20'' casing is always used as Surface Casing Pipe in a oil well completion and steel grade generally chosen

J55 or K55 , ERW and Seamless both applicable.  

Surface Casing pipe is installed to : 

1 prevent poorly consolicated shallow formations from sloughing into the hole

2 protect fresh water sands from contamination by drilling mud 

3 provide protection against hydrocarbons found at shallow depths 

4 provide initial support for the blowout preventers

5 support the wellhead system and all subsequent casing strings 

The main size of 20'' is as below: 

    OD                             WT(MM)          KG/M             PPF              Length (M)      Threads

20''/508mm                 11.13              139.89            94                R3/R2            BTC/STC/LTC

20''/508MM                  12.70               158.49          106.50          R3/R2           BTC/STC/LTC

20''/508MM                  16.13               197.93          133.00         R3/R2            BTC/STC/LTC

There are also other sizes which can be used as Surface Casing Pipe such as 16'', 13-3/8'' etc. It is commonly based on the

well designs and technical requirements. 

            shipment by bulk vessle