La. Admin. Code tit. 56, § I-327 - Screen
A. Every water well
shall be provided with an appropriate screen. It shall be the responsibility of
the driller to determine the type of screen required, screen material, slot
openings, entrance velocity, screen length and setting, and whether or not the
well is to be gravel packed.
B.
Type of Screen. The type of screen used is governed by cost, the contractor's
experience with handling a specific type of screen, water quality, length of
screen required, proposed well yield, and the required structural strength of
the screen. The screen selected shall be strong enough to withstand external
pressures and vertical load due to the weight of drill stem used to set the
screen and the casing above the screen, if set in one continuous
string.
C. Screen Material. The
type of screen material is generally dependent upon cost and the quality of
water to be pumped. If the water contains a relatively high concentration of
carbon dioxide, dissolved solids or hydrogen sulfide, corrosion-resistant
materials should be used in the construction of the screen. If a corrosive
environment is present, the screen should be made entirely of the same
material, and the lap or extension pipe (for not less than 5 feet) above the
screen and blank pipe, if used, should be made of the same material as the
screen. The likelihood of corrosion and encrustation can also be decreased by
maintaining the entrance velocity within acceptable limits, 0.1 foot per second
or less.
D. Among metal alloys
available with varying degrees of corrosion resistance are the stainless steels
which combine nickel and chromium with steel and the various copper-based
alloys. Manufacturers can be expected to provide advice on the type of metal or
metal alloys that should be used if supplied with the results of a water
analysis. Nonmetal screens made of polyvinyl chloride (PVC) have been used as
an alternative when corrosive conditions exist.
E. In contrast to "corrosive waters",
encrusting waters are usually alkaline, have excessive carbonate hardness and
contain iron and/or manganese. Encrustation, which reduces the open area of the
screen and the specific capacity of the well, is the deposition of undesirable
material about the screen openings. Efficient well development, which will
decrease excessive head losses or pressure differentials across the face of the
screen, will minimize the precipitation of encrusting minerals.
F. Screen Slot Openings. The selection of the
screen openings, which shall be based on the results of mechanical analysis of
the formation samples collected during drilling, is dependent upon the
percentage of material that will be allowed to pass through the openings in the
development process. Generally, the percentage of material that will be
permitted to pass through the screen openings is related to the intended use of
the water. Although proper screen selection and well development should
eliminate the pumping of sand during normal operations, cyclic pumping and
increased pumping rates sometimes cause a well to yield some sand. Sand pumping
by wells used to supply public and domestic water systems cannot be tolerated,
whereas some sand in water used for irrigation is generally acceptable. Other
factors involved in the selection of the slot openings are the uniformity of
the material, the uniformity coefficient, the type of overlying sediments and
the desired entrance velocity.
G.
Properly designed slot openings should allow the water to flow freely from the
formation into the pump area while preventing clogging and sanding.
H. Entrance Velocity. To minimize the
potential for encrustation, corrosion and "sanding", the entrance velocity
should not exceed 0.1 foot per second. The entrance velocity is calculated by
dividing the yield expressed in cubic feet per second (gallons per minute
divided by 448.8 equals cubic feet per second) by the total area of the screen
openings in square feet. The total area of the screen openings is the area of
the openings provided per foot of screen multiplied by the length of screen in
feet. Most manufacturers provide tables listing the open area for screen
diameter and slot openings.
I.
Screen Length. The length of the screen is influenced by cost, aquifer
thickness, desired well yield and the estimated pumping level. The screen
length should represent a compromise between cost and well efficiency. Well
yield is more effectively increased by increasing the length of the screen than
by proportionally increasing the diameter.
J. Screen Setting. Installation of the screen
should be based upon an evaluation of all data collected during drilling and a
detailed interpretation of the driller's and geophysical logs, if available.
Care should be exercised to avoid damaging any part of the screen and to ensure
that the setting is correct.
K.
Gravel Pack
1. If the interval to be screened
consists of a fine uniform sand or consists of thin alternating layers of fine,
medium and coarse sand, it may be desirable to gravel pack the screen. The
objectives of gravel packing are to increase the permeability of the material
in the zone immediately surrounding the screen, to minimize the chances of sand
pumping, to reduce the entrance velocity at the face of the screen, to reduce
the chances of error where a screen is set opposite alternating beds of sand of
different grain size and clay, and to allow the installation of a small
diameter screen in relatively thick aquifers.
2. If required, a properly graded gravel pack
shall be selected based upon an evaluation of the sieve analysis for the sands
in the formation. The uniformity coefficient (see
§113 of this Chapter for glossary of terms)
of the selected gravel pack material should be 2.5 or less. The gravel
envelope, usually 3 to 8 inches thick, should consist of clean, well-rounded
siliceous material that will permit the selection of screen openings that will
retain 90 percent or more of the gravel pack material by size. Limestone and
shale shall not be used as a gravel pack.
L. Formation Stabilization. If the hole
drilled to accommodate the screen is much larger (4 inches or more) than the
diameter of the well screen, it is sometimes necessary to stabilize the
extension pipe with a material such as sand or gravel to prevent caving or
slumping of silt, sand, and clay from above the aquifer. Formation
stabilization should not be confused with gravel packing. In contrast to gravel
packing, the material used as the formation stabilizer is not specially graded.
In addition, commercially available equipment, such as shale packers or
metal-petal baskets, are commonly used to prevent sloughing or caving into the
producing formation.
Notes
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