A Guide To Water Well Casing and Screen Selection   -   17

7.0  SCREEN DESIGN

7.1  Torch Cut Slots

With the preceding section in mind, analysis may be made of the types of screens in common use. The first considered represents, under any circumstances, and undesirable choice. This is casing with vertical slots perforated by a cutting torch. Fortunately this practice has fallen into disfavor, and is infrequently used today. The only reason for inclusion of such material in a water well is lack of an alternative. The disadvantages of torch cut slots are low area of opening, high corrosion at the ragged torch cut edges, slag, irregular uncontrolled openings, weakness particularly in collapsing strength, and a great tendency to clog. Even low cost is not an advantage of this material, since torch cutting with oxygen-acetylene gas is a slow process.

7.2  Milled Slots

Another vertically slotted screen is manufactured from casing by milling openings with axially oriented cutters. This product is designed for use in oil wells where fluid production is very low. Its disadvantages include clogging due to the parallel surfaces within the opening. Since slot clogging is directly related to wall thickness, thicker material encourages greater plugging. Some vertical mill slotted casings are machined with an undercut to reduce this tendency. A second drawback is low area of opening, although this can be overcome at higher cost by increasing the number of slots. Collapsing strength of vertically slotted casings, however, is substantially reduced when the number of openings is increased.

Field experience has proved that development is generally slower in wells screened with milled slot casings. Another aspect of vertical slots compared with horizontal openings is that sand and gravel control is less positive. A smaller aperture and/or larger gravel must be specified. The filter pack should be relatively coarse, with at least 90% retention, rather than the conventional 65 to 80%.

The chief positive characteristics of milled slot casings are low cost, uniform openings and availability in a fairly wide range of apertures and patterns.

Back   |   R & D Home   |   Next