reply to post by PuterMan
Sure I can elaborate on MWD/LWD. It's the signal that you're referring to. I think wikipedia has a good answer, but I'll give a brief one too,
since it's what I used to do.
MWD/LWD is Measurement While Drilling/Logging While Drilling. A basic MWD tool reads direction (azimuth), inclination, and background gamma readings
(every rock gives off some level of background radiation, read the gamma and you have an idea if you're in sand or shale). More advanced tools
included resistivity of the formation, porosity of the formation, pressure of the annulus, neutron porosity, and sonic readings.
The readings from these tools are sent back up hole in a few different ways. The most typical and widely used is through the drilling mud, by
restricting and allowing flow through the drillstring you create a small pressure wave ~2-30 psi which can be interpreted on surface. There are EM
tools which send a low frequency EM wave through the formation, however that isn't as likely to work underwater. A newer technology sends the data
back through the drill pipe, like a wire.
A lot of it is extremely accurate and can tell you right where you are at all times. Some of the wells I've seen drilled have taken some crazy
courses and directions.
However when it comes to Direction and Inclination, a lot of tools rely on magnetometers which reference to magnetic north for azimuth. Some use
north seeking gyros. Gyros lack accuracy once you reach a certain level of inclination in the hole. Magnetometers can have accuracy issues due to
magnetic interference from the drillstring or casing. If they are drilling with a magnetometer they will know when they are getting close to the
other well due to the readings going crazy, like a compass would, however to find an exact location based on that interference is guessing. They can
use a combination of the other tools to hone in those readings, however they are only going to tell them when they are close enough to the original
well... if that makes sense.
Once they get in the general area it might not be too bad, they'll have to make a bunch of attempts, like was suggested by cementing and sidetracking
over and over again. How long it could take depends on how far back they have to pull after each attempt. It could be a short amount of time for
each, as it's a lot easier to sidetrack with the well profile they appear to be using. Or it could take a long time, as I can say that once you're
within about 50' of the target well the magnetic interference shoots way up and your direction becomes unclear. Picture being blindfolded in a 50
cubic foot room with a bucket in the middle. You know where the bucket should be before you're blindfolded, but then someone spins you around to
make you dizzy. Now find that bucket. You could walk right past it, you could walk into it, you could glance off of it, you could do lots of
I'm really bringing this up to point out that there is no real guarantee of two things. 1. They'll be able to hit the target on the first few
tries because it's a small target so far away, and the interference is going to mess with them. 2. Nobody has said, and nobody has released that we
actually know the well is where it should be. We know where the top part is, but due to uncertainty in the readings while drilling, we don't
actually know where the rest of the well goes. There is a term called vertical section, it's measured horizontally, and basically the difference in
location from the top of the hole to the bottom. As you drill, even straight down, it's hard to maintain exactly 0 degrees of inclination. The
target may not be exactly where they think, but be within a certain area... maybe 500' from where they think or it's supposed to be.