26″ Unusual Bit Failures

Hello all,
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!
Regards,
Mike

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