All screens were initially prepared for model testing in an identical
fashion, exposing half the screen to flow. In subsequent testing, two-thirds of the
exposed screen section was masked to expose 60°. This latter series of tests resulted
in a more realistic model / aquifer scaling ratio by eliminating the previous distortion
of one-sixth of the aquifer flowing into one-half of the well. Masking the well screens
was accomplished in a symmetrical fashion to provide for uniform exposure. Ordinary
duct tape was used on the outside of the well screen to selectively block off portions
of the screen apertures.
For initial model testing, it was felt that aquifer materials from
field formations should be used as representative of prototype conditions. Two
different aquifer materials were located which met the requirements. Both formations
are well known producing aquifers, and have substantially different geohydraulic
properties.
4.3.1 Santa Barbara Formation
The first aquifer material tested in the model was a Quaternary
geologic formation common in the Santa Barbara area of California. The Santa Barbara
formation is a "tight" fine-grained sand with low hydraulic conductivity,
representative of the coastal area of California near Santa Barbara. Average grain
size is .15mm and the uniformity coefficient is 1.3. The aquifer material was obtained
from an outcrop location in Santa Barbara and installed in the model in Los Angeles.
Following a sieve analysis (see Figure 9), tests using a "Darcy
apparatus" were made, establishing an initial hydraulic conductivity of 50 gallons per
day per square foot. Hydraulic conductivity as determined from model tests was somewhat
lower, averaging 29 gpd/ ft2 (see Figure 10).
Figure 9
Figure 10
A gravel pack (Monterey #1) was selected to give a pack/ aquifer
ratio of 7:1 (see Figure 9).
Due to the extremely low hydraulic conductivity of the aquifer
material, only 2 gpm could be produced through the model, with maximum drawdowns at
the well approaching 55 ft. This model yield of 2 gpm would be equivalent to a well
in the field screened in 5 ft of aquifer producing 12 gpm. Field tests from the Santa
Barbara area confirm these results.
4.3.2 Silverado Aquifer
The second series of model tests incorporated a much more permeable
material known as the Silverado formation, which underlines much of the West Coast
basin of Los Angeles.
The Silverado aquifer was obtained from a well under construction,
using a mud scow, a drilling tool known for its ability to remove formation intact
rather than pulverizing it.
Mechanical grading analyses of the Silverado formation used in the
model, along with two different gravel packs, are shown in Figure
11. The difference between the field aquifer and the model aquifer can be
explained by rearrangement of the material when introduced into the model. The
average grain size of the Silverado model aquifer is .87mm with a uniformity
coefficiency of 7.3.
Figure 11
