard 10426-1, Petroleum and natural gas industries - Cements and materials for well
cementing - Part 1: Specification.

Implication of cementing for well production and performance:

Production optimization begins with a good completion, and a good completion depends on the integrity of the primary
cement job. About 15% of primary cement jobs fail, costing the oil and gas industry an estimated USD 450 million
annually in remedial cementing work (Newman et al., 2001). Figure 1-1 shows the percentage of wells experiencing
sustained casing pressure (SCP) versus age for the 22,000 wells in the U.S. Gulf of Mexico (personal communication, J.
Levine, 2003). This means that there are 8,000 to 11,000 wells in the Gulf of Mexico with sustained SCP. In 1999, there
were 3,810 wells with surface casing vent flows and 814 wells with gas migration problems in Alberta, Canada (Alberta
Energy and Utilities Board, 1999). The primary reason for these failures is improper balancing of the pressures, which
allows gas and fluid influx into the cement-filled annulus during the primary cement job, and movement of pipe, cement,
or both in the wellbore during depletion. Methods to improve primary cementing practices and prevent zonal-isolation
failures will be discussed in later chapters. In this introductory chapter, the effect of cement job quality on the long-term
performance of a well will be outlined.

Zonal isolation:
Complete and durable zonal isolation is the foremost goal of the cement job. During the life of a producing oil and gas
well, the quality of the cement job has a direct impact on the economic longevity of the well. From the time the well is
first produced until the well is abandoned, appropriate cement-slurry design and placement techniques will affect well
productivity, both physically and economically. If allowed to set undisturbed, Portland cement systems of normal
density (≈16.0 lbm/gal or 1,930 kg/m3 ) usually exhibit extremely low matrix permeability. The literature quotes values
in the microdarcy range. However, during the productive life of a well, the cement is subjected to various severe
conditions that can affect the longevity of this low matrix permeability. The first condition, termed “cracking,” is caused
by thermal or pressure fluctuations in the well caused by the production process. For example, gas wells are subjected
to large variations in drawdown pressure and temperature as the gas demand changes. Depending on the magnitude
and frequency of these production variables, the casing and cement sheath expand and contract in different ways
(Chapter 8). This causes stress gradients that gradually crack cement, with the subsequent loss of cement integrity (Fig.
1-2). The second condition, termed “debonding,” occurs when the bond between either the cement/rock or the
pipe/cement interface fails. Several production practices can cause debonding: ■ the gradual pressure decrease as a well
is produced ■ casing movement as subsidence occurs ■ cement shrinkage with time ■ temperature and pressure
fluctuations ■ stimulation practices, such as hydraulic fracturing. The third condition, called “shear failure,” typically
results in complete failure of the cement sheath. Shear failure is normally caused by effective-stress increases around a
wellbore caused by rock subsidence and movement as the reservoir is depleted. This effect can also be caused by
vibrations from downhole pumps or gas-lift operations. Any of these conditions will result in flow paths in the form of
discrete conductive fractures in the cement, or microannuli. These paths, and their effective widths, create cement
permeabilities that far exceed the intrinsic permeability of the undisturbed cement. Even a small microannulus results in
a large effective permeability along the cement sheath.

Portland cement manufacture:

Portland cement is produced by pulverizing clinker. Clinker is the calcined (burned) material that exits the rotary kiln in
a cement plant. Clinker consists primarily of hydraulic calcium silicates, calcium aluminates, and calcium aluminoferrites.
One or more forms of calcium sulfate (usually gypsum, CS_ SH2 ) are interground with the clinker to make the finished
product. Materials used in the manufacture of Portland cement clinker must contain appropriate amounts of calcium,
silica, alumina, and iron compounds. During manufacture, frequent chemical analyses of all materials are made to
ensure uniformity and high quality.

