cementing Problems Related to Cementing and their Solutions

Cementing

1. CEMENTING
Oil well cementing is the placement of cement slurry inside or outside the casing in the well. As
oil/gas wells are drilled, it is necessary todeterminethe casing setting depths of the different
casing strings, run the casing strings and cement the casing in place. Cementing oil and gas wells
involves the displacement of cement slurry down the tubing or casing to pre-determined sections
of the annulus and allowing the cement to set.
Materials used in oil well cementing vary from basic Portland cement used in engineering
construction of all types, to highly sophisticated special-purpose resin-based or latex cements.
Portland cement (API Classes A, C, H, and G), Blast furnace slag (BFS) and Pozzolans(fly ash),
ASTM Types C and F are the main cementing materials used in oilfield applications

2. Cement
Cements are made from limestone or other material containing a high percentage of
calcium carbonate and clay or shale. The chemical composition may be adjusted if
necessary for instance iron and aluminum oxides may be added if not present in sufficient
quantity in the clay or shale.
Cement used in the Industry
SlagCement: Blast furnace slag (BFS) is a byproduct obtained in the manufacture of pig-
iron in a blast furnace in the steel industry. The material on top of the molten steel is
removed and cooled, then quenched with water and ground. The material composition is
mainly monocalciumsilicate, dicalciumsilicate and dicalciumaluninosilicate. These silicates
set very at room temperature when mixed with water.
PozzolanCement: Pozzolansare siliceous/aluminous materials that react with calcium
hydroxide (lime) and water to form a stable cement. Naturally occurring pozzolansare
normally created during volcanic activity while lyash, a waste product from coal-burning
power plants is the most common pozzolan. These non-combustible materials are collected
before going out of the smoke stacks. Pozzolancan be mixed with Portland cement or with
Portland cement and lime.
Other types of cement include ultrafine cement, epoxy cement, perlite cement, diesel oil
cement, latex cement, radioactive cement, Cal cement (fast setting Plaster of Paris).

3. Portland Cement
Portland cement is manufactured by fusing with heat calcium carbonate (limestone) and
aluminum silicates (clay), with a small amount of iron when required. The dry materials are
finely ground and mixed thoroughly in the required proportions to obtain a kiln feed, which
is fed into the rotary kiln and heated to temperatures of 2600-2800oF (1427-1538oC).
The molten rock is cooled, and the resulting material which is referred to as clinker, is then
ground with a controlled amount of gypsum(CaSO4. 2H2O) to form Portland cement.
Portland cement consists of different components, the most abundant being tricalcium
silicate. The main compounds found in Portland cement and their functions are:
-DicalciumSilicate (Ca2 S) is a slow hydrating compound and results in a gradual gain in
strength, which occurs over an extended period.
-TricalciumSilicate (Ca3 S)is the main strength-producing material, which is responsible
for early strength (1 to 28 days).
-TricalciumAluminate (Ca3 Al) promotes rapid hydration and controls the initial setting
and thickening time. It is also controls the resistance or susceptibility of cement to sulphate
attack. High sulphateresistant cements have 3% or less Ca3 Al. -
TetracalciumAluminoferrite(Ca4 AF) gives color to the cement. An excess of iron oxide will
increase the amount of Ca4 AF. All the classes of Portland cement are basically
manufactured in the same way using the same ingredients, but in varying proportions. The
table below shows the content of the different components in the API classes of cement

5. CementSlurry: Water is added to the cement to convert it to a hydrous form
called cement slurry. The water is added in order to hydrate the cement and
make it pumpable. The water requirement of each type of cement varies
with the fineness of grind. Though only about 25% water by weight of
cement is needed to hydrate the cement, more water is needed to make the
slurry pumpable.
Setting and hardening reactions begin immediately when water is added to
cement. The chemical compounds in the cement undergo hydration and re-
crystallization, resulting in a set product.
Portland cement does not get hard through a drying process but through the
chemical reaction when thewaterwith which it is mixed reacts with
tricalciumsilicate to form calcium silicatehydrate(CSH). The calcium silicate
hydrate has the physical strength that makes Portland cement hard at room
temperature in several hours.

