Double walled screen-filter with perforated joints

A well screen-filter includes a pair of substantially spaced concentric screens defining an annular filtering space therebetween, connected with perforated joint(s) closing the lower end of the filtrating space. The annular space is filled with filtrating materials (gravel or synthetic balls) as a pack and an upper joint acts as a cover cap of the annular filtrating space to seal the pack. The lower perforated joint includes holes for passage of fine particles to a sedimentation tube, preventing clogging of the pack and enabling proper functioning of the double walled screen-filter.

TECHNICAL FIELD 
The present invention relates to well strainers, screen-filters and the 
like, employed in equipping oil, gas and water wells to protect sanding 
and silting in. 
BACKGROUND ART 
Conventional screen-filters are used in many gas, oil and water wells to 
reduce sanding and silting in. These screen filters generally employ as 
filter material any of an artificial gravel pack poured into an annular 
space between the wall of the hole and the outside space of the screen, or 
a gravel pack formed by the formation gravel grains during development of 
the wells or gravel grains or grains formed of other materials, so that 
the filter material is applied as a bonded-on pack into the slots of the 
screen or bonded-on as a coated pack on the outside space of a single 
screen with a special type of resin. 
The wells with a gravel pack in the annular space must be drilled with a 
large diameter to accommodate a minimum of three inch gravel pack 
thickness from each side and to ensure that a required envelope of gravel 
will surround the entire screen. But generally, even in the large diameter 
hole having an adequate annular space, gravel is improperly packed. 
Therefore, this approach is costly and not effective to prevent sanding 
and silting in, particularly for deeper wells. 
The pack formed by development from formation gravel grains, also requires 
a large hole diameter and a very strong and special type of screen for 
long lasting development and very often fails to be fully satisfactory. 
Therefore, this approach is more costly and is not effective for 
protection of sanding and silting in. 
A pack bonded-on the outside or bonded-on into the screen openings from the 
gravel grains or grains from the other materials is very costly, fragile 
and very sensitive to transportation and installation into the hole. 
Such tightly packed grains reduce by about three times the free flow areas, 
compared with that of a loosely packed grains. 
Fