Louisiana Administrative Code › Title 56 - PUBLIC WORKS › Part I - Water Wells › Chapter 3 - Water Well Construction › Section I-327 - Screen

Current through Register Vol. 50, No. 6, June 20, 2024
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.