Centralisers are hinged metal ribs which are installed on the casing string as it is run (Figure 7). Their function is to keep the casing away from the borehole so that there is some annular clearance around the entire circumference of the casing.
The proper use of centralisers will help to:
• Improve displacement efficiency (i.e. place cement all the way around the casing)
• Prevent differential sticking
• Keep casing out of keyseats
Centralisers are particularly required in deviated wells where the casing tends to lie on the low side of the hole. On the high side there will be little resistance to flow, and so cement placement will tend to flow up the high side annular space. Mud channels will tend to form on the low side of the hole, preventing a good cement job. Each centraliser is hinged so that it can be easily clamped onto the outside of the casing and secured by a retaining pin. The centraliser is prevented from moving up and down the casing by positioning the centraliser across a casing coupling or a collar known as a stop collar. The spacing of centralisers will vary depending on the requirements of each cement job. In critical zones, and in highly deviated parts of the well, they are closely spaced, while on other parts of the casing string they may not be necessary at all. A typical programme might be:
1 centraliser immediately above the shoe
1 every joint on the bottom 3 joints
1 every joint through the production zone
1 every 3 joints elsewhere
A float collar (Figure 6) is positioned 1 or 2 joints above the guide shoe. It acts as a seat for the cement plugs used in the pumping and displacement of the cement slurry. This means that at the end of the cement job there will be some cement left in the casing between the float collar and the guide shoe which must be drilled out.
The float collar also contains a non-return valve so that the cement slurry cannot flow back up the casing. This is necessary because the cement slurry in the annulusgenerally has a higher density than the displacing fluid in the casing, therefore a U-tube effect is created when the cement is in position and the pumps are stopped.Sometimes the guide shoe also has a non-return valve as an extra precaution. It is essential that the non-return valves are effective in holding back the cement slurry.
The use of a non-return valve means that as the casing is being run into the borehole the fluid in the hole cannot enter the casing from below. This creates a buoyancy effect which can be reduced by filling up the casing from the surface at regular intervals while the casing is being run (every 5 – 20 joints). This filling up process increases the running in time and can be avoided by the use of automatic or differential fill up devices fitted to the float collar or shoe. These devices allow a controlled amount of fluid to enter the casing at the bottom of the string. The ports through which the fluid enters are blocked off before the cement job begins. The use of a differential fill-up device also reduces the effect of surge pressures on the formation.
A guide (Figure 5) shoe is run on the bottom of the first joint of casing. It has a rounded nose to guide the casing past any ledges or other irregularities in the hole.
The objective of a primary cement job is to place the cement slurry in the annulus behind the casing. In most cases this can be done in a single operation, by pumping cement down the casing, through the casing shoe and up into the annulus. However, in longer casing strings and in particular where the formations are weak and may not be able to support the hydrostatic pressure generated by a very long colom of cement slurry, the cement job may be carried out in two stages. The first stage is completed in the manner described above, with the exception that the cement slurry does not fill the entire annulus, but reaches only a pre-determined height above the shoe. The second stage is carried out by including a special tool in the casing string which can be opened, allowing cement to be pumped from the casing and into the annulus. This tool is called a multi stage cementing tool and is placed in the casing string at the point at which the bottom of the second stage is required. When the second stage slurry is ready to be pumped the multi stage tool is opened and the second stage slurry is pumped down the casing, through the stage cementing tool and into the annulus, as in the first stage. When the required amount of slurry has been pumped, the multi stage tool is closed. This is known as a two stage cementing operation and will be discussed in more detail later.
The height of the cement sheath, above the casing shoe, in the annulus depends on the particular objectives of the cementing operations. In the case of conductor and surface casing the whole annulus is generally cemented so that the casing is prevented from buckling under the very high axial loads produced by the weight of the wellhead and BOP. In the case of the intermediate and production casing the top of the cement sheath (Top of Cement – TOC) is generally selected to be approximately 300-500 ft. above any formation that could cause problems in the annulus of the casing string being cemented. For instance, formations that contain gas which could migrate to surface in the annulus would be covered by the cement. Liners are generally cemented over their entire length, all the way from the liner shoe to the liner hanger.
Dispersants are added to improve the flow properties of the slurry. In particular they will lower the viscosity of the slurry so that turbulence will occur at a lower circulating pressure, thereby reducing the risk of breaking down formations. The most commonly used are:
• Polymers 0.3 – 0.5 lb/sx of cement
• Salt 1 – 16 lb/sx
• Calcium lignosulphanate 0.5 – 1.5 lb/sxg)
Fluid loss additives are used to prevent dehydration of the cement slurry and premature setting. The most common additives are:
• Organic polymers (cellulose) 0.5 – 1.5%
• Carboxymethyl hydroxyethyl cellulose (CMHEC) 0.3 – 1.0% (CMHEC will also act as a retarder)
Heavyweight additives are used when cementing through overpressured zones. The most common types of additive are:
• Barite (barium sulphate) – this can be used to attain slurry densities of up to 18ppg. It also causes a reduction in strength and pumpability.
• Hematite (Fe2O3) – The high specific gravity of hematite can be used to raise slurry densities to 22 ppg. Hematite significantly reduces the pumpability of slurries and therefore friction reducing additives may be required when using hematite.
• Sand – graded sand (40 – 60 mesh) can give a 2 ppg increase in slurry density.
Extenders are used to reduce slurry density for jobs where the hydrostatic head of the cement slurry may exceed the fracture strength of certain formations. In reducing the slurry density the ultimate compressive strength is also reduced and the thickening time increased. The use of these additives allows more mixwater to be added, and hence increases the amount of slurry which is produced by each sack of cement powder (the yield of the slurry). Such additives are therefore sometimes called extenders.
The most common types of lightweight additives are:
• Bentonite (2 – 16%) – This is by far the most common type of additive used to lower slurry density. The bentonite material absorbs water, and therefore allows more mixwater to be added. Bentonite will also however reduce compressive strength and sulphate resistance. The increased yield due to the bentonite added is shown in Table 4.
• Pozzolan – This may be used in a 50/50 mix with the Portland cement. The result is a slight decrease in compressive strength, and increased sulphate resistance.
• Diatomaceous earth (10 – 40%) – The large surface area of diatomaceous earth allows more water absorption, and produces low density slurries (down to 11 ppg).
In deep wells the higher temperatures will reduce the cement slurry’s thickening time. Retarders are used to prolong the thickening time and avoid the risk of the cement setting in the casing prematurely. The bottom hole temperature is the critical factor which influences slurry setting times and therefore for determining the need for retarders. Above a static temperature of 260 – 275 degrees F the effect of retarders should be measured in pilot tests.
The most common types of retarders are:
• Calcium lignosulphanate (sometimes with organic acids) 0.1 – 1.5%
• Saturated Salt Solutions
Accelerators are added to the cement slurry to shorten the time taken for the cement to set. These are used when the setting time for the cement would be much longer than that required to mix and place the slurry, and the drilling rig would incur WOC time. Accelerators are especially important in shallow wells where temperatures are low and therefore the slurry may take a long time to set. In deeper wells the higher temperatures promote the setting process, and accelerators may not be necessary. The most common types of accelerator are:
• Calcium chloride (CaCl2) 1.5 – 2.0%
• Sodium chloride (NaCl) 2.0 – 2.5%
It should be noted that at higher concentrations these additives will act as retarders.