Chemical notation of cement:

A special chemical notation established by cement chemists is frequently used in this textbook. The chemical formulas of
many cement compounds can be expressed as a sum of oxides; for example, tricalcium silicate, Ca3SiO5, can be written
as 3CaO • SiO2. Abbreviations are given for the oxides most frequently encountered, such as C for CaO (lime), S for SiO2
(silica), and A for Al2O3 (alumina). Thus Ca3SiO5 becomes C3S. A list of abbreviations is given below. C = CaO F = Fe2O3
N = Na2O P=P2O5 A = Al2O3 M = MgO K = K2O f = FeO S = SiO2 H =H2O L = Li2O T = TiO2 Others are sometimes used,
such as S_ S = SO3 and C– = CO2. This convention of using a shortened notation was adopted as a simple method for
describing compounds whose complete molecular formulas occupy much space when written.

Rheology of Well Cement Slurries:

A proper understanding of cement slurry rheology is important to design, execute and evaluate a primary cementation.
An adequate rheological characterization of cement slurries is necessary for many reasons, includingevaluation of slurry
mixability and pumpability, determination of the pressure-vs-depth relationship during and after placement, calculation
of the return rate when free fall is occurring, prediction of the temperature profile when placing cement in the hole, and
design of the displacement rate required to achieve optimum mud removal. Despite a great .amount of research
performed during the past 50 years, a complete characterization of the rheology of cement slurries has yet to be
achieved. This is due to the complexity of cement slurry rheological behavior, which depends on many different factors
such aswater-to-cement ratio, specific surface of the powder, and more precisely the size and the shape of cement
grains, chemical composition of the cement and the relative distribution of the components at the surface of the grains,
presence of additives, and mixing and testing procedures. The influence of these factors on cement slurry properties is
described elsewhere (Chapters 2,3, and 5, and Appendix B). This chapter concentrates on the rheological
characterization and flow behavior of cement slurries inawellbore.

Mud removal:
Mud removal has been a subject of intense interest in the well-cementing community for many years because of its
effect on cement quality and zonal isolation. The main objective of a primary cement job is to provide complete and
permanent isolation of the formations behind the casing. To meet this objective, the drilling mud and the preflushes (if
any) must be fully removed from the annulus, and the annular space must be completely filled with cement slurry. Once
in place, the cement must harden and develop the necessary mechanical properties to maintain a hydraulic seal
throughout the life of the well. Therefore, good mud removal and proper slurry placement are essential to obtain well
isolation. Incomplete mud displacement can leave mud channels or mud layers on the walls across the zones of interest,
thereby favoring interzonal communication. Therefore, bonding of the cement to the pipe and formation, as well as
cement-seal durability, are affected by the efficiency of the displacement process. Mud removal in cementing
operations is not fundamentally different from mud-to-mud displacement or mud-to-completion fluid displacement,
although the means and objectives are slightly different. ■ For a cementing engineer, the heart of the mudremoval
process consists of optimizing casing centralization, selecting the sequence of fluids, determining the volume and
properties of each of the fluids, and to some extent determining the pumping rate. Often, these are the only variables
the engineer can control. Ideally, after determining the optimal fluid properties, one can determine the correct fluid
compositions. In the end, the final objective is to achieve the most efficient zonal isolation. Success is most often
evaluated using cement logs (Chapter 15). ■ Mud displacement while drilling has a different objective: to replace one
drilling fluid with another with a minimum amount of intermixing. Also, the conditions are different (e.g., the pipe- to
hole-diameter ratio is much smaller), and the evaluation method is direct: One monitors the amount of mixed fluid
recovered on the surface. ■ Mud displacement by completion fluids has yet another objective: to remove the maximum
amount of suspended solids in the wellbore fluid. The result is evaluated by monitoring of the clarity of the returned
fluid. In all the above three domains—replacement of a drilling fluid by a cement slurry, another drilling fluid, or a
completion fluid—the outlined technical objective is balanced by economic considerations, which include the rig time
required for the displacement operations, the cost of the cleaning products, and the eventual loss of recovered fluid
because of intermixing.

