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Drill Bit selection- Conglomerate layer
05 December 2017
I would like to here about drilling solutions for tacking a coarse conglomerate layer.

Drillbit and mud solutions to maintain hole stability through this layer of up to 15m.

This is shallow water drilling.
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3 answer(s)
Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 361
Join Date: 10/01/05
Hi Jeremy form the Kingdom (of Fife :) )

From my experience of drilling conglomerate and similar Glacial drift/ debris, in North Atlantic, WOS, Faeroes, Norwegian sea, Davis Straights, East Coast of Canada, North Falklands basin regions etc. 

Best practise rule of thumb is adhere to 'how to drill a useable hole as written in late 60's' Where J & G (i.e. stiffness and rigidity) principles is the key to operational success.

e.g. If bit/BHA tools and equipment are not SMART enough, in design and engineering considerations. i.e. not stiff (at times we have include 11inch pony DC's) nor rigid (properly stabilised an rigidised) vibrations tendencies will not be suppressed and downhole often results and break loose downhole. BHA will wander micro doglegs will be created, axial, torsional and lateral vibrations all can and will result.

From the three drilling speeds probable to choose from. (i.e. slow, dead slow, or the more common one today in such a technological and TOFS world i.e. stop)

Drilling conglomerates will have to be conducted in the dead slow mode. Doing the right things and getting things right first time the assured way to safe, effective and efficient results.

The key and important objective aspect to minimise drilling difficulties and further operational problems, equipment failures etc resulting from experiences are;

Prioritise to drill manage and maintain a true in gauge 'Peters perfect cylinder'

Case study Example: The Heidrun platform was notorious for major glacial drift (boulder pebble bedded intervals). A stiff rigid assembly evolved in this region that included a 11n pony a 36" spiral blade stabiliser that sat a fairly short distance above the hole openers etc.  During batch drilling this assembly drilled 3-4 top holes followed by running 3-4 conductors with few reported problems resulting due to perfect cylindrical wellbores that were drilled.

This was a 25-30ft BHA that cam out to the rig premade we simply picked it up put on the bit, RIH, removed the handling sub then ran the other DC's etc needed. In reverse process we POH and sent it back to town. This saved about 6-8hrs rig floor handling time. 

Note: Problem with modern rigs is often that skates generally cannot handle such a high BHA weight and bulky dimensional BHA load etc.. So another way to get this to the drill floor via a cran etc may be needed.

Hole opener cutters and bearing were changed and improved from standard norm. Where stuff that comes off the shelves as standard may fall apart in such circumstances) 

Engineered choice in my reckoning has always been a  rotary assembly (no motors to be involved, where note all three major 3rd parties will try and sell you this as the best solution, but cannot produce the evident in my experience to sanctify this.) 

Another lesson learned is that resonant speeds often result in these sections. Be careful of this and change rotary by about 20-30rpm. 

With stiff and rigid assembly its not going to build angle so you can apply a far bit more weight. Peripheral speeds of hole opener cutter even on a rotary assembly will be 4-5times faster than rotary speeds so 40-60rpm is often more than enough rotary.

For top and surface hole although it is difficult in theory to lift large pebbles cobbles form the hole after they have been drilled.
Sweeps, adequate pump and due to short distance involved (assuming you are drilling these sections riserless) cuttings transport should not be an issue.)  

In a cased hole and marine riser situation larger particle sizes will take a lot longer to come out than normal cutttings. On one campaign we were in fact plagued by conglomerates and did not manage to set one seal assembly due to rogue particles that exists somewhere in the well (likely in SSBOP cavities etc.)

During 1998 Falklands campaign we hit conglomerate at TD of every well. Using principles as above we drilled these sections relatively free using insert bits, stiff rigid rotary assemblies and if I remember we also successfully took 3-4 conglomerate cores to satisfy regulatory authorities needs so we could TD wells early.

Technological additions HR or shock sub type device may also be beneficial in some situational, conditional circumstances to be assessed on a well by well basis.

Merry Xmas.

Drilling Supervisor
Total Posts: 59
Join Date: 11/09/10
Some more questions... how thick is the conglomerate and hoe far do you have to drill once through it?  Neil
Drilling Supervisor
Total Posts: 59
Join Date: 11/09/10
What hole size are you planning to drill through the conglomerate with?  Neil

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