Replies to post #5928 on Zion Oil & Gas Inc (ZNOG)

[BlissBull]

Not exactly, tisdal... Smaller pipe means less volume, but the pressure is still high because most of the casing strings run all the way to the surface inside the larger ones...

In fact, if the pressure in the lower zone is too much higher than the 4 upper zones, the "single string multiple zone completion" may not work... So they would have to use two separate strings...

http://www.drillingformulas.com/multiple-zone-completion/

As KyOil said, its a "MONSTER WELL!!!"

[KyOil]

tisdal, you ask a good question about other motivations for going to a smaller casing.

We've assumed that the smaller casing was simply just necessary after deciding to case the 4,400' above the cap rock because a smaller diameter drill bit and casing (5.5") is needed to get past the installed 7" casing.

But, could have something motivated them to install smaller casing for well control purposes? I've never encountered this issue, so cannot comment from experience. From a static pressure standpoint (no moving fluids), the pressure gradient from the bottom of the well to the top is the same independent of casing diameter. In the event of a blowout, having a smaller hole in the producing formation would help limit flow. But, I suspect they are trying to prevent a blowout, not just limit the damage if one occurs. Another possible factor is that for given steel strength and wall diameter, a smaller diameter casing can withstand higher pressure than a large one. So, this could have been an economic and availability issue. It might have been that the 7" casing they had on-hand did not have a high enough rating for the pressures now being anticipated, so that was a factor in deciding to upgrade the well control (casing is the second most critical component of well control after the surface valves). Hard to say without knowing what they had on-hand and what pressures were being observed.