7. Properties of Cement Slurries
•Pumping or thickening time
•Setting time
•Sulphateresistance

8. Standardization of cements
There are 2 major classification systems for cement, the ASTM (American Society for
Testing Materials) and API classifications.
The ASTM specification classifies Portland cements as types I, II, III, IV and V they are
manufactured for use at atmospheric conditions while the API classification has 8 or 9
classes as shown below.
Class A: Designed for use from surface to 6,000 ft(1,830 m) depth where special
properties are not required.
Class B: Intended for use from the surface to 6,000 ft(1,830 m) depth where conditions
require moderate to high sulphateresistance.
Class C: Designed for use from surface to 6,000 ft(1,830 m) depth where conditions
require high early strength.
Class D: Intended for use from 6,000 to 10,000 ft(1,830 to 3,050 m) depth, under
conditions of moderately high temperatures and pressures.
Class E: Intended for use from 10,000 to 14,000 ft(3,050 to 4,270 m) depth, under
conditions of high temperatures and pressures.
Class F: designed for use from 10,000 to 16,000 ft(3,050 to 4,880 m) depth, under
conditions of extremely high temperatures and pressures.

9. Standardization of cements. Ctnd.
Class G: Intended for use from surface to 8,000 ft(2,440 m) depth as manufactured, or can be used
with accelerators and retarders to cover a wide range of well depths and temperatures.ClassG is
the most common type of cementusedin most areas.
Class H: Intended for use as basic well cement from surface to 8,000 ft(2,440 m) depth as
manufactured, and can be used with accelerators and retarders to cover a wide range of well depths
and temperatures. Class H has a coarser grind than Class G and givesbetterretarding properties in
deeper wells.
Class J: Intended for use as manufactured from 12,000 to 16,000 ft(3,660 to 4,880 m) depth under
conditions of extremely high temperatures and pressures or can be used with accelerators and
retarders to cover a range of well depths and temperatures.
The API Classes A, B and C correspond to ASTM Types I, II and IIIrespectivelywhile The API Classes D,
E, F, G, H and J are cements manufactured for use in deep wells and tobesubjected to a wide range
of pressures and temperatures and have no corresponding ASTM types.
The chemistry of each of these grades is similar, but the quality control is most stringent for API
Classes G and H. Also, the particle size for each of these classes is different, with API Class C being
the finest. Increasing coarseness dictates that less water is required to wet the particles to create
pumpableslurry.

10. Cement Additives
Accelerators: These are reaction rate enhancers. Accelerators are used
to reduce the thickening time of cement slurry, shorten the setting time
and increase the rate of early strength development. They are usually
applied at low temperatures to reduce waiting on cement (WOC) time
because the cement slurry takes a longer time to achieve the required
compressive strength at lower temperatures. Accelerators are usually
applied in slurries to be used for conductor and surface casing.
Accelerators are mostly inorganic materials that have the ability of
reacting with the cement. Common accelerators include calcium
chloride, sodium silicate, sodium chloride, seawater, gypsum and
ammonium chloride. Most salts are used as accelerators in low
concentrations but as retarders in high concentration.

11. Retardersare used to slow down the setting of cement and to increase
the thickening time invariably increase the setting time. Retarders are
used to counter the effect of temperature with increase in well depth.
They are used in cement slurries for intermediate and production
casings, squeeze cementing and cement plugs. Typical retarders include
sugar, lignosulphonates(low concentration) egcalcium
lignosulphonate, and cellulose derivatives egcarboxy-
methylhydroxyethylcellulose (CMHEC), and organic compounds. In
high concentrations, salts can be used as retarders.

12. Extendersare lightweight additives, which increase the slurry yield and
lower the slurry density to allow weak formations to be cemented
without being fractured by the cement column. Extenders include
bentonite, sodium silicates, pozzolans, nitrogen and ceramic
microspheres.
Fluid-loss additives are used to prevent slurry dehydration and reduce
loss of fluid to the formation. Most fluid-loss additives increase the
slurry viscosity, although some retard it to some degree. Long chain
polymers such as CarboxyMethyl HydroxyEthyl Cellulose (CMHEC),
CMC, hydroxyl-ethyl cellulosearealso used to reduce filtrate loss.
Bentonitecan also be used to control fluid loss but is not as effective as
the long chain polymers.
Dispersantsare materials that reduce the viscosity of cement slurry.
They tend to improve fluidlosscontrol and are also useful in the design
of high-density slurries. Common dispersants are calcium
lignosulphonatesand polymers

13. Lost Circulation Control Agents are materials used to control the loss
of cement slurry to weak or fractured formations. Ground coal (kolite),
ground walnut hull and cellophane flakes are lost circulation control
additives. Weighting Materials are high-density particulate materials
used in the formulation of high-density slurries. High-density slurries
are required when the bottom-hole pressure in a well is high. These
materials include barite, hematite and sand.
Saltsmodify the ionic strength of the mix water. They are used while
cementing through immobile salt zones and also to improve bonding to
shale sections.
Strength Retrogression Control Additives: Normal cements develop
high permeability and reduction in strength at temperatures above
230F. The addition of 30-40% silica flour BWOC (by weight of cement)
prevents both strength reduction and development of permeability at
high temperatures.

