The Well / Aquifer Model (Initial Test Results)   -   23

7.0   SIGNIFICANCE OF TEST RESULTS AS RELATED TO WELL DESIGN

7.1   Concept of Effective Area of Opening

An important observation resulting from the initial testing occurred during the 10^th, 11^th, and 16^th tests (see Figure 16 and data records 10, 11, and 16 in Appendix III). In these three tests, a combination of variables produced a unique hydraulic situation in the well/ aquifer model. The aquifer type in the model was the Silverado formation, and the gravel pack was Monterey #2 (see Figure 11). The resulting pack/ aquifer ratio was 2:1.

The well screens tested had an opening fraction of 50% (i.e., 180⁰ or half the screen open area was available for flow). During normal testing procedure, when well screens with percentage open areas of 1% or less were used (e.g., milled slot and standard shutter), an interesting phenomenon developed. Initially, flow in the model well was similar to that of other tests. However, as time progressed, well discharge decreased and at the same time head loss across the screen increased. The effect was much more pronounced with milled slot screen than standard shutter screen. An unstable condition apparently occurred around the well screen, caused by fine-grained material "bridging" or blocking the screen’s open area. This blocking effect became progressively worse as flow diminished and pressure drop across the well screen increased. This "pressure induced bridge" of the sand grains in effect reduced the maximum open area of the screen slots by as much as 90%. In the case of the milled slot screen, some slots were completely sealed off.

The net result was that maximum open area available for flow into the well, as measured from screen aperture dimensions, was reduced considerably. This led to a concept of "effective open area" or the area of the screen aperture which is open to flow (analogous to effective porosity).

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