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Dealing with "Gumbo attacks"
08 July 2009 quite a few useful points.
As was stated in 2009 and recent post.
There are 'limits to drilling clay' that will evidently result in gumbo.
Cause and effect that in fact can be quite readily determined and evaluated.
We know this because in the late 90's we could not figure or understand out why Unocal and others could drilling top hole (predominantly claystone) so fast. e.g. typically deepwater 17 1/2" (slender well sections) at 80-110m/hr, as this was the max limits where above drillers had noted problem then tended to result.
This was for Sedco 600 and 601 2nd gen semi's with limited pumping capacity i.e 1400hp mud pumps.
To analyse this I created a simple Mathcad top hole drilling model to see where limits were based on 4-6% by cuttings concentration that historic books have stated is the limit for clay drilling) that I still use today. The limits predicted not surprisingly tied in with what the drillers wells ROP limits as experienced.
From the model therefore we can state (quite conservatively) what APWD EMW will result (note: this evidently changes as will max ROP permissible as section deepens. Where as section length increases ROP will likley may have to be cut back as hole cleaning and bottoms up times becomes more increasing difficult.)
If we don't account for this (note we see very little drilling programs stating this) then we are simply asking for problems to result.
Above these limits? drillers informed us they started to have mud balls, mud rings, gumbo events etc. So rest assured there are 'soft problem' limits when drilling clay. Mud rings, mud balls, pack offs gumbo etc.
So if your rig has bigger mud pumps with more HHP psi and pump rate capabilities, the max ROP is likely far higher.
If you are drilling a 26in top hole ROP's are likely halved.
i.e. 40-60m/hr max at same pump rate i.e. 900-1000gpm.
Rest assured Gumbo does not simply just happen. It is evidently caused by drillers not understanding what causes and effects the problem.
Therefore understand the evident causes, that today one can readily measure downhole conditions i.e. with a APWD sub, to stay in full control, drill at the max limits predicted, and prevent versus cure what often results w.r.t soft claystone.
Wishing you success on future wells.