14. Reasons for cementing
1) To bond the casing to the formation thereby supporting and strengthening
it.
2) Seals against contamination of fresh water zones and protects other oil
and gas zones not being produced.
3) Slows down corrosion of the casing by minimizing contact between the
pipe and formation fluids.
4) Helps to prevent blowout from high-pressure zones behind the casing.
5) Seals off lost circulation zones and other troublesome formations in order
to continue drilling.
6) Prevents vertical migration of formation fluids behind the casing.
7) Provides a base for squeeze cementing, fracturing and future work-over
during the life of the well.
8) Protects the surface and intermediate casing string while drilling the
additional hole, as un-cemented pipes are severely shock loaded.

15. Types of Cementing Jobs
Cementing jobs are classified either primaryor secondary.
Primary cementing refers to the cementing of casing or liner strings in
a well. Primary cementing is the process of placing cement between
the casing and wellbore wall.
Primary Cementing Techniques
Following are the three most commonly used techniques
1. Single stage cementing
2. Multi stage cementing
3. Liner cementing

16. Primary (single stage) cementing procedure
During primary cementing, the casing string is lowered into the
wellbore using elevators and drawworksdisplacing the mud in the bore
as the casing is lowered. The casing string is left suspended by the
elevators so that it can be rotated or reciprocated. The cement head is
then made up to the upper end of the string and then connected to
flow lines coming from the pumping truck. The cement slurry is mixed
in a recirculating blender, which provides constant cement slurry of
specific weight. The blender is then connected to a cement pump
which pumps the cement at low circulation rates (and high pressures if
necessary) to the cementing head at the top of the casing string and
down into the well through the tubing. A spacer (usually water) is
pumped ahead of the cement slurry to aid in removing the drilling mud
in the annular space.

17. A bottom wiper plug is sent ahead of the spacer to separate the drilling
mud and the spacer thus preventing the contamination of the spacer by
the drilling mud. After displacing the required quantity of cement
through the casing head, the top wiper plug is then sent down the well
behind the slurry. The diaphragm of the bottom plug ruptures as a
result of increase in pressure when the plug reaches the float collar,
which is usually placed in the final or last but one joint of the casing
string. The spacer and cement slurry then flow through the float collar,
down the last or last two joints, and into the annulus between the
casing and wellbore wall. The volume of spacer and cement slurry
displaced is what is required to fill the annular space or to reach a
height that is sufficient to accomplish the cementing job. The top plug is
sent after the cement and drilling mud is displaced after the top plug.
When the top plug reaches the float collar, it is bumped on top of the
bottom plug and the cement pump is shut down.

18. Conventional Primary Cementing Procedure (Single Stage)

20. Summary of procedure for single stage
cementing
1. Circulate the casing and annulus clean with mud (one casing volume
pumped)
2. Release wiper plug
3. Pump spacer
4. Pump cement
5. Release shut-off plug
6. Displace with displacing fluid (generally mud) until the shut-off plug lands
on the float collar
7. Pressure test the casing

21. Cementing Equipment
Cementing Head: It connects discharge line from cement unit and top
of casing.
Scratchers: its designed to remove mud cake and break up gelled mud.
Guide Shoe: Placed on the bottom of first joint of casing. It guides and
centralize casing.
Wiper plugs: is the first cement plug that is pumped ahead of spacer to
clean inside the casing.
Shut-off plug: is the second plug that follows cement slurry. When it
reaches the float collar, It lands on the wiper plug and this marks the
end of displacement process.

22. Centralizer
Centralizers: Used to keep casing away from
the borehole so that there is some annular
clearance around the entire circumference of
the string.
Centralizer helps to
1.Place cement all the way around the
casing
2.Prevent differential sticking
3.Keep casing out of keyseats.
1 centralizer is placed immediately above the
shoe, 1 every joint on the bottom 3 joints, 1
every joint through the production zone, 1
every 3 joints elsewhere.

23. Float collar
Float collar-A float collar is positioned 1 or 2
joints above the guide shoe. It acts as a seat
for the cement plugs used in the pumping
and displacement of the cement slurry. This
means that at the end of the cement job
there will be some cement left in the casing
between the float collar and the guide shoe
which must be drilled out. The float collar
also contains a non-return valve so that the
cement slurry cannot flow back up the casing.

24. Cementing Calculations
•Cement slurry requirement
•No. of Sacks of cement
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•Mixwaterrequirement
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•Additive requirement
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•Displacement volume for conventional cementing operation
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