When Rocks Leak
Apr. 2nd, 2008 09:02 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
hertinknessSo, I finalized most of the figures for the report today- and they all worked, except one: the cross section.
I based my cross section on a variety of sources- the existing geologic maps, some structural measurements and rock descriptions that my boss did, and the drilling logs for the big well. It mostly worked- it shows that one of the wells that we drew down shares a water bearing bedding plane with our well, and better yet, if we just drill him a new one a maximum of a hundred feet deeper down, he won't have any more problems when the production well goes online. However, the other well that we drew down? Not even close.
We have leakage between bedding planes.
This isn't necessarily a bad thing- if we can quantitatively estimate how much water is leaking through, we can get a clearer picture of the recharge area for this well, and possibly get more water out of it. The problem is convincing the regulatory agency that this is indeed what is happening.
Fortunately, there's a neat little mathematical trick that we can use to estimate this. Transmissivity is basically the ease with which water flows through the aquifer to the pumping well, and there are several ways to estimate this. The first two rely on pumping test data from both the pumping and recovery phase, and are estimates of the average transmisivity (these don't always match, so there's a whole new level of agony trying to figure out which number you should use, but fortunately that didn't happen here). The second one you can do if you've drawn down a monitoring well during the test, and that will give you transmissivity in that direction.
We already know that the aquifer is strongly anisotropic- that means that the speed at which water flows through the rock varies greatly depending on which direction it's going. And we have an average value, and two directional values in this case. So, what we can do is construct an ellipse with the transmissivity values and their directions from the well, and that'll give us an estimate of transmissivity in all directions, not just in the directions of wells we know about. From that, we can figure out how long it will take for the water to get to our well, and more importantly, where it's coming from.
Disclaimer: Apparently, this isn't a very common phenomenon- or it is, it's just that the signal from other processes overwhelm it most of the time. But in this formation, it's business as usual- the last time the senior hydrologist had to pull this little trick, it was in the same rock. Go figure.
I based my cross section on a variety of sources- the existing geologic maps, some structural measurements and rock descriptions that my boss did, and the drilling logs for the big well. It mostly worked- it shows that one of the wells that we drew down shares a water bearing bedding plane with our well, and better yet, if we just drill him a new one a maximum of a hundred feet deeper down, he won't have any more problems when the production well goes online. However, the other well that we drew down? Not even close.
We have leakage between bedding planes.
This isn't necessarily a bad thing- if we can quantitatively estimate how much water is leaking through, we can get a clearer picture of the recharge area for this well, and possibly get more water out of it. The problem is convincing the regulatory agency that this is indeed what is happening.
Fortunately, there's a neat little mathematical trick that we can use to estimate this. Transmissivity is basically the ease with which water flows through the aquifer to the pumping well, and there are several ways to estimate this. The first two rely on pumping test data from both the pumping and recovery phase, and are estimates of the average transmisivity (these don't always match, so there's a whole new level of agony trying to figure out which number you should use, but fortunately that didn't happen here). The second one you can do if you've drawn down a monitoring well during the test, and that will give you transmissivity in that direction.
We already know that the aquifer is strongly anisotropic- that means that the speed at which water flows through the rock varies greatly depending on which direction it's going. And we have an average value, and two directional values in this case. So, what we can do is construct an ellipse with the transmissivity values and their directions from the well, and that'll give us an estimate of transmissivity in all directions, not just in the directions of wells we know about. From that, we can figure out how long it will take for the water to get to our well, and more importantly, where it's coming from.
Disclaimer: Apparently, this isn't a very common phenomenon- or it is, it's just that the signal from other processes overwhelm it most of the time. But in this formation, it's business as usual- the last time the senior hydrologist had to pull this little trick, it was in the same rock. Go figure.
