Problem 1a

The table on the right shows the results of a formation sample hypothetical sieve analysis. (Grain sizes listed in the table are conjecture, and in no way are intended to represent actual sieve sizes.) Use the data to determine filter pack type and well screen slot size for the installation of a monitoring well.

Grain Size

Wt. Retained

Cumulative Wt.

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Cumulative %

Retained

65 52 52

40 23 75

10 109 184

4 13 197

3 1 198

Pan 2 200

Calculate the cumulative percent retained for each successive sieve

Grain Size Wt. Retained Cumulative Wt. Retained

Cumulative % Retained

65 52 52 28

40 23 75 37.5

10 30 184 92

4 55 197 98.5

3 1 198 99

Pan 2 200 100

Step 1.

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Step 2.

Plot the data as Grain size vs. Cumulative percent retained

Note that 90% retained is the same as 10% passed, 30 % retained is the same as 70% passed, etc.

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Effective grain size = D10 = .0113” D60 = 0.038”

Step 4.Determine the Uniformity Coefficient. The Uniformity Coefficient is D60 /D10

Uniformity Coefficient = .038/.0113 = 3.36

Step 3. Determine Effective Grain SizeEffective grain size is equal to the sieve size that retains 90% of the formation material, or passes 10%, hence, D10

If the effective grain size exceeds .01 inches,

AND

The Uniformity Coefficient is greater than 3,

then you can naturally develop the well.

Effective grain size = .0113”Uniformity coefficient = 3.36

You can naturally develop this well.

You must now determine screen slot opening size.

• To determine the screen slot opening size for a naturally developed well, use the following guidelines:

If the uniformity coefficient > 6, slot size is equal to D50

If the uniformity coefficient is >3 or < 6, slot size is equal to D40

If the uniformity coefficient is <3, slot size is D30

3 < 3.36 < 6, therefore screen opening slot size is equal to D40

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D40 = .027”

Well screen slot opening size = .027”However, this size opening is not available commercially, so you must choose the next smaller available size, which is .025”.

Answer 1a.

• Naturally develop the well

• Use a screen slot size of .025”.

Grain Size Wt. Retained

Cumulative Wt. Retained

Cumulative % Retained

18 0.25 0.25 0.125

14 3.75 4 2

12 16 20 10

10 50 70 35

8 90 140 70

6 40 180 90

2 18 198 99

0.5 2 200 100

Problem 1b.

The table below shows the results of a formation sample hypothetical sieve analysis. (Grain sizes listed are conjecture, and in no way are intended to represent actual sieve sizes.) Use the data to determine filter pack type, filter pack grain size, and well screen slot size. Assume the material is fine grained and fairly well sorted.

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Grain and Slot Opening Size (thousandths of an inch)

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Step 1. Plot the data and determine D10 and D60: = .006, and .01Step 2. Determine Uniformity coefficient = .0010/.006 = 1.67

Effective size is less than .010”Uniformity coefficient is less than 3Therefore, use an artificial filter pack for this well.

Now determine the filter pack grain size for this well.

Well screen slot size for an artificial filter pack

Step 1. Use the multiplier

Use a multiplier on the D30 grain size of the formation sample.If the sample material is relatively fine grained and well sorted, use 3.If the sample is relatively coarse grained and less well sorted, use 6.

This formation is fairly well sorted and fine grained, we will use 3.D30 = .008”D30*3 = .024”This answer is used to generate the grain distribution curve for the filter pack

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Step 2. The answer from the multiplier is the first point plotted on theGrain distribution curve for your filter pack. This point is D30 for the filter pack.

Step 3. Determine D10 and D60 for the filter pack.

To determine these two variables, use trial and error using grain sizes around D30.

The ratio of D60 to D10 is set at 2.5

D30 = 24

(24+x)÷(24-x)=2.5

24+x=60-2.5x

3.5x=36

X=10.2857

D10= 24-10.2857=13.714 (.013714”)

D60= 24+10.2857=34.2857 (.0342857”)

34.2857/13.714=2.5

Step 4. Plot D10 and D60 on the grain size distribution curve for the filter pack.

