Intermediate Casings

Question 1

What is the purpose of the intermediate casing?

Answer 1

1. Control the hole and seperate the well into workable drilling segments
2. Case off lost circulation zones, water flows, ect.
3. Isolate salt sections
4. Protects open hole from increases in mud weight
5. Prevents flow from high pressure zones if mud weight must be reduced
6. Basic pressure control casing. BOP always installed on the intermediate string.
7. Supports subsequent casings
8. Control high pressure zones
9. Protect formations from high to low pressure caused by drilling or cementing operations
10. Can be used as production casing
11. Provide a place to hang a production liner

Question 2

What cements are recommended or intermediate casing?

Answer 2

1. Because large volumes of cement are used, both filler lead cement and tail neat cement is used. Sometimes three slurries are needed.
2. Single stage cement slurries (which are similar to surfacing casing slurries) filler slurries need to be light enough to not break down the weaker formation. Completion slurries needs to have enough strength to hold the pipe and provide a good seal between pipe and formation.

Question 3

Why would only the bottom pipe be cemented (usually 1,000 to 3,000 feet cemented)

Answer 3

1. Cost

2. Un-cemented section of casing might be reclaimed from the well and re-used

Question 4

What additives are found in most intermediate string cement slurries

Answer 4

1. Retarders- Retarders insure sufficient pumping time to the cement in place and impart some friction reducing properties to the slurries.
2. Friction Reducers- Lower Pump Pressure and reduce chance of losing fluids in a lost circulation zone
3. Fluid Loss Additives- prevent slurry loss into lost circulation zones and dehydration it the annulus due to permeable zones resulting in cement bonds
   4.

Question 5

What are major Intermediate String Characteristics

Answer 5

1. Casing sizes range from 6 5/8” to 20.” Most common are 9 5/8”, 10 3/4” and 13 3/8”
2. Some sections may erode severely, particularly salt sections and rotten shale sections
3. Casing strings tend to be very heavy and set on bottom
4. Cement Volume is dictated by wellborie conditions
5. Guid shoe/float shoe and float collar are commonly used
6. Cement volumes are the largest for intermediate string
7. Often cemented in stages
8. Damage is common
9. Prolonged drilling through this section
10. Completion may be made in this section.

Question 6

What are cement best practices for intermediate casing?

Answer 6

Bottom and top plugs are used to minimize cement contamination. Bottom plug wipes all mud film off the casing ahead of the cement and top plug seperates cement from displacement fluid. Plug containers that hold both plugs should be used. Stage tools are sometimes used when weak formations are experianced. Multiple slurries are often used