This post a while back on water handling reminded me of another geologic feature that's a pain to drill through: faults. (see my last post on this topic and some links to older posts here).
Now, I haven't overseen drilling into any spectacular or famous faults, per se. But I have very rarely drilled into faults of at least local significance - local enough to be mapped and named, anyway.
Drilling into faults or fault zones at any depth of significance (say, more than 50 feet or so) leads to two major issues: water production and borehole instability.
Larger scale faults tend to be more than just a single fracture in the bedrock. They may include a fault zone, where the rock for a certain distance is much more fractured and may possibly have a different structure than the parent rock. This may hold a lot of water. And if you're flying along, doing some sort of fast, aggressive drilling such as air-rotary, and creating a borehole with a reasonably wide diameter, you may end up tapping into a lot more water than expected. And depending on the air pressure the driller's using, the drilling rate, and the structure of the rock, the water flow may not just stop once the driller turns off the air pressure. Nothing like watching your frac tanks fill up with contaminated water while you wait for the water to stop pouring out of the borehole!
So, water production is a thing that you can deal with. You get the frac tanks on-line, make sure that you have pumps capable of moving a lot of water, and maybe stop drilling for a bit every once in a while to see how much water you're getting back. Borehole instability is another problem.
Once we install a borehole, we usually like to do a bunch of testing, which involves lowering instruments down there to collect samples and geophysical measurements, installing packers to seal off certain zones for testing, and maybe putting in a permanent system with multiple sample ports. If you have a fracture zone that's at a reasonably steep angle, and bedrock that is not super hard (like a siltstone or sandstone) you may find that the walls of the borehole pinch back in almost immediately. This makes it hard to fish the drill rods out of the borehole, let alone any $10,000 geophysical tooling you'd like to use. You can always try and bang in some steel casing past the obstruction, but at that point you may have shrunk the effective size of the borehole so you can't get the other stuff you need down there, and then you've shut off the rest of the bedrock from evaluation. And multiple boreholes get expensive fast. Another option is to be a lot more cautious up front, and do all your sampling/testing in 10-foot intervals as you drill (with casing above the interval in question), but that does slow the drilling process down and requires much more coordination between multiple contractors, all of whom are being paid for their standby time.
Intercepting a fault/fault zone actually can tell us quite a bit about the regional geology and the structure of the bedrock. We just have to be able to get a borehole in there long enough to do the evaluation.