I- Introduction
The cement is used as a sealing material while drilling an oil and gas well. Pumping cement behind the casing provides a barrier to the fluids flow from or into the formation, and bonds the casing to the formation. This is called a primary cement job. Cement is also pumped for other reasons like sealing off perforated casing or plugging zones (Click Here).
Fig 01- Oil Well Sketch
II- Oil Well Cement Functions
There are many reasons for pumping cement slurry while drilling operations, the most important functions are:
- Preventing fluids flow from one formation to another or to the surface. The fluids can flow between the casing and the formation.
- Bonding the casing to the formation
- Supporting the casing string
- Protecting the casing from the corrosion caused by formation fluids
- Sealing off the troublesome zones
A successful primary cementing job is crucial for the well integrity to allow continuing the drilling operations. In other cases, cement has to be pumped into the well and forced to flow into the formation; this process is called secondary or squeezing cementing job.
III- Cement Powder Classes
There is a verity of cement powder which is used for cementing an oil and gas well. Each cement powder differs from other powders when mixed with water due to the chemical components.
Class A & B
They are cheap powders and can be used only for shallow depths where there is not any special requirement. The difference between A class and B class is that B class can resist to sulphate.
Class C
This powder can harden quickly due to the high concentration of C3S
Class D,E and F
These are expensive powders due to the properties which they can provide. These classes are known as retarded cements which make them suitable for cementing deep wells at high level of temperatures and pressures.
Class G & H
These are the most compatible powders with most of additives and can be used for wide range of pressure and temperature. The most common used class is class G, it can be used at temperature up to 200F° without adding modifiers. Class H has coarser grind than class G which gives it better retarding properties for cementing deep wells.
The cement powders mentioned above are API classified cements. There are other non-API cement:
- Pozmix cement: It is a result of mixing Portland cement with pozzolan and bentonite. It is used for shallow wells due to its light weight.
- Gypsum cement: it is formed by mixing Portland cement and gypsum. This type of cement is used for remedial jobs; it can develop a high early strength. It is very appropriate for sealing off lost circulation zones.
IV- Mix-water Requirements
The water which is used to prepare the cement slurry has to be well quantified to provide appropriate cement mixture. If The amount of water is too much than the accurate percentage, the cement barrier will not be strong and impermeable and if it is less than designed, the slurry density and viscosity will increase, also the pumpability will increase.
The amount of water is designed based on these issues:
- Getting an easy slurry to pump
- Hydrating all the cement powder which provides hard cement
- No free water has to be present after the cement hardened
V- Cement Properties
At designing stage, fundamental properties have to be considered for any cement slurry.
Compressive Strength
After pumping the cement slurry and place it in the annulus, drilling operations should not be started till the cement reaches an appropriate level of compressive strength. The cement should not disintegrate due to vibrations when drilling next section. The compressive strength depends on several variables: temperature, pressure and amount of mix-water.
Thickening Time
The thickening time is the range of time which the cement slurry can be pumped and displaced. The thickening time should be enough to allow the slurry to be mixed, pumped into the casing and then displaced by drilling fluids till placed it in the annulus. When designing the cement slurry, any delays or interruption should be considered to get a safe thickening time.
Wellbore conditions are also important factors which can affect the thickening time. Reduction in this parameter can be caused by increase in pressure or temperature. These conditions should be simulated and tested in the laboratory before performing the job.
Slurry Density
Cement slurry density can vary from 11 ppg to 18.5 ppg. The density is controlled by adjusting the amount of mix-water or additives. The cement slurry density is relatively higher than the density of drilling fluids which has to be considered when planning for cement job especially when formations with normal pore pressure trend.
Water Loss
The setting of cement is a result of cement powder hydration. Slurry should not lose its water before place it at its planned position because it can affect its pumpability. The fluid loss property should be determined at laboratories under standard conditions. The fluid loss depends on the type of the job; it is not crucial to control fluid loss in primary jobs but for the squeezing jobs it should well defined.
Permeability
The permeability of cement after hardening will be very low (less than 0.1 milli-darcy); this level is much lower than production formations permeability. In some cases, the permeability of the cement can be affected: for example by gas intrusion which lead to higher permeability and then the barrier of cement can be less effective.
VI-Cement Additives
Slurry properties are adjusted by adding additives; this adjustment is performed to optimize the cementing jobs. The cement additives are used to:
- Varying the slurry density
- Changing the compressive strength
- Accelerating or retarding the setting time
- Controlling fluid loss
- Reducing the slurry viscosity
Accelerators
When cementing shallow wells, where the temperatures are low, the thickening time can be decreased to avoid any additional rig time to wait for cement hardening. The reduction is reached by using what are called accelerators
Common accelerators are sodium chloride, potassium chloride and gypsum
Retarders
The temperature increases with depth. The high temperature in deep wells reduces the thickening time which can lead to cement setting into the casing. Retarders are used to extend to the thickening time to avoid such risks.
Common retarders are organic acids, modified cellulose and lignosulphonates.
Extenders
They are used to reduce the slurry density in case when there is a risk of exceeding the fracture pressure of some formations. The use of this type of additives can also reduce the compressive strength and increase the thickening time. They called extenders because they allow more mix-water and as a consequence more slurry volume.
The most common extenders are the bentonite, pozzolan and diatomaceous.
Heavy additives
Cementing through over-pressured formations can be crucial. The slurry density in these cases is increased by adding barite, hematite and sand. The use of barite and hematite can cause a significant reduction in pumpability.
Fluid loss additives
They are used to avoid cement dehydration and premature setting. Organic polymers are the most common fluid loss additive.
Dispersants
They are used to lower the slurry viscosity in order to improve the flow properties. The most common dispersants are the salt, polymers and calcium lignosulphanate.
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