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.