Rupert Sutherland, GNS Science and Victoria University of Wellington
John Townend, Victoria University of Wellington
Virginia Toy, University of Otago
|HWT (4.5”, 114 mm) steel casing. 13/12/14. R. Sutherland.|
It was a complicated casing operation last Wednesday, and it was done carefully. We lowered PWT casing (5.5”, 140 mm) with only a minor hitch – the float shoe apparently failed with a third of the pipes still on the rack. Before this, mud had been displaced from the borehole as each piece was added. A float shoe is a one-way valve at the bottom of the pipe that stops mud entering, giving the pipe some buoyancy (air inside it) and reducing its effective weight, and preventing backflow when the pipe is cemented.
|Float shoe for PWT casing. 10/12/14. R. Sutherland|
Next, a BQ steel pipe (2.25”, 56 mm) had to be threaded 890 m to the bottom of the borehole (8.5”, 210 mm diameter), running alongside the PWT casing (5.5”, 140 mm). Simple maths tells you that a 14 mm tolerance over 890 m of rubbing against jagged rocks had a small chance of success. But it went in!
Next, a stainless-steel tube with delicate optical fibres inside was threaded into the BQ pipe. That worked, and we tested our sensors. OK too!
At 4 a.m. on Thursday, it seemed that everything had gone as planned. Just in time too, because the trucks showed up at 5 a.m. to set it all in concrete.
Concrete appears at wellhead. PWT and BQ pipes visible, and fibre optic cable held by clamp.
11/12/14. R. Sutherland
By 9 a.m., the joyous mood had changed. Cement appeared back at the surface much earlier than it should have, and pump pressures were lower than predicted. We quickly deduced that the PWT steel casing had broken.
Our immediate action was to flush cement out as best we could before it set. Fortunately, we had waste pits prepared for just such an emergency.
|Cement pumped from the hole. 11/12/14. R. Sutherland|
We have analysed records of pump volumes and pressures, and digital data from the drill rig computer. This shows that a significant event occurred at 11:42 a.m. on Wednesday, and it seems this is when the casing broke apart. The depth to the broken joint is estimated to be 436 m.
The drillers made up an HWT drill-string (4.5”, 114 mm) that fits snugly inside PWT pipes, and carefully lowered this into the borehole with a core barrel and bit. We touched an object at 436.0 m, pushed it to 437.0 m, and then cored it. It was the cementing plug. This was the last thing pumped after the cement. It floats, so its position seems consistent with our depth estimate for the break.
We advanced farther and eventually started coring rock below 470 m. This is bad news. It means that the pipes are not aligned.
It is still too early to tell exactly what this means for the project. Several options are available to us. Our top priority is to remediate the immediate situation and secure the upper casing. However, we need to think through all possibilities before putting more cement in the borehole.
Everyone is looking forward to a break over Christmas. It has been physically and mentally very demanding for the last 16 weeks, and it still seems like there is so much more to do.
|The latest addition to our team is as big as Virginia but not as ferocious.|