WEBVTT

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

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

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greater than the drawdown that
would be expected otherwise.

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

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

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

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

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this dashed line.

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

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

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

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

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due to the well skin.

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

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

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

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

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this with the Jacob analysis.

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

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

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So, the important thing to recognize is
right here.

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

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

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well, where our data were made.

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

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

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gonna use to determine the observed
drawdown.

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

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

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calculated using a monitoring well out
here in the formation.

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

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

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

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formation properties.