Cementing technical systems:

1. Thixotropic cements: A thixotropic system is fluid under shear, but develops a gel structure and becomes self-
supporting when at rest (Shaw, 1970). In practical terms, thixotropic cement slurries are thin and fluid during
mixing and displacement but rapidly form a rigid, self-supporting gel structure when pumping ceases. Upon
reagitation, the gel structure breaks and the slurry regains fluidity. Then, upon cessation of shear, the gel
structure reappears and the slurry returns to a self-supporting state. This type of rheological behavior is
continuously reversible with truly thixotropic cements.
2. Freeze-protected cements: Permafrost zones in Alaska, northern Canada, and Siberia present unique
cementing difficulties. Permafrost is defined as any permanently frozen subsurface formation. The depths of
such formations vary from a few meters to 2,000 ft [600 m]. Below the permafrost, the geothermal gradients are
normal. Permafrost sections vary from unconsolidated sands and gravels containing ice lenses to ice-free,
consolidated rock.
3. Salt cement systems: Cement systems that contain significant quantities of NaCl or KCl are commonly called
“salt cements.” Salt has been used extensively in well cements for three principal reasons. ■ In certain areas
(e.g., offshore), salt is present in the available mix water. ■ Salt is a common and inexpensive chemical that,
when used as an additive, can modify the behavior of the cement system. ■ During cementing across massive
salt formations or water-sensitive zones, cement slurries containing large amounts of salt prevent dissolution of
the salt formation and prevent clay swelling.
4. 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.
5. 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.
6. BFS systems: The use of BFS systems in well cementing is not a new concept; indeed, patents covering slag-
cement compositions for wellbores appeared as early as 1958 (Harmeen and Stuve, 1958). BFSs can be used
alone as a cementitious material or blended with Portland cements (called Portland slag cements or slag
cements). Slags are also used to convert drilling fluids into a cementitious material. In this case, chemical
activators are required to speed up slag hydration kinetics. The use of BFS for well cementing has increased
 significantly since the early 1990s. This section discusses the production and chemistry of BFS, as well as the use
of BFS in various well-cementing applications.
7. Ultralow-density cement systems: Formations that have a low fracturing gradient or are highly permeable,
vuggy, or cavernous pose difficult cementing situations. Such formations are often unable to support the
annular hydrostatic pressure exerted by a conventional cement slurry. Some formations will not even support a
column of water. The density of conventional cements mixed with water always exceeds 8.33 lbm/gal [1,000
kg/m3 ]. In reality, the density of conventional systems with acceptable properties usually exceeds 11 lbm/gal
[1,320 kg/m3 ]. Therefore, there are situations in which it is impossible to perform a successful cement job with
a conventional slurry. Ultralow-density cements provide a solution to such problems. In general, the ultralow-
density category refers to systems with densities less than about 10 lbm/gal [1,200 kg/m3].
8. Microfine cements: Microfine cements are composed of very small particles (generally 4 to 15 μm). Therefore,
the surface area is very large (500 to greater than 1,000 m2 /kg). The most common microfine cements are very
fine Portland cements (Ewert et al., 1991; Bensted and Barnes, 2002); however, slag cements may also be
incorporated (Clarke and McNally, 1993; Section 7-8). The advantage of such cements is their improved ability to
penetrate and flow through tight spaces and porous media. The Portland cement–based microfine cements
generally behave similarly to their conventional counterparts; however, owing to their higher reactivity,
additional gypsum or retarders may be necessary to achieve predictable rheological performance and setting.
9. Acid-soluble cements: As discussed in Chapter 6, lost circulation during drilling and cementing is a common
problem. Most common remedies, such as lost-circulation materials (e.g., flakes and fibers) and thixotropic
cements, remain in the thief zone permanently. This is not a problem unless the thief zone is also the producing
zone. Such situations include gas-injection and gas-storage wells. Therefore, there is a need for lost circulation
materials that can easily be removed after well construction is complete.
10. Chemically bonded phosphate ceramics: Chemically bonded phosphate ceramics (CBPCs) are binders that fall
between sintered ceramics (e.g., pottery, porcelain) and chemically bonded systems (e.g., Portland cement).
CBPCs are formed by acid-base reactions between an acid phosphate (e.g., Mg, Ca, or Al) and a metal oxide (e.g.,
MgO, CaO, or ZnO2) (Jeong and Wagh, 2003).

