Analysis and Comparison of the Wells in the Pleasant Valley Area of Ventura County   -   3

3.0   COMPARISON OF WELL YIELDS

3.1   Specific Capacity per foot of Screen

It is an established fact that the specific capacity of a well (Discharge in gpm/Drawdown in ft) is directly dependent upon the total thickness of aquifer penetrated by the well. For exapmle, a well constructed with 1000 feet of screen will yield approximately twice the discharge of one with only 500 feet of screen if all other properties are held constant (e.g. drawdown and aquifer parameters).

Because of this fact, and in order to make a fair comparison between wells in the Pleasant Valley area, specific capacity per foot of well screen were calculated for each of the wells. (The method of calculation is shown in Appendix). Figure 7 (Appendix) is a list of Specific Capacities per Foot of Well Screen in order from highest to lowest. Figure 3 is a simplified bar chart of the same data showing the relationship for the 34 wells.

As can be seen, there is no relationship between well screen open area and specific capacity. In fact, the lowest percentage open area screens show the lowest percentage open area screens show the highest specific capacity per foot. This last observation confirms field and laboratory studies which show that above 3-5% open area, no significant gain in well efficiency is obtained.

3.2 Relationship of Specific Capacity to Aquifer Properties

To confirm the relationship between high yielding aquifers (e.g. sands and gravels) and high yielding wells, a plot of specific capacity versus aquifer Hydraulic Conductivity was made. Figure 4 clearly shows the direct dependence of specific capacity on aquifer properties. In other words, wells penetrating high yielding aquifers result in high discharges as compared to wells producing from lower yielding aquifers (for the same length of screen).

3.3 Specific Capacity and Length of Well Screen

A comparison between specific capacity and length of installed well screens was made for the 34 wells in the Pleasant Valley area. The wells were grouped into three principle screen types:

1. Shutter Screen
2. Milled or punched slotted casing
3. Continuous wire wrap type screen

Data on specific capacity were analyzed with the results shown graphically in Figure 5. As can be seen, the more well screen installed in a well, the higher the specific capacity and consequently the well yield. The following simple example illustrates the importance of this point:

Using the average of the wells completed with Shutter Screen, the specific capacity for a well with 300 feet of screen would be 40 gpm/ft (see uppermost curve on Figure 5). If it were desired to pump this well at 2000 gpm, the total drawdown would be 50 feet (2000/40).

Given the same well with 900 feet of screen, the specific capacity as obtained from the graph on Figure 5 would be 120 gpm/ft. For the same discharge of 2000 gpm, the drawdown in this case would be only 17 feet. This amounts to a savings of 33 feet of pumping lift. Over the lifetime of this well, the savings in pumping costs would more than pay for the additional length of screen and borehole.

Simple analyses such as these illustrate the importance of proper well screen length regardless of open area in well design.

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