Primary cementing is performed after the well is drilled and the casing is installed.
The cement slurry is pumped down the casing and circulated up the annulus.
This is done to seal the annulus, provide support for the casing, create zonal isolation and protect the casing from possible corrosion.
Secondary cementing operations are performed after the casing has been cemented in the well.
Some examples of these operations are plugging water zones, cement squeezes, and repairing casing leaks.
Related: What is Casing?
How is Primary Well Cementing Performed?
After the required volume of cement is calculated and mixed, it is pumped into the casing between two wiper plugs.
When the first plug reaches the toe of the well it breaks apart and allows cement to go into the annulus.
After the second plug reaches the toe of the well, there is a pressure signature at the surface.
The role of the wiper plugs is to clean the casing walls before the cement plug and behind it.
There are also other ways to cement the casing such as pumping cement through the tubing or even placing cement into the wellbore before the casing is installed.
The amount of time it takes for cement to harden depends on the type of cement used and downhole conditions. Usually, it is between 12 to 24 hours.
After cementing job is performed, the quality of cement is evaluated with Cement Bond Logging (CBL) tool run on coiled tubing or wireline.
This tool uses acoustic waves to determine cement integrity.
It is important to pump the right volume of cement to avoid leaving a void space in the annulus or allowing cement to get trapped inside of the casing.
If problems arise during the cementing operation that results in cement hardening inside of the casing, it can decrease the usable length of the well.
In many cases, it might require cement milling which can be a pretty slow process.
During improper cement operations up to 1000-2000 of the casing can get cemented and require milling.
In extreme cases, it might take weeks to mill out all the cement inside of the casing.
On the other hand, if not enough cement is placed it can potentially result in hydrocarbons moving up the annulus.
To get a better estimate of the amount of cement required for the job, caliper logging is performed on the open hole before the casing is installed.
The type of cement used will depend on the downhole conditions such as temperature, pressure, and formation type.
Related: How Coiled Tubing Milling Operations Are Performed?
Secondary Cementing Operations
Many secondary or remedial cementing operations are performed to resolve problems created during improper primary cementing jobs.
This is usually accomplished by either pumping cement plug down the well, performing squeeze cementing, or both.
What is Squeeze Cementing?
If there are issues during a primary cementing job, sometimes more cement is squeezed into the specific zone to fix the problem.
This is performed by spotting cement at the required depth and then applying pressure at the surface to force it into the formation.
To ensure that cement slurry goes into the target zone sometimes packer tools and plugs are used to isolate the zone.
In some cases, coiled tubing can be used to pump cement into the target zone.
Some examples of the issues that might require cement squeeze are cracks in the cement in the annulus or void space that was not filled by cement during the initial cementing operation.
Related: 18 Most Common Coiled Tubing Applications
Sealing Lost Circulation Zones
If there is a low-pressure zone that is taking all the fluid, cementing can be used to seal that zone.
Repairing Casing Leaks
The casing can get damaged during frac operations or due to other factors that may arise during the life of the well.
No matter what the cause, the leak usually needs to be repaired.
One way to accomplish this is to squeeze cement into the formation around the leaking casing.
There are also more sophisticated methods like placing a casing patch to cover the leak but these methods are more expensive and might not work for older wells.
Zonal Isolation
If one of the zones in the well poses problems to production (ex. producing too much water), it might have to be shut down.
This can be achieved either by performing a cement squeeze or placing a cement plug over a target zone.
Well Abandonment
At the end of its life before the well can be abandoned, usually a cement plug is placed inside of the well to prevent any fluid flow to the surface.
Related: Well Abandonment and Orphan Wells In Oil And Gas Industry
What Cement Slurry is Made Of?
The main components of cement slurry are usually gypsum, lime, or pozzolans.
Other chemicals are often added to control the properties of the cement:
Density control – bentonite, barite
Accelerators – used to decrease thickening time (calcium chloride, sodium chloride)
Retarders – used to increase thickening time by decreasing cement hydration rate (calcium lignosulfonate)
Viscosity control – calcium lignosulfonate, sodium chloride
Filtration control – polymers, latex
Lost circulation control agents – gilsonite, sawdust, walnut shells
Typical Cementing Unit Equipment
High-Pressure Pump – pushes cement slurry into the well.
Blender – used for mixing dry cement with water and other additives to create cement slurry.
Cementing head (plug retainer)– a connection that allows cement to be displaced from the pump and into the casing.
Read next: Most Common Types of Wireline Logging Operations
References:
https://petrowiki.spe.org/PEH:Cementing