Disadvantages of PTFE well casing and screen materials:

May sorb/desorb organic constituents from/into solution;

Only slotted casing is available for screens;

Ductile behavior of PTFE ("creep" or "cold flow") may result in the partial

closing of well intake openings (i.e., screen slots);

PTFE's extreme flexibility may result in non plumb and bowed wells;

Non stick nature of PTFE may cause annular seal failure;

Moderate weight and low strength per unit length;

PTFE casing and screen is unsuitable for driven wells; and

Higher cost relative to stainless steel and PVC.

Structural strength of screen materials is primarily a problem only with PTFE screen

materials, which are affected by a phenomenon known as "creep" or "cold flow." Under

constant stress through time, such as continuous loading of the entire length of casing, PTFE

can deform plastically (i.e., it retains the deformed shape after the stress is removed), and in

screened casings made of PTFE, the result can be partial or complete closure of the slots, thus

effectively ruining the well's usefulness for monitoring purposes. This is a problem,

however, only when the wells are relatively deep (250 feet or deeper); in shallow wells the

physical resistance of PTFE to compression is greater than is its tendency to deform

plastically (Du Pont, reference 1).

If PTFE is to be used in deeper wells, structural strength problems can be avoided by

using slightly larger slots; larger slots may be narrowed slightly because of cold flow,

however they will not be completely sealed shut. It also may be possible to obtain PTFE

casing that has been modified by the use of fillers. Fillers can be used to increase the

resistance to cold flow by approximately a factor of 2 (Du Pont, reference 1), thus limiting

the deformation that will occur in the screened casing. More information about "cold flow"

phenomena is available from the manufacturer (Du Pont, reference 2).

Metallic Materials

Metallic well casing and screen materials available for use in monitoring wells include

carbon steel, low carbon steel, galvanized steel, and stainless steel. Well casings and screens

made of any of these metallic materials are generally stronger, more rigid, and less

temperature sensitive than thermoplastics, fluoropolymer, or fiberglass reinforced epoxy

November 1992

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