Monday, 30 October 2017

CASING CEMENTING OVERVIEW

CASING CEMENTING GUIDELINES 

casing and cementing on onshore offshore drilling


Why We Need To Cement The Casing

The objective of primary cementing is to provide zonal isolation. Cementing is the process of mixing a slurry of cement, cement additives and water and pumping it down through casing to critical points in the annulus around the casing or in the open hole below the casing string.

The Two Principal Functions Of The Cementing Process Are:

  1. To restrict fluid movement between the formations
  2. To bond and support the casing

If this is achieved effectively, other requirements imposed during the life of the well will be met, including:
  • Economic
  • Liability
  • Safety
  • Government regulations

First Of All Prepare Your Casing

Drilling mud (also referred to as drilling fluid) is circulated down through the casing to condition and clean the well. This is usually done by the rig.

A pressure test is done by the pumping unit, checking that there are no leaks in the lines that connect the pumping unit to the well. Then a chemical wash and a spacer are pumped in. 

The chemical wash is a fluid that helps thin and disperse the drilling mud in the wellbore and annulus ( the space between the casing and the formation). The spacer is also a fluid. Its job is to keep the drilling mud and slurry separated. 

The wash and spacer may also be designed to induce a turbulent flow that helps the slurry efficiently displace the drilling mud in the annulus. 

Next a bottom plug, also called a wiper plug, is dropped from the cement head. This bottom plug cleans (wipes) the sides of the casing as it descends and provides a barrier between the mud and the cement slurry, which is pumped next.

Preparing the casing and mud Dropping bottom plug 



Pump The Casing Cement 

Once the slurry is mixed to the correct density, it is pumped from the cementing unit through the cement head and into the casing, and then displaced to the annulus. In the case of continuous mixing, mixing while pumping can continue according to the requirements of the job. The cement head is a container mounted on the casing. It is attached to the cementing unit and allows for the release of fluids and plugs in a pre determined sequence. 

The slurry pushes the wash, the spacer and bottom plug ahead of it into the casing. When the bottom plug reaches the float collar, the plugs diaphgram ruptures and the wash, spacer and slurry proceed to the casing shoe. 

There are two kinds of slurry, lead and tail slurry. Lead is a low density slurry for covering the upper part of the annulus. Tail slurry is a higher density slurry for covering the shoe.



Now Perform Cement Displacement Process
After the slurry is pumped into the casing, the top plug is dropped. The top plug separates and protects the slurry from contaminants from the displacement fluid that will be pumped. The displacement fluid, typically drilling mud, pushes the top plug and slurry down into the casing. 

The slurry is forced out the bottom of the casing and up into the annulus. When the top plug reaches the bottom plug it is sealed or "bumped" with the bottom plug by a pressure increase. The process is complete. When there is a pressure increase indicated at the surface, the displacement is complete. 


Finish Performing Cement Job 
At the end of the job, it is normal for some displacement fluid to return through the treating lines, when you open the valves on the cementing unit. However, if the returns are greater than 3 to 5 barrels, it could be an indicator that the float collar is malfunctioning. 

The float collar contains a check valve to prevent flowback up the casing. If it malfunctions, slurry can push the plugs up into the casing due to a u-tubing effect. When the cement slurry hardens in the annulus, it forms a barrier between the casing and the formation. This barrier, or sheath, isolates the casing from the formation fluids. This completes the cementing job.


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