Speaker: Nay wells are affected by a well
skin, a low permeability layer that

surrounds the well and causes the drawdown
in the skin to be less than- er to be

greater than the drawdown that
would be expected otherwise.

So, we can see this in the, in the sketch.

This line here is the expected drawdown
using, uh, the Jacob analysis, or

perhaps some other analysis, but as we
get right in the vicinity of the well, we

see that there's a low permeability zone
here, and the head goes like so, follows

this dashed line.

And as a result, this is the expected
drawdown based on our theoretical analysis.

It is using the properties of the aquifer,
uh, out here away from the well

[stammering] in this region, but in fact
we observe that the drawdown at the well

is here, so the drawdown is greater, um,
and that results from the extra headloss

due to the well skin.

So we want to characterize this, and one
way to characterize it is to use the well

efficiency, which is the ratio of the
expected drawdown from our theoretical

analysis to the observed drawdown,
what actually occurs in the field.

So we need a way to calculate what the
expected drawdown is, and we can do

this with the Jacob analysis.

What I'm showing here is a version of the
Jacob analysis that's set up to calculate

the head- er I guess this is the drawdown
here, um, as a- at a particular time.

So, the important thing to recognize is
right here.

The radial distance that we're using here
is the radius of the well.

What we used in the previous analysis
was the radial distance of the monitering

well, where our data were made.

In this case, we need to use the radial-
the radius of the well itself.

This time here, that's the time, the
elapsed time, for a data point that we're

gonna use to determine the observed
drawdown.

The calculation goes like so: we put in
the observed time and the radius

of the well, and everything else is pretty
much the same, the s and the t we've

calculated using a monitoring well out
here in the formation.

The performance here of the monitoring
well, the head in the monitoring well, is

not effected by the skin, so when we
calculate TNS from the monitoring well

data, we're getting something that's
really just affected by the, um,

formation properties.