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

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

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greater than the drawdown that[br]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[br]using, uh, the Jacob analysis, or

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

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see that there's a low permeability zone[br]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[br]drawdown based on our theoretical analysis.

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

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

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is here, so the drawdown is greater, um,[br]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[br]way to characterize it is to use the well

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

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

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So we need a way to calculate what the[br]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[br]Jacob analysis that's set up to calculate

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

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So, the important thing to recognize is[br]right here.

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

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What we used in the previous analysis[br]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-[br]the radius of the well itself.

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

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gonna use to determine the observed[br]drawdown.

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

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

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calculated using a monitoring well out[br]here in the formation.

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The performance here of the monitoring[br]well, the head in the monitoring well, is

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

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

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formation properties.