A Bit of Disappointment

What’s the story?

When we entered the Hole U1383B with the 14.75-inch bit, Charlie, the driller, found cement extending up from the casing shoe inside the 16-inch casing. This was a good sign that we likely had cement extending up on the outside of the casing as well. We need this to get a good seal for the experiment and to help support the entire seafloor structure.  We drilled through the cement, then lowered to the base of the 18.5-inch rat hole and started drilling. At that point, it was drilling slow and steady at ~2 m/hr. Our operations superintendent had about 32 hrs in the plan to drill ~69-194 mbsf—but this was calculated at 4 m/hr. We decided to drill a fair amount, but perhaps not that far. A lot would depend on penetration rate and hole conditions. Another issue was how long the bit will last. Bits typically last 40-60 rotating hours. However, in a hole like this, we will tend to not go too far past the lower limit—the last thing we want is to have a roller cone fall off!

Over the next five hours, we were not able to advance the hole any deeper than 89 mbsf. Included in this time was a short wiper trip to clean out the hole. Given this, we were not confident in the bit and decided to trip the bit back up to the ship to see what was going on (and ensure all the bit pieces were still there!). Once the bit reached the ship, we would most likely assemble a new 14.75-inch bit and head back down to drill deeper.

Unfortunately, when we recovered the bit, two of the Tri-cone structures were completely missing and the third cone had all the inserts missing (these are the little teeth on the cones; they are tungsten carbide, I believe). These were all broken off in the bottom of the hole. Given the way the hole was drilling, we believe that there must have been some imperfection in the bit. The drillers said they’ve never seen a 14.75-inch fail in this way – in relatively good, consistent—albeit slow—drilling basement with only a small number of hours on the bit.

broken drill bit
Photo by Adam Klaus, IODP/TAMU

What do we do now?

CHOICE #1: Whether (1) to try to fish the metal pieces out of the hole, or (2) Start a new hole.

Option #1: Fishing the metal pieces out of the hole would have taken at least 3 round trips with a magnetic fishing tool and we’d have to hope that the pieces would come out. There is simply no guarantee that it would only take three trips, but likely many more and we still never would be able to be sure we could get it all out. Also, if it all didn’t come out, we’d likely have to run in with a milling bit to grind it all up. Another problem is that the broken off metal pieces are very large and might have been jammed into the hole. These are really big and hard pieces—not an easy thing to mill out. The drillers and engineers were not optimistic about the ability to rehabilitate this hole.

Option #2: Start a new completely new hole re-entry cone and casing. We know we can do this and get to the same place that we are now without junk in the hole.

DECISION #1: We all agreed that Option #2 is the only sensible way to go.

CHOICE #2: Whether (1) to install a triple-cased hole like the one were doing or (2) to install a double-cased hole (reentry cone with 16-inch casing and then 10.75-inch casing into basement).

Option #1. If we were to go with the triple casing option, it was estimated to take all the time left in the expedition plus 1 more day. (So if anything went wrong, we would have very little flexibility.)

Option #2. If we do the double casing option, this will fit into the schedule and we would still have time for contingency and/or other activities.

DECISION #2: We decided on the double casing option. It leaves time in case we encounter problems (not that this ever happens!) and still allows us a chance at a full coring depth, logging/packer tests, and installing the CORK.

CHOICE #3: The only thing left to decide was where to drill the next hole: we’re going back to the location of the original jet-in test and it will be Hole U1383C.