- Rig down drilling equipment and rig up cementing head.
- Spacer is pumped.
- Bottom wiper plug is dropped.
- Bottom wiper plug reaches the float shoe, the plug is “bumped”. The diaphragm in the plug is ruptured and allows the cement to pass through and enter the annulus.
- Lead Cement is plugged.
- Tail cement is pumped.
- The second/top plug is dropped.
- Mud is used as a displacement fluid to push it down until the plug is seated in the float collar.
- Cement now occupies the annulus and the volume between the float collar and the shoe.
- The cement pumps are then used to pressure test the casing to ensure the plugs are holding.
- Pressure is then bled off.
- Cementing head is then rigged down and casing head rigged up.
To support the vertical and radial loads applied to the casing
Isolate porous formations from the producing zone formations
Exclude unwanted sub-surface fluids from the producing interval
Protect casing from corrosion
Resist chemical deterioration of cement
Confine abnormal pore pressure
Cement is introduced into the well by means of a cementing head. It helps in pumping cement between the running of the top and bottom plugs.
The most important function of cementing is to achieve zonal isolation. Another purpose of cementing is to achieve good cement to pipe bond. Too low an effective confining pressure the cement may become ductile.
For cement one thing to note is that there is no correlation between the shear and compressive strength. Another fact to note is that cement strength ranges between 1000 – 1800psi and for reservoir pressures > 1000psi this means that the pipe cement bond will fail first. This would lead to the development of micro-annuli along the pipe.
There are 8 general categories of additives.
- Accelerators – Reduces setting time and increases the rate of compressive strength build up.
- Retarders – Extends the Setting time.
- Extenders – Lowers the density.
- Weighting Agents – Increases density.
- Dispersants – Reduces viscosity.
- Fluid Loss Control Agents.
- Lost Circulation Control Agents.
- 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). Increase the hydration of Aluminate phase/gypsum system. Accelerate the hydration n of C3 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 phase 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.
Inorganic Compounds –
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.
Other Cement Additives
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.
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.
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
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.
These are cement slurries that upon entering the formation they begin the gel and eventually become self-supporting.