Surface Casing

Question 1

What are the main purposes of surface casing

Answer 1

1. Protects fresh water sands
2. Case unconsolidated formations
3. Provide Primary Pressure Control- BOP stack is usually nippled on surface casing
4. Support future casing
5. Case off potential lost circulation zones
6. Casing head and other completion fitting used in completing the well are attached later

Question 2

Characteristics of surface casing

Answer 2

1. Casing sizes usually run from 7 5/8” for shallow wells to 20” on deep multistring wells
2. Hole may be severely eroded.
3. Shallow surface strings can be pumped out easily
4. Drilling muds often viscous with little water loss control
5. Casing may stick easily in unconsolidated formations
6. Loss circulation may be a problem
7. Most areas want cement to surface for surface casing
8. Guide shoe or float shoe, float collar, cementing plugs, scratchers, and centralizers are commonly used

Question 3

What is the most important feature of surface casing

Answer 3

Protecting fresh water reservoirs

Question 4

What are recommended cement properties for surface casing

Answer 4

• Completion cements with accelerated thickening times and compressive strengths are used.
• Deeper strings use lightweight lead cements followed by heavier-weighted completion (tail) cement
• With a Lead and Tail Cement Blend, lead filler cements (High Water Ratio) followed by neat or high strength tail cement that is accelerated
• Lost circulation material is usually added
• High strength cements should be used on deep well surface casings to support future strings

Question 5

What are surface casing cementing practices

Answer 5

1. Large Diameter surface strings are cemented through drill pipe with sealing sleeve
2. Bottom and top plugs are used to reduce mud contamination
3. Bottom joints are centralized and thread locked to prevent backing off while drilling
4. Regulatory rules usually require a WOC time of 4 to 8 hours or minimum of 500 psi compressive strength cement before resuming rig operations
5. Avoid using bottom plug when using lost circulation material
6. Differentail pressure may require that the pipe be chained down to prevent pipe pump out
7. If inn string method is used, enough weight needs to be applied to the drill pipe so it won’t be pumped off the landing surface

Question 6

Where is the float collar placed on surface casing

Answer 6

Two joints above the guide show so as to prevent mud from contaminating the cement around the shoe joint

Question 7

Why are scratchers used

Answer 7

Scratchers help circulate mud from formation walls

Question 8

Why are centralizers used

Answer 8

Centralizers center the casing in the hole to help circulate cement completely around the pipe