Function of cementing

In an oil/ gas well, the primary functions of cement are:
1. Provide zonal isolation
2. Support axial load of casing strings
3. Provide casing protection against corrosive fluids
4. Support the wellbore

Cement Manufacture &Chemistry

Cement is made from calcareous and argillaceous rocks such as limestone, clay and shale
and any other material containing a high percentage of calcium carbonate. The dry material
is finely ground and mixed thoroughly in the correct proportions.The chemical composition
is determined and adjusted if necessary. This mix is called the kiln feed.




The kiln feed is then heated to temperatures of around 2600-2800 F (1427-1538 C). The
resulting material is called clinker. The clinker is then cooled, ground and mixed with a
controlled amount of gypsum and other products to form a new product called Portland
cement. Gypsum (CaSO4. 2H2O) is added to control the setting and hardening properties of
the cement slurry and to prevent the flash setting cement. Figure (1) shows a flow diagram
of the manufacturing process and the chemical composition of the clinker.
Cement slurry is the mixture produced when dry cement is mixed with water.

Oil well cement is manufactured to API Specification 10 and is divided into 8 classes (A-H)
depending upon its properties. Class G and H are basic well cements which can be used with
accelerators and retarders to cover a wide range of depths and temperatures. The principal
difference between these two classes is that Class H is significantly coarser than Class G.

The classes are:

• CLASS A: Intended for use from surface to a depth of 6,000 ft (1,830 m), when special
properties are not required. Similar to ASTM (American Society for Testing Materials) Type I
cement.
• CLASS B: Intended for use from surface to a depth of 6,000 ft (1,830 m). Moderate to high
sulphate resistance. Similar to ASTM Type II, and has a lower C3A content than Class A.
• CLASS C: Intended for use from surface to a depth of 6,000 ft (1,830 m) when conditions
require early strength. Available in all three degrees of sulphate resistance, and is roughly
equivalent to ASTM Type III. To achieve high early strength, the C3S content and the surface
area are relatively high.
• CLASS D: Intended for use from 6,000 ft (1,830 m) to 10,000 ft (3,050 m) under conditions of
moderately high temperatures and pressures. It is available in MSR (moderate sulphate resistance)
and HSR (high sulphate resistance) types.
• CLASS E: Intended for use from 10,000 ft (3,050 m) to 14,000 ft (4,270 m) under conditions
of high temperatures and pressures. It is available in MSR and HSR types.
• CLASS F: Intended for use from 10,000 ft (3,050 m) to 16,000 ft (4,880 m) depth under
conditions of extremely high temperatures and pressures. It is available in MSR and HSR types.
• CLASS G + CLASS H: Intended for use as a basic well cement from surface to 8,000 ft (2,440
m) as manufactured, or can be used with accelerators and retarders to cover a wide range of well
depths and temperatures. No additions other than calcium sulphate or water, or both, shall be
interground or blended with the clinker during manufacture of Class G and H well cements.
They are available in both MSR and HSR types.

Cementing additives

Additional chemicals are used to control slurry density, rheology, and fluid loss, or to
provide more specialised slurry properties.
Additives modify the behaviour of the cement slurry allowing cement placement under a
wide range of downhole conditions. There are many additives available for cement and these
can be classified under one of the following categories:
Accelerators: chemicals which reduce the thickening time of a slurry and increase the rate
of early strength development.They are usually use in conductor and surface casing to reduce
waiting on cement time (WOC). Calcium chloride (CaCl2), sodium chloride (NaCl) and sea
water are commonly used as accelerators.
Retarders: chemicals which retard the setting time (extend the thickening) of a slurry to aid
cement placement before it hardens.These additives are usually added to counter the effects
of high temperature.They are used in cement slurries for intermediate and production
casings, squeeze and cement plugs and high temperature wells. Typical retarders include:
sugar; lignosulphonates, hydroxycarboxylic acids, inorganic compounds and cellulose
derivatives. Retarders work mainly by adsorption on the cement surface to inhibit contact
with water and elongate the hydration process; although there are other chemical
mechanisms involved.
Extenders: materials which lower the slurry density and increase the yield to allow weak
formations to be cemented without being fractured by the cement cloumn.Examples of
extenders include: water, bentonite, sodium silicates, pozzlans, gilsonite, expanded perlite,
nitrogen and ceramic microspheres.
Weighting Agents: materials which increase slurry density including barite and haematite,
see Chapter Seven for full description.
Dispersants: chemicals which lower the slurry viscosity and may also increase free water by
dispersing the solids in the cement slurry. Dispersants are solutions of negatively charged
polymer molecules that attach themselves to the positively charges sites of the hydrating
cement grains.The result is an increased negative on the hydrating cement grains resulting in
greater repulsive forces and particle dispersion.
Fluid-Loss Additives: Excessive fluid losses from the cement slurry to the formation can
affect the correct setting of cement. Fluid loss additives are used to prevent slurry
dehydration and reduce fluid loss to the formation.Examples include: cationic polymer, nonionic
synthetic polymer, anionic synthetic polymer and cellulose derivative.
Lost Circulation Control Agents: materials which control the loss of cement slurry to weak
or fractured formations.

