Friday, January 24, 2014

gone fishing

A long time ago, I mentioned the gear used to fish stuff out of monitoring wells. But I never did write the intended follow-up: what kinds of things have been permanently lost?

I could fill a book with the list of items that can be lost in the ground, so I figured I'd restrict myself to items that either I have lost or the contractors I'm overseeing have lost and not been able to retrieve. In order from least to most expensive/problematic:

1. These can be retrieved if you get a big enough pump (a vac truck or a drill rig with a Moyno pump will work), but the nuts and bolts for flush mount roadboxes tend to, um, disappear over time. If I have a site that's been monitored for more than 10 years or so, I often advocate for a full well rehab with something big enough to suck out all the hardware that accumulates over time.

2. The driller's weighted tape gets stuck while installing a monitoring well. Technically, it was stuck above the screened interval, so we switched to the bentonite seal material a little sooner than planned and entombed it in bentonite before cutting the tape. If it gets stuck next to the screened interval, then you're abandoning the boring and re-drilling.

3. A water level probe that somehow got wedged in the screen of a steel monitoring well. We eventually tugged a little too hard on the cord and it broke off right at the probe. There's nothing in a probe tip that would impact groundwater, right?... Right?

4. A check valve used to develop a former injection well popped off. The check valve was plastic, so no potential contamination issues, but it got stuck at the top of the screen, rendering the well useless. In an attempt to at least unplug part of the well screen, we reversed direction and tried to push it down the rest of the way, but only got it more firmly wedged about 2 inches down. We decided we didn't need that point so much after all.

5. Similar to #4 above, but for a much deeper well: the tubing for a bedrock well was cut too short and fell down the well. Apparently it was irretrievable (this was before I came along).  Without knowing there was more tubing wedged at the target interval,  I went to add tubing on my own. I got my tubing wedged with the other stuff cluttering the well. When we tried to pull the whole mess up, we overstretched the tubing we had (or rearranged things?), and when we let go, it sproinged back down the well just out of reach. Now there's really no way to fit tubing down to where it needs to go.

6. Various lengths of direct-push rods bounce off boulders and cobbles and become too bent to pull back. Luckily, direct-push rods are cheap... unless you had a fancy probe at the end.

7. When drilling in an alluvial valley, we installed a monitoring well screened from something like 110 to 120 feet below grade. When the drillers pulled up the temporary 6-inch casing, the bottom 60 feet were missing. The borehole had apparently collapsed above the casing, and since we had no idea if the bottom casing was still blocking the well screen, we had to abandon the well and re-drill. 60 feet of lost casing = cranky drillers.

8. We were rock coring close to a fracture zone, and an impressive borehole deviation developed. At about 200 feet below grade, the angle of the borehole became too much for the core barrel, and in the course of pulling out *most* of the core barrel, they sheared a solid steel coupling on the top of the drive shaft (which is now an especially impressive paperweight in my collection). We abandoned the borehole with the rest of the core barrel remaining in place.

If I cast my mind back, I could find an even 10, but I think this is long enough. Just remember - you can always fix something that's permanently lost, even if it's by grouting up the borehole and re-starting. It just gets... expensive.

1 comment:

Rock Head said...

We were drilling a 6-inch monitoring well through coal cyclothems in WV with an air hammer. As the driller was pulling up the drill string, the hammer got caught on a blocky piece of sandstone about 60 feet down.

It was a Friday afternoon and everyone wanted to get home, so he was popping the hammer up and down, rotating and counter-rotating to try and get it past the obstruction. I guess he counter-rotated a little too much one time, and the hammer came and fell to the bottom at 280 feet.

The drillers never could get the hammer out and had to grout it up and abandon the hole.