Mechanical properties of well cement:

Until recently, the well cementing industry focused on one principal mechanical property of set cement— unconfined
uniaxial compressive strength—to qualify a cement design. The uniaxial compressive strength is determined by a cube-
crushing test (Appendix B) and is used to estimate the ability of set cement to support casing and survive the perforation
process. When combined with water- or air-permeability measurements (Appendix B), one can also estimate the
cement’s ability to provide zonal isolation and resist attack from formation fluids. Indirect ultrasonic techniques using
tools like the ultrasonic cement analyzer (UCA) (Appendix B) allow one to monitor the evolution of strength with time.
With the UCA, one can determine the curing time required for a cement system to attain a given compressive strength.
In recent years, the well cementing industry has begun to concentrate on set cements’ ability to provide zonal isolation
throughout the lifetime of the well. This was triggered by the observation that, even in situations in which the cement
was properly placed and provided a good initial hydraulic seal, zonal isolation disappeared with time (Goodwin and
Crook, 1992; Jackson and Murphey, 1993). Zonal isolation loss has been attributed to several causes. ■ Gas-migration
problems that were not initially detected ■ Loss of cement-bond log response with time ■ Fracturing the wrong zone
during a stimulation treatment ■ Extreme downhole temperature or pressure changes ■ Chemical attack ■ Pressure
migration to shallower zones.

Cement behavior:
A cement sample, like any material, deforms when subjected to stress. Determining a relationship between stress and
strain is an important aspect of solid mechanics. This relationship is called the constitutive equation of the material
under consideration, and various theories have been developed to describe it in a simplified way. The simplest one is the
theory of elasticity, which assumes a unique relationship between stress and strain (and that the behavior is reversible).
This theory is usually sufficient to analyze cement failure in tension or in compression at ambient conditions. Other
theories, such as the theory of elastoplasticity, have been developed to take into account nonreversible behaviors that
are observed in materials before failure. Significant nonreversible behavior is observed in cements subjected to
confining pressure.

OBJECTIVES OF CEMENTING:
The objective of casing cementing is to ensure that the whole length of the annulus is completely filled with sound
cement that can withstand long term exposure to geothermal fluids and temperatures (Hole, 2008). The most important
functions of a cement sheath between the casing and the formation are (Rabia, 1985):

a) To prevent the movement (migration) of fluids from one formation to another or from the formations to the surface
through the annulus;

b) To hold the casing string in the well;

c) To protect the casing from corrosive fluids in the formations and buckling;

d) To support the well-bore walls (in conjunction with the casing) to prevent collapse of formations; e) To prevent
blowouts by forming a seal in the annulus;

f) To protect the casing from shock loads when drilling deeper.

Cementing is also used to condition the well:

a) To seal loss of circulation zones;

b) To stabilize weak zones (washouts, collapses);

c) To plug a well for abandonment or for repair;

d) To kick-off side tracking in an open hole or past a junk;

e) To plug a well temporarily before being re-cased.

cement design:
To design a cement system that resists the downhole conditions, the following methodology is proposed. ■ It is first
essential to have a database of cements or alternate sealants with the desired mechanical properties (elastic
parameters, compressive strength, tensile strength, shrinkage/expansion) at various temperatures and confining
pressures. For example, having a variety of cement systems with low, medium, and high Young’s moduli is
recommended. The database allows us to know what can be achieved under various constraints such as slurry density,
slurry rheology, and cost. Rock properties are also needed and can be obtained from sonic logs.