Strength Retrogression: At temperatures above 230 F, normal cement develop high
permeability and reduction in strength. the addition of 30-40% BWOC (by weight of
cement) silica flour prevents both strength reduction and development of permeability at
high temperatures.
Miscellaneous Agents: e.g. Anti-foam agents, fibres, latex.

Slurry testing

Cement tests should always be performed on representative samples of cement, additives
and mix water as supplied from the rig. Cement tests are detailed in API 10, references a & b.1 1-THICKENING TIME
Thickening time tests are designed to determine the length of time which a cement slurry
remains in a pumpable state under simulated wellbore conditions of temperature and
pressure. The pumpability, or consistency, is measured in Bearden Consistency units (Bc);
each unit being equivalent to the spring deflection observed with 2080 gm-cm of torque
when using the weight-loaded type calibration device. The measure takes no account of the
effect of fluid loss. Thus, thickening times in the wellbore may be reduced if little, or no,
fluid loss control is specified in the slurry design.
Results should quote the time to reach 70 Bc - generally considered to be the maximum
pumpable consistency.
2- FREE WATER AND SEDIMENTATION
The separation of water from a slurry, once it has been placed, can lead to channel formation
and gas migration problems - particularly in deviated wells. The free-water test is designed
to simulate this using a 250 ml graduated cylinder in which slurry is left to stand for two
hours under simulated wellbore conditions. The volume of water collected after this period is
expressed as a percentage by volume.
For deviated wellbores, a more critical test is to incline the column at 45 degrees. However,
care should be exercised with the results from inclined tests as the migration path for the
water is significantly reduced and thus the free water measured will increase. Downhole, this
may not be the case due to the presence of the casing string.
The reporting of free-water should be as a percentage. When 'traces' are reported, definition
of this term should be sought. For liners and in wells where gas may be present, a zero freewater
slurry should be used.
The amount of sedimentation of the slurry should also be reported by measurement of the
variation of density over a sectioned column of set cement.
3- FLUID-LOSS
Fluid-loss tests are designed to measure the slurry dehydration during, and immediately after
cement placement. Under simulated wellbore conditions, the slurry is tested for filtrate loss
across a standardised filter press at differential pressures of 100 psi or 1000 psi. The test
duration is 30 minutes and results are quoted as ml/30 min.
4 -COMPRESSIVE STRENGTH
The measurement of the uniaxial compressive strength of two-inch cubes of cement provides
an indication of the strength development of the cement at downhole conditions. The slurry
samples are cured for 8, 12, 16 and 24 hours at bottom-hole temperatures and pressures and
the results reported in psi. Dynamic measurements using ultrasonic techniques correlate well
with API test results, but can lead to over-estimation of the strength.
5- RHEOLOGY
Ensuring that the rheological behaviour of the slurry downhole is similar to that specified in
the design is essential for effective cement placement. The slurry viscosity is measured using
a rotational viscometer, such as a Fann. The slurry sample should be conditioned for 20
minutes in an atmospheric consistometer before measurements are taken.
Readings should be taken at ambient conditions and at BHCT when possible. Measurements
should be limited to a maximum speed of 300 rpm (shear rate 511 1/s). Readings should also
be reported at 200, 100, 60, 30, 6 and 3 rpm.