Cementing Additives and Mechanisms
There are 8 general categories of
additives.
Ø Accelerators
– Reduces setting time and increases the rate of compressive strength build up.
Ø Retarders
– Extends the Setting time.
Ø Extenders
– Lowers the density.
Ø Weighting
Agents – Increases density.
Ø Dispersants
– Reduces viscosity.
Ø Fluid
Loss Control Agents.
Ø Lost
Circulation Control Agents.
Ø Specialty
Agents.
Can be added, to shorten the
setting time, or to accelerate, the hardening process.
Calcium Chloride – under the right conditions it tends
to improve the compressive strength and significantly reduces the thickening
and setting time. Used in concentrations of up to 4.0%.
The mechanism is difficult to
understand but there are four major theories put forward.
It affects the hydration phase by
one of the following theories;
Cl– ions enhance the formation
of ettingite (crystalline) Tenoutasse 1978.
Increase the hydration of Aluminate
phase/gypsum system. Traettenber & Gratten Bellow 1975.
Accelerate the hydration n of C3S.
Stein 1961
Changes the C-S-H structure.
Controls the diffusion of water and
ionic species.
C-S-H gel has a higher area and will
react faster.
Diffusion of the chloride ions;
Cl– ions diffuse into the
C-S-H gel faster this producing the precipitation of portlandite sooner.
The smaller size of the Cl– ions
causes a greater tendency to diffuse into the C-S-H membrane. Eventually the
C-S-H membrane bursts and the hydration process is accerated.
Changes the aqueous pahse
composition.
Calcium chloride also produces a
high heat of hydration. This heat could accelerate the hydration process.This
heat will cause the casing to expand and contract as it dissipates. The
differing rates of expansion and contraction could result in the casing pulling
away from the cement and lead to the formation of micro-annuli.
It also has the ability to affect
the cement rheology, the compressive strength development, produce shrinkage by
10-15%, increases the permeability with time and lowers the sulphate
resistance.
They work by one of 4 main
theories;
Adsorption Theory – the retarder is
adsorbed & inhibits water content.
Precipitation Theory – reacts with
aqueous phase to form an impermeable and insoluble layer around the cement
grains.
Nucleation Theory – retarder
poisons the hydration product and prevents future growth.
Complexation Theory – Ca+ ions
are chelated by the retarder. A nucleus can’t be properly formed.
Lignosulphonates
– Wood pulp derived polymers. Effective in all Portland cements and added in
concentrations of 0.1% to 1.5% BWOC. It absorbs into the C-S-H gel and causes a
change of morphology to a more impermeable structure.
Hydroxycarboxylic Acids – They have
hydroxyl carboxyl groups in their molecular structure. Below 93°C they can
cause over retardation. They are efficient to temperature of 150°C. One acid
used in citric acid with an effective concentration of 0.1% to 0.3% BWOC.
Saccaride
Compounds – Sugars are excellent retarders of Portland cement. Such compounds
are not commonly used due to the degree of retardation being very sensitive to
variation of concentration. It also depends on the compound’s susceptibility to
alkaline hydrolysis.
Cellulose
Derivatives – Polysaccharides derived from wood or vegetal matter, and are
stable to the alkali conditions of the cement slurry.
Organophosphates – Alkylene
phosponic acids.
Acids and accompanying salts
Sodium Chloride, used in
concentrations of up to 5.0% used with bottom hole temperatures less than 160
deg F. it will improve compressive strength and reduce thickening and setting
time.
Oxides of zinc and lead.
Reduce slurry density –
reduces hydrostatic pressure during cement.
Increases slurry yield – reduces
the amount of cement required to produce a given volume.
Water Extenders –
Allows/facilitates the addition of water to help extend the cement
blend/slurry.
Low Density Aggregates –
Materials with densities less than Portland cement (13.5 g/cm3)
Gaseous extenders – Nitrogen
or air can be used to prepare foam.
Clays – Hydrous aluminum
silicates. Most common is bentonite (85% mineral clay smectite). Can be used to
obtain a cement of density 11.5 to 15.0ppg, with concentrations up to 20%. Used
with an API ratio of 5.3% water to 1.0% bentonite.
Bentonite – this is added in
conjunction with additional water, used for specific weight control but will
make for poor cement.