■ The second step is to properly understand and describe the downhole situation to identify the cause of potential
problems. This requires a review of all the events that may occur after the cement sets, such as a change of wellbore
fluid, change of temperature, formation creep, subsidence, or other tectonic event. In situations in which the casing can
be damaged by the external loading, determination of the casing failure mode (Section 8-4.7) from downhole
information is necessary. Example: A casing has been cemented with a 0.821-psi/ft or 15.8-lbm/gal [1,900-kg/m3 ]
cement displaced with a 0.509-psi/ft or 9.8-lbm/gal [1,174-kg/m3 ] mud. The reference pressure is the pressure applied
inside the casing by the mud during cement setting, not the pressure applied by the cement in the annulus. The drilling
of the next section, in which overpressured shales are encountered, requires a mud-weight increase to 0.914 psi/ft or
17.6 lbm/gal [2,109 kg/m3 ]. The maximum pressure increase is at the shoe, located at 3,000 ft [914.4 m]. The pressure
increase is
■ The required cement mechanical properties to survive such a change of downhole conditions are then determined
using a stress model of the cemented cased wellbore. The input data to the model include the well geometry; steel,
cement, and rock properties; and the expected variation of downhole conditions. The cement properties are obtained
from the database mentioned above.

■ Cement selection using such a model is performed with an iterative process, though one can imagine that, in the
future, the model will be able to determine the best configuration automatically. The best solution is the most cost-
effective cement that does not fail under the input loading condition. The model can be used to provide
recommendations on cement design by tuning the properties of the cement to minimize the expected problem.
Unfortunately, in complex situations, such as in tectonically active areas, the modeling can be very involved.

■ The model must also be able to predict whether the loading can induce the formation of a microannulus, and in that
case it is necessary to select the cement accordingly (for example expanding cement with a Young’s modulus lower than
the rock properties).

■ Finally, the integrity of the cement sheath should be monitored over time to validate the selection. To help the field
engineers design cements based on mechanical considerations, software tools that follow the above guidelines have
been developed. An application of such software tools for the cementation of steamassisted gravity drainage wells is
given in Stiles and Hollies (2002). This stress analysis model software contains databases on cement, rock, and steel
mechanical properties; a calculator to determine downhole stress conditions; a stress model to predict whether the
cement will fail (as described in Section 8-4); and a tool to help in designing the cement. This software tool greatly
facilitates the design of special cement for long-term zonal isolation, and this type of software tool is recommended to
design critical cases.

The figure below show the cementing system design:

