Cement 20" casing which is stuck in 26" open hole

Hi All,

We were running 20" casing in 26" vertical hole and got stuck 160m from bottom. Attempts to free the string were not successful and we have decided to cement the casing in place. Anyone has experience with such cement job? How far can the cement fall down the rathole? Some numbers:

• KCL mud 1.12sg
• hole was backreamed from 250-350m when POH 26" BHA (we did experience drag when RIH casing afterward)
• hole was 20-25% excess from a dye tracer pill circulation exercise
• HiVis pill was spotted on the bottom 50m of the hole
  
  Since the rathole volume is quite large, it'd be helpful if we can have a more accurate idea about how much cement is likely to sink down, in order to prepare a more realistic cement volume. Thank you very much!

Regards,

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