Engineering Redesign for Bottom hole Balling in high MW environment
We have an issues that we have identified as a unique problem for high mud weight Wells. We drill in North of Pakistan where we require high MW in OBM environment due to tectonics stresses (foothills of Himalaya). We started implemented a crude version of Limiter Redesign process and the team is making great strides in it.
The problem we have is that now that we have started re-designing our WOB limiters, we are stuck on the BHB (bottom hole balling issue). We are running OBM of 1.96 SG from 2000 to 4000m with ROP of 10-15m/hr in the start ( with aggressive 5-19 PDC) and the in the next 60-80 m or sometimes in 100+m, the ROP dies down to 1-3m/hr. We have pulled around 5 PDC bits and all of them have chamfer on their cutter and very slight chamfer.
We have tried SMITH/Baker/NOV/HAL-616/716/519 , matrix/steel body/ Planer/3D/4D/ridge shaped you name it. With the metal body 519 the ROP is better it also dies down after 80-100m.
We are drilling clay with max 50% silty claystone with occasional 5-10% sandstone. The drilling manual -12th Edition chapter -12 -Drilling Practice identifies that Bottom hole balling is a complex phenomenon that can slightly be improved by:
- increasing HSI/GPM ( we can't go much due to 5K psi pumping system)1-1.5 HSI is max allowable at this depth with RSS+MWD+Thruster BHA.
- Increase RPM-Does not yield any increase in our case.
- Engineering Redesign.-We were earlier running the matrix body 716/616 bits stitched to metal body bits from planner cutters to 3D cutters/ridge shaped Smiths ones,4D Nov and shaped cutters from Baker and upped the aggressively from 616 to 519 single row.
There is no vibration damage as we obviously stay away from resonant frequencies, low to none SnS, no chipping or any vibration related damage just chamfer on taper, nose and gauge cutter that just drop the ROP from 10-12 to 1-2 and most of the ROP before TOH is less than 1m/hr.
Do we have something that we can do, to redesign this limiter? I mean we can go to 5 blade-23mm PDC bit, I think there is more to it than increasing the cutter dia.
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