26â€³ Unusual Bit Failures
11 January 2013
Would appreciate any comments/opinions on a couple of unusual 26" bit failures we have had in the past few days.
We are drilling 26" hole at around 1,000m (say around 3,300ft). The well is vertical, in fact hanging around 2.5degs, probably following formation dip. We have a packed BHA including a 9½ Shock Sub and parameters are WOB 55-50KLbs, RPM 120, GPM 950, PSI 2350, so nothing excessive.
ROP is around 1.5m/hr; higher bit weights do not give higher ROP and lower rotary speed reduces ROP. Bits are 1.1.5, not ideal for our formations but in a remote location we have supply issues.
Mud is 10.5ppg weighted native clay, formation is alternating claystone & sandstone.
We pulled one bit due to high torque after about 40 hours and found the spearpoint and the next row of cutters on the no. 1 cone missing, with associated junk damage to the rest of the bit and evidence of skidding. At the time we attributed this to a metallurgical/manufacturing fault.
Later we pulled another bit (a different manufacturer), after 60 hours, again due to high torque, and again found the spearpoint of the No. 1 cone missing.
There is no H2S to cause embrittlement and there seems to be no correlation between the onset of high torque (cone failure) and transition from claystone to sandstone - both are equally hard.
Loss of a spearpoint is not something I have experienced before, never mind twice and any input would be welcome. Great way to start the New Year!
A few thoughts on the unusual 26" bit performance:
1. 26" tricone bit. Is it genuine? When I was operationally active, only two manufacturers could provide genuine 26" bits; the other, would provide "padded" bits, i.e. cones from smaller bits welded to the pin using fillers or pads. Obviously, their performance was sub-optimum.
2. 30" shoe track drillability. Often the float equipment or the material left inside the conductor was not easily drillable. I have read reports of loss of spear points and locked cones as a direct reult of drilling shoe tracks which were not "drillable".
3. Bit hydraulics. Experience indicates the mandatory use of "Centre Jets" for better removal of debris from the hole centre. This can be enhanced further either by blanking off one of the peripheric nozzles (Paper SPOE 8379 JPT October, 1980) or using an upwards looking jet (i.e. SMFi "ARTEP" bit also referred to as a "reversed nozzle").
PS Sorry for the late reply. Only today I became aware of the query...
Sorry I'm as bit late in jumping into this discussion.
The Shock tool can easily be modelled to determine the float position of the mandrel (ie - is it fully extended or not).
Also - Shock Tools can be built with different spring constants to accomodate different scenarios.
I actually wrote an Excel Spreadsheet that details Shock Tool conditions for a given BHA and gives recommendations for the psring constant.Dont have the spreadsheet with me at the moment, but your coordinator should have a copy.
Hope this helps.......Hendo.
I tend to disgaree with the provious comments about high WOB - a 26" Bit (even a 1-1-5) should be able to run that WOB for days on end.
Also erosion should be more evident on the rest of the bit - it wouldn't just cause the spear point to break off.
I'd look more at your hydraulics and BHA - mainly at the shock sub.
On the hydraulics, if not already doing, I'd look at setting up for cross flow, otherwise you may actually just be floundering the bit and getting an artificially low ROP, because you are re-grinding the cuttings all the time.
I'd certainly look at that if you are not seeing any difference at all in ROP over formation changes.
With that GPM and SPP, I'm gessing you have a reasonable pressure drop across the bit, so the shock sub may be fully extended all the time and not actually doing anything.
Another thing is that if you are running 9"" DC's, then you should be looking at a 12" Shock sub (the mandrel is the weak point), otherwise the bit might be wobbling.
Similarly, you mention a 'packed' BHA - which can mean different things to different people. If the shock sub is more than a joint above the bit, then you might as well not run it - it's too far back to do any good. I suggest you contact the shock sub supplier to get more information on suggested running parameters and BHA's.
Another thing is to make sure your supervisors are looking properly at the stabs. Your ROP sounds awfully low considering you are using native mud (which I've always taken to mean you can drill at very high ROP's), so maybe the stabilisers are hanging up on ledges at the sand / clay interface, or perhaps much of the WOB is being taken up by the stabilisers all the time?
I have seen a simlar problem (17-1/2" bits in chalk).
I think you have the answers already. Wrong series of bit with too much WOB.
Does the shock sub work in your application? Where in the BHA do you run it? Do you get bit bounce? if so, that might explain the broken spear point.
I agree with Sadiq's comments about potential bit erosion. The nozzles are probably too far away from the bottom of the hole to have much impact force (what's the rule of thumb on that again - 10 x nozzle diameter or is it less than that?), so do you really need all that SPP?
Really feeling with you. ROP of 1.5 m/hr is quite low thereby increasing the load stress on the bit's cones at 55Klb WOB and 120 rpm (the bit will rapidly accumulate Krevs). The parameters are not excessive but the combination of high rpm and high WOB if placed on journal bearing rock bit are damaging. Did you observe any sign of erosion/wear on the bit? You did not mention this but my experience is that the sand/sandstone in surface hole are very 'erosive'. Staying long on bottom (implied fromt he very low rop) accelerates wear on the bit (body and teeth). The nozzles TFA may also have to be increased, if possible. Another thing to find out may be the age of the bit.
Consultant DD (Eclipse Energy Services Technology Ltd/ Baker Hughes)