Cement material:
API Class G oil well cement (OWC) was used in this study. Calcium sulfoaluminate (CSA) cement and gypsum were
introduced as mineral additions. A synthetic powder retarder and a melamine formaldehyde-based powder
superplasticizer (SP) were used as chemical additives. The chemical composition of the cements used are given in Table
1. The C3S, C3A, and SO3 contents of the Class G cement used in this study are 62.6%, 1.4%, and 2.5%, respectively, and
the C2S, C2F, and MgO contents are 23.2%, 7.0%, and 0.1%, respectively. The cement satisfies the chemical
requirements (C3S 65%, C3A 3.0%, SO3 3.0%) of Class G cement specified in API Specs 10A [34]. The mixture
proportions of the OWC slurries are summarized in Table 2. The API Class G standard slurry was prepared with a w/b of
0.44 as specified in API Specs 10A [34]. Slurries with two water-tobinder (w/b) ratios, 0.32 and 0.40, were prepared for
static gel strength tests and to compare their gel transition time in order to evaluate which w/b is more suitable for the
targeted application as a plugging material. For all other tests, a w/b of 0.40 was used for practical considerations of
workability and shorter test durations. The sum of CSA cement and gypsum is 1, 3 or 5 percent by weight of binder
(%BWOB). The ratios of CSA cement to gypsum are 100:0, 70:30, or 50:50. While the dosage of retarder was varied to
adjust for consistency, SP dosage was held at 0.5 %BWOB. The following notations are used to identify samples
throughout this paper: ‘APIG’ refers to an API Class G standard cement slurry without superplasticizer or retarder. ‘C’
followed by a number indicates the total weight of CSA cement and gypsum in %BWOB. ‘H’ and ‘L’ indicate a CSA
cement to gypsum ratio of 50:50 and 100:0, respectively. Samples without ‘H’ or ‘L’ have a CSA cement to gypsum ratio
of 70:30. The last number denotes the retarder dosage in % BWOB.
Cementing units:
The various components of cementing units, which fabricate and inject the cement slurry, illustrates the combination of
the components to assemble a basic cementing unit. A variety of configurations and compositions exists, tailored to the
type of rig to be serviced and the redundancy, versatility, and mobility required. The various configurations are
described below, according to the type of rig to be serviced. Skid-mounted units: Illustrated, skidmounted units are most
applicable to isolated land rigs, offshore rigs, cementing barges (lakes and rivers), and open-sea cementing vessels.
Truck-mounted units, such units are suitable for almost any land rig. However, the chassis must be adapted to the type
of surface upon which the unit will travel. The “standard” unit is designed to travel on roads and must conform to local
road regulations. The “off-road” unit is built for more difficult terrain. The “desert” unit can be driven over soft surfaces,
even sand dunes. Semitrailer-mounted units: Like the truck-mounted units, semitrailer-mounted units are appropriate
for almost any land rig. They can be drawn by many types of tractors, providing a logistical advantage. A heavy tractor-
drawn unit with five axles has a better weight distribution ability than the corresponding truck with only three. The
maximum authorized payload is greater than that of the truck, which allows the loading of more equipment on the same
chassis. Helicopter units: Helicopter units are intended for rigs totally inaccessible by land or water. The units, the mixing
equipment, and the cement silos are designed to be transported by helicopter. They can be dismantled into smaller
components, incorporating lifting frames, and are often made of lighter materials to reduce weight. Traditionally, a
cementing unit contains two of each vital item. This redundancy is necessary because a well can be severely damaged or
lost if it becomes impossible to complete a job after it has commenced. The extra equipment serves as an “insurance
policy” to protect the operator’s investment.

Remedial cementing tools:

Remedial cementing tools are mechanical or hydraulic devices used downhole to assist in the placement of cement
during plugback or squeeze cementing operations. They generally isolate areas of the casing from treating pressures and
cement. Some are available in retrievable or drillable designs, each being suited for a particular set of well conditions.
Remedial cementing tools are generally provided with service. Details of a specific tool’s operation or limitations should
be obtained from the service company or manufacturer.
Conclusion:
The main conclusions of this work can be summarized as follows:

• A good and sound cement sheath is achieved when the wellbore is well conditioned before running the casing. It must
be ensured that drilling mud cake is removed as much as possible as this ensures proper bonding between the casing
and formation.

• The casing string should be well centralized in the wellbore to attain a good cement sheath all around the casing. This
will be achieved by ensuring the minimum stand-off of 70% in critical sections of the well. • Bottom hole circulation
temperature is very critical in the design of cement slurry and accurate prediction is necessary as it affects the slurry
thickening time and rheology.

• The inclusion of silica flour in the design of cement slurry for geothermal wells ensures longevity of the well since it
helps prevent strength retrogression which occurs when cement sheath is exposed to elevated temperatures of more
than 120°C.

• The conditions encountered in the wellbore vary from one well to another and therefore pilot tests of cement slurry
should be conducted for each cement operation.

• Cementing calculations have to be done cautiously, taking into consideration the experience of the field being drilled,
to ensure that correct volumes are pumped to avoid cases of wet shoe that come as a result of over-displacement of
cement. This also ensures that pressure limits are not exceeded resulting in burst casings or fractured formations.

Reference:

1. GEOTHERMAL WELL CEMENTING, MATERIALS AND PLACEMENT TECHNIQUES

2. Experimental design of a well cement slurry for rapid gel strength development
3. Well Cementing(book).