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D10 D60

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Step 5. Draw a smooth curve through the three points.

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Step 6. Specify and draw the limits (range) of the grain size envelope, 8% on either side of the distribution curve.

Filter pack design is now complete.

Range of filter pack grain sizes = .0046” to .054”.

Next determine screen slot size for the well• Design the screen to retain 90% to 99% of

the filterpack.

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To retain 99% of the filter pack, screen slot size should be approximately Equal to D1, to retain 90% slot size should be equal to D10. These values are taken from the filter pack grain size distribution curve.

D1 = .006” D10 = .01374”

For D1, screen slot size is .006” which is available commercially as listed above.

For D10, screen slot size is .01374”, so the screen slot size of .012”as listed above would be used.

For 99% retention

For 90% retention

Answer 1b.

• Use an artificial filter pack

• Grain size range for the filter pack is.0046” to .054”.

• Use slot size .006” for D1, and .012”

for D10.

Question 2

Discuss the impact of screen length on monitoring well efficacy.

Answer, Question 2Well screens, part of the intake system of a monitoring well, are designed to provide access to a specific target zone for data gathering and sample collection and to provide openings through which groundwater can efficiently enter or pass through the well. Of all the factors that control the efficacy of a monitoring well to provide a true representative groundwater sample or hydraulic head measurement, the length of the screen is perhaps the most important. It is essential to understand that any measurement taken from a well represents a composite average of the conditions existing over the length of the screen. Because concentrations of contaminants can vary by orders of magnitude within only a few centimeters, it is critical to define the objectives of the monitoring well before you design the screen.From the outset it is important to identify whether the well will be used merely detect the presence of a contaminant or provide a measure of absolute concentration. Long screen wells are effectively used as tools to screen groundwater and detect the presence of contaminants, whereas short screen wells are designed to sample more specific target zones for concentration measurements. The differences in chemical concentrations the above methods would return could be profoundly different, and prompt very different decisions.Contaminant plumes can have complex three dimensional distributions, and can often be driven beneath the water table. Short screen wells can accurately measure flow direction and contaminant distribution, and are recommended for programs where the objective is to define the distribution of contaminants and hydraulic head where complex geologic conditions occur. Long screen wells cannot provide discrete data of necessary quality to define three dimensional distributions of contaminants, hydraulic head, or groundwater chemistry because of the averaging effects over the length of the screens. However, if the goal of the study is to identify the presence of a contaminant in an aquifer or monitor the presence of LNAPLs, a longer screen can be used. If designed for detection of LNAPLs, the well screen would need to be of sufficient length to maintain presence within the water table during both extreme highs and extreme lows.A lack of planning and foresight or vague project objectives can lead to a monitoring well design providing at best, misleading data, and an equal measure of professional embarrassment.

Question 3

List three monitoring well types, other than multilevel monitoring systems, that can be used to monitor multiple vertical intervals? What distinguishes them from one another?

Answer 3

Well ClustersWell clusters are used to monitor several vertical zones in the same location.

Each single casing, single screen assembly is built in a separate bore hole, each of varying depths. Each well has only one annular seal layer.

Answer 3

Well NestA well nest is another well type that can monitor several vertical zones

in the same location. Multiple single casing single screen wells of varying lengthshare one large bore hole. Multiple layers of annular seal are required to prevent hydraulic communication from adjacent horizons.

Answer 3

Single-Casing, Multiple-Screen WellConsists of alternating sections of well screen adjacent to zones of interest

and well casing between the zones of interest all within a single bore hole. Multiple layers of annular seal are installed to prevent hydraulic communication between zones of interest. Packers must be installed in the blank casing zones. You can save money drilling the small, single hole, but those savings are often consumed in the purchase of pumps and pressure transducers.