Well screen segments are placed through each water-bearing formation of a well. Although several different types of well screens are available, continuous wire wrap screens are generally preferred as the wire wrap screens provide the highest percentage of open area. Wire wrap screens have up to 37% open area while perforated pipe screens, the other type of commonly available well screen, have only up to 12% open area. It is desirable to install well screens with the highest open area possible as a higher open area allows water to enter the well more slowly. Excessive corrosion of the well occurs when water enters the well screen at a velocity greater than 0.1 foot per second. At speeds greater than 0.1 foot per second, solids drop out of the water and create too much encrustation and corrosion. Although debris can plug any well screen, the higher percentage of open area in wire wrap screens allows the wire wrap screen to continue to function because of its greater percentage of open area when a perforated pipe screen would be completely plugged.
Continuous wire wrap screens are made by wrapping a wire having a generally V-shaped cross-section around a series of longitudinal support rods to which the narrowest portion of the cross section of wire is welded or otherwise attached at every intersection to form inwardly opening flow slots. The wire and rods are generally made of metal, for its strength and corrosion resistance. Metals commonly used for well screens are mild steel, copper bearing steel, cor-ten steel, stainless steel and bronze. Well screens are manufactured in segments of a predetermined length. The segments of well screen are then attached by end fittings so that the section ofjoined well screen extends to the desired length.
The end fittings are conventionally made of the same metal as the screen and may be threaded at their outer ends to permit a threaded pipe or another length of well screen to be attached thereto. Alternatively, the outer ends of the end fittings are beveled to facilitate a fall penetration weld and then welded to another pipe or length of well screen.
When a well screen is installed in the borehole of the well, three forces act on the screen: column load, which is the vertical compression on the screen; tensile load, which is the extending force acting on the screen; and collapse pressure which is horizontal force acting on the screen. The well screen and end fittings which join the well screen segments must be strong enough to withstand these forces. The deeper the well, the greater the column and tensile load the well screen and casing must support. One segment well screen has to support the entire weight of the pipe extending above and below that segment. This burden exerts a column load on the screen, which is not a great force as the screen should be installed so that it hangs from of the pipe with no weight from the pipe resting on the screen. A tensile load is exerted on the screen when long sections of screen and casing are installed. The screen must have enough tensile strength to permanently hold any casing or screen suspended below it, which can be as much as 159,000 pounds to 209,000 pounds in extreme conditions. After the borehole annulus is gravel packed, earth pressures exert horizontal stresses on the screen. The screen must have adequate collapse resistance to withstand both earth and hydraulic pressures. In wells where the well borehole bends or slants, the well screen and its attaching end fittings are subject to even greater stresses. For continuous slot screens, the weight of the pipe column is supported by the cross-sectional area of the longitudinal rods and the end fittings. Because of the tremendous forces which end fittings must withstand, end fittings, especially in deeper wells or corrosive water, are made of metal.
End fittings are usually welded to the ends of the support rods and often must also be welded around of the periphery of the screen to the end wrap of wire. The welding operation adds greatly to the expense of attaching end fittings to well screens. Moreover, the melting caused by the welding process results in a loss of strength in the metal forming the longitudinal rods at the point of the weld. During the manufacture of the longitudinal rods, the strength of the rods is enhanced by work hardening. When the longitudinal rods are welded to an end fitting, the rods become partially annealed by the heat of the welding operation and lose part of their strength. As a result of the weld, longitudinal rod strength is diminished by at least 30%. Engineers typically estimate that another 50% of longitudinal rod strength is lost due to welding errors. As a result of the loss in strength caused by the welding process, well screen segments can break apart at the point of the weld to the end fittings. Accordingly, it is desirable to develop a process for attaching end fittings to support rods which does not require welding and which retains 100% of the longitudinal rod strength.
One method is taught by U.S. Pat. No. 4,819,722 to Daly. That patent teaches a plastic end fitting and method of attaching the plastic end fitting to a metal well screen. In Daly, the plastic fitting is secured to the well screen by melting the outer surface a portion of the fitting with a induction heating element which is wrapped around the well screen and fitting. Because metal end fittings provide greater strength and are more resistant to deformation by heat than plastic end fittings, it is still desirable to join a metal end fitting to a metal well screen without welding.
The metal end fitting of the present invention over the end fittings and means for joining end fittings to well screen taught in the prior art as the metal end fitting of the present invention can be joined to a well screen without the need for welding. The end fitting of the present invention is generally hollowly cylindrical in shape and sized so that the outer diameter of the end fitting is slightly larger than the outer diameter of the well screen to which the fitting is to be joined. Bores are formed in the body of the end fitting. The number and placement of bores in the end fitting should be such that the number and placement of bores in the end fitting match the number and placement of longitudinal rods in the well screen. The end fitting is mounted to the well screen by means of inserting each of the alongitudinal rods of the well screen into each of the bores in the end fitting. The end fitting is permanently secured to the well screen by swaging the end fitting so that the fitting is compressed about the longitudinal rods. The swaging process distorts the rod and bore from vertical in once or more areas, which binds the rod within the bore. The bond created by the swaging process is able to withstand greater tensile and collapse pressure than the longitudinal rods themselves. Alternatively, an adhesive may be used to secure the longitudinal rods in the bores of the end fitting. In an alternative embodiment of the invention, the bores are drilled completely through the end fitting. The longitudinal rods are then inserted through the bores. To secure the rods within the bores, a swaging tool is used to deform the end fitting about the rods. Alternatively, the protruding ends of the longitudinal rods are welded to secure the rods in the end fitting. Following the welding operation the end fitting and longitudinal rods may optionally be swaged for a more secure attachment.