La. Admin. Code tit. 56 § I-327

Current through Register Vol. 50, No. 11, November 20, 2024

Section 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.

La. Admin. Code tit. 56, § I-327

Promulgated by the Department of Transportation and Development, Office of Public Works, LR 1:249 (May 1975), amended LR 11:956 (October 1985), repromulgated by the Department of Transportation and Development, Office of Public Works, LR 31:942 (April 2005), amended by the Department of Natural Resources, Office of Conservation, LR 37:910 (March 2011), amended by the Department of Natural Resources, Office of Conservation, LR 37:910 (March 2011), LR 37:3529 (December 2011).

AUTHORITY NOTE: Promulgated in accordance with R.S. 38:3091-R.S. 38:3098.