A number of components are fitted to the casing string to enable it to be cemented in place. In order to successfully cement each casing string, casing accessories should be installed and the necessary components are listed below;
A float shoe is a short and rounded shape component with non-return value inside which is installed at the end of the casing. The advantages of a float shoe are as follows;
- Prevent mud flowing back while running casing and prevent cement from outside U-tubing back into casing due to unbalanced conditions while performing cementing operation.
- Help running casing to the well. The round shape of a float shoe prevents a casing string from hanging up and guiding a string into a wellbore. Some float shoes are made of high strength drillable material and can be used to reciprocate and rotate to pass any obstructions in a wellbore.
A float collar is also a non-return valve which normally installed one or three joints above a float shoe. The advantages of a float collar are as listed below;
- Prevent mud and cement from U-tubing back into a casing string and float casing if required. This is the same advantage as a float shoe, and this also serves as a backup check valve in the casing string. If the check valve in a float shoe fails, a check valve in a float collar still performs the same purpose.
- Land cement wiper plug. Some models of float collars have non rotating profiles. A cement plug landed into the profile will have fewer tendencies to be spun while drilling out. This will minimize time to drill out cementing plug because a cement wiper plug will not be spanned.
- Contain contaminated cement. The space between a float shoe and float collar called a “shoe track” will contain any contaminated cement when the top plug wipes any residual mud inside the casing. This will prevent bad cement at a casing shoe and help operators to achieve good formation integrity test (FIT) or leak off test (LOT) of the next well section.
A centralizer is a device to keep a casing string out of the well bore wall. The advantages of casing centralizers are listed below;
- Centralize casing string and minimize contact between casing string and wellbore
- Achieve proper cement around casing string and reduce cement channelling
- Minimize differential sticking and drag while running in hole
A wire scratcher is sometimes installed with casing string to help remove filter cake in the wellbore while a casing string is run in the hole. This will help improve cement bond quality.
Numbers of centralizers and centralizer placement are very important parts to achieve casing running operation and get good cement jobs. Excessive casing jewelleries installed in the casing string can lead to an operational issue while running casing. Therefore, compromising the quantity of installed centralizers and jewelleries is imperative to meet all required objectives.
Baker, R. (2001) A primer of oilwell drilling: A basic text of oil and gas drilling. 6th edn. Austin, TX: Petroleum Extension Service, Continuing & Extended Education, University of Texas at Austin.
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Bourgoyne, A.T. (1986) Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers.