In order to carry out a conventional primary cement job some special equipment must be included in the casing string as it is run. For this reason we try to post downhole cementing equipment
Downhole cementing equipment include :
Guide shoe – A guide (picture bellow) 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.
Float collar – A float collar (picture bellow) 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 annulus generally 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 .
• Centraliszr – these are hinged metal ribs which are installed on the casing string as it is run (picture bellow). 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 centralizers will help to:
• Improve displacement efficiency (i.e. place cement all the way around the casing)
• Prevent differential sticking
• Keep casing out of key seats
Centralizer 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 centralizer is hinged so that it can be easily clamped onto the outside of the casing and secured by a retaining pin. The centralizer is prevented from moving up and down the casing by positioning the centralizer across a casing coupling or a collar known as a stop collar. The spacing of centralizer 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 program might be:
1 centralizer immediately above the shoe
1 every joint on the bottom 3 joints
1 every joint through the production zone
1 every 3 joints elsewhere
• Wipers/scratcher – these are devices run on the outside of the casing to remove mud cake and break up gelled mud. They are sometimes used through the production zone.