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Overbalanced drilling - optimizing HSI and impact force or jet velocity
20 October 2015

We just ran a 6'' Talon bit to drill horizontal - the formation is weak and we run highly overbalanced with MW 1.45 and Formate mud - geomechanics study advised high MW. SPP is limited and the HSI planned for the PDC is just 1.0 HP/in2. 

Because we drill highly overbalanced ROP will be slowed down rapidly - cuttings will be held on bottom -  "chip hold down" effect - for my understanding impact force and HSI would help to prevent drilling the same chips again and again - of cause not with significant  ROP increase because overbalanced  pressure effect is exponential -

But if we drill at 2.0m/h instead of 1.7m/h for 570m we will save 2 days. Are there any papers or publications showing that even with high overbalance (300 bar above pore pressure) HSI and impact force should be high as well or will better bit hydraulics not help to get better ROP in this case. 

7 answer(s)
Short Bit & Tool Co.
Total Posts: 17
Join Date: 11/05/13
I would add more food for thought
Most PDC drill bit designs have fluid that is directly jetted for each blade where the energy created by velocity and volume of the fluid impacts primarily into the formation. Does that energy act to move the cuttings? My challenge to other bit fabricators is to simply give the cuttings the least distance to travel under relatively high pressure into a low pressure area. Current designs all have one thing in common, and that is they have to move cuttings from where they are created 2-3" across the paths of all the other cutters in that blade. Isn't there a better way to move the chips out of the way? It worked well 50 years ago.

Short Bit & Tool Co.
Total Posts: 17
Join Date: 11/05/13
Food for thought.
Before the time of PDC bits and their jetted hydraulics the diamond bit industry had the same chip hold down problem to deal with. In the 1960's there were considerable efforts to improve ROP in the overbalanced Gachsaran formations in Iran. Here we are 50 years later and the basic problem is the same. The chips that are generated have to travel in a radial direction from center to gage all in the jetted high pressure channel between blades. This is why they are re-drilled. Now I don't have a solution but I  suggest that work needs to be done in general to offer a modern cross flow PDC bit.
Bill Short
Documents uploaded by user:
Cross Flow Example.pdf
Drilling Supt
Nostrum Oil & Gas
Total Posts: 45
Join Date: 14/11/10


for me it is still not clear what the optimum solution is - so what to optimize - flowrate and bit nozzle size or flowrate w/o nozzle size.  
The minimum flowrate for hole cleaning is calculated with 560 lpm - but depends on ROP and cutting size  - which I do not know exactly in advance  - means I use worst case fastest ROP and worst case biggest cutting size in the planning phase.  
Flowrate is depending as well on MWD/LWD minimum and maximum range.  
That results in available flowrate between 720 and 890lpm because SPP of the rig pumps is limited and 5 1/2'' and 4'' DP instead of 5'' and 3 1/2'' DP is ot available. 
Apart from that it could be necessary to increase mud weight because  of well bore stability problems.
The paper from Germany (Kai-Uwe and Michael :-) - what a small world) states that running overbalanced  reduced depth of cut and increase in flowrate helped. But this is for impregnated bits which have a fixed TFA - so that I think it cannot be applied as a rule for PDC bits.

PS: the system to drill the Middle and Upper Bunter in Germany has been optimized -  no longer impregnated bit and HS motor - new generation PDC (Quantec - now Talon) with RSS and limited rpm from surface but high WOB alnost doubled ROP. 

This article shows Quantec as well as Vortex jets.

Viscosity is definive a parameter to have a look on to prevent drilling on chipped cuttings and get them off bottom. 

Has anybody drilled with different  HSI, JIF, Flowrate in same formation high over balanced getting increase in ROP from higer flowrate or better bit hydraulics  ?  
Higher flowrate will have the following effects: 
1. Bit cooling in the abrasive sandstones will be more effective with higher FR and it can increase bit durability
2. High Flow rate will give us high velocity and the flow will be close to the turbulence. It should improve hole cleaning and slightly decrease friction
3. With high FR it will be easy to move out possible cavings
4. Safety space from minimum required FR

So what is left now for increasing bit pressure getting higher HSI, jet velocity  and impact force ?
Рақмет, Спасибо, Thanks, Gracie, Danke, Takk, Matonto Mingi

Consultant [retired Shell staff]
Total Posts: 240
Join Date: 02/09/05
This, I believe, is an ideal application of the French SMFi ARTEP bit concept, i.e. having one nozzle pointing upwards (backwards, in your case).
Total Posts: 18
Join Date: 17/09/07

Following on from Rodman's advice, I'd talk to your Bit people.  You have stated that the formation is 'weak' so I'd be wary of too much, well pretty much anything.  I'd suggest that you are less cutting the formation as blowing it away, with all the potential issues that brings.  It may be too late but I'd suggest you see how you can optimize the bit/formation/hydraulics to maximize ROP even if that produces some counter-intuitive answers.

Good Luck!


Drilling tech advisor
Franks International
Total Posts: 13
Join Date: 13/03/14
Please consider looking up 'Vortexx' (note xx) nozzles for PDC bits, we developed these special type nozzles for massively over balanced mud situations.
The idea of this nozzle is that it generates a localized low pressure zone just in front of the nozzle on the formation making it appear that the rock is being drilled with effectively under-balanced mud.
Because of this phenomena you can dramatically improve the RoP.
Nozzles can be orientated so that the flow is directed down along the channels between the blades on PDC bits to ensure good cuttings removal. Best results where the blade height is low so that jet impact force is no lost. These nozzles have a thread-form designed for SDBS bits only !
This is the perfect application for this tech and it will mean nothing more than acquiring a set of nozzles to fit your standard SDBS PDC/impreg bit.

Drilling Specialist/Well Engineer/Training Consultant
Kingdom Drilling
Total Posts: 353
Join Date: 10/01/05

Ask yourself, at 2m/hr (with likely very small cuttings particulates being generated?)

What flowrate is required to effectively evacuate the bit, remove and transport cuttings efficiently from your wellbore?

Here the likely flowrate reduction needed to clean the hole effectively will release xxxpsi standpipe pressure energy to then afford to your bit i.e. smaller nozzle selection tin increase JIF, HSI etc. 

My understanding is that the higher energy expended at the bit can make a significant differences in >ROP in low ROP formations and there is documented evidence to this I am sure.

Other areas to explore are
- improved bit selection and design.
- Mud rheology fine tuning. 

I did find journal examples in these matters and a 'basic of chip hold down' removal paper in my archives as attached.

It would be interesting to hear what changes you made and what were your concluded findings and results.  

Wishing you success in your ventures.
Posted by

Peter-Joern Palten

Drilling Supt

Nostrum Oil & Gas

Total Posts: 45
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