Pozzolan – finely ground pumice of
fly ash. Pozzolan costs very little but does not achieve very high weight
reduction of the slurry.
Diatomaceous Earth – also
requires additional water to be added. Properties are similar to that of
bentonite.
Silica – α quartz and
condensed slilica fume. α quartz is ised to prevent strength retrogression in
thermal wells. Silica fume (micro fume) is highlt reactive the most effective
pozzolanic material available. The high surface area increases the water demand
to get pumpable slurry. Such a mixture can produce a cement slurry as low as
11.0ppg.
Normal concentration = 15%
BWOC but can be as high as 28% BWOC.
Can sometimes be used to prevent
annular fluid migration
Expanded Pearlite – Used to
reduce the weight as water is added with its addition. Without bentonite the
pearlite separates and floats to the upper part of the slurry.
Can be used to achieve a slurry
weight as low as 12.0ppg. Bentonite in concentrations of 2-4% is also added to
prevent segregation of particles and slurry.
Gilsonite – Used to obtain
slurry weights as low as 12.0ppg. in high concentrations mixing is a problem.
Powdered coal – Can be used to
obtain a slurry with a density as low as 11.9ppg, 12.5-25lbs per sack are
usually added.
Microspheres – Small gas
filled beads that promote densities as low as 8.5ppg., they can be either glass
or ceramic.
Nitrogen – Nitrogen is used
as the density reducing medium. The base slurry needs to be homogenous with
high compressive strength and low permeability. Could achieve densities as low
as 7.0ppg.
Ilmenite – Can attain
densities in excess of 20.0ppg. The viscous nature of the slurry may promote
sedimentation. It must be adjusted.
Hematite – Used to increase the
specific weight of the cement. It is an iron oxide ore. Has minimal effect on
the thickening time or compressive strength of the cement. Can prepare slurries
up to 19.0ppg but can go as hig as 22.0ppg. A much finer particle size
distribution.
Barite – Requires more water
to be added to the slurry and as such the compressive strength of the cement is
reduced. Can prepare slurry weights as high as 19.0ppg.
Limenite – requires no
additional water to be added to the slurry. Minima effect on the thickening
time or compressive strength.
Sand – no additional water is
needed and it has little effect on the pumpability of the cement. When set the
cement will form a very hard surface.
Gypsum – blended with
Portland cement to produce a cement blend with reduced thickening and setting
time for low temperature applications. i.e. less than 140 deg F. However a
significant amount of water is needed when using gypsum.
Sodium Silicate – used for
great depths. Used to retard the thickening and setting time, especially good
at very low concentrations. For high temperature applications it is necessary
to add organic acid.
Highly concentrated
suspensions of solid particles in water. With concentration as high as 10%.
When cement goes across a
zone the aqueous phase of the slurry goes into the formation, leaving the
cement particles.
As the aqueous phase
decreases, the slurry density increases and the slurry performance diverges
from the original design. If enough fluid is lost the slurry becomes difficult
to pump to the point where it may be able to be pumped.
To maintain API standards for
adequate slurry performance you need a fluid loss rate of less than 50ml/30min.
Such fluid loss matter act by ;
Filter cake formation across the
zone.
Reducing the permeability of the
filter cake.
Increasing the viscosity of the
aqueous phase.
Particulate Materials
Uses latex additives to
achieve fluid loss. Emulsion polymers are supplied as suspensions of polymer
particles. They contain about 50% solids. Such particles can physically plug
the pores in the filter cake.
Water Soluble Polymers
They increase the viscosity
of the aqueous phase and decrease the filter cake permeability.
Cellulose Derivatives
Organic proteins
(polypeptides). Not used above temperatures of 93°C.
Non-Ionic Synthetic Polymers
Can lower fluid loss rates from
500ml/30min to 20ml/30min.
There is also Anionic
Synthetic Polymers and Cationic polymers.
Lost Circulation Prevention
Bridging agents
The addition of materials
that can physically bridge fractured or weak zones. Eg Gilsonite and Sellophane
flakes added in quantities of 0.125-0.500lbs/sack.
Thixotropic Cement
These are cement slurries
that upon entering the formation they begin the gel and eventually become
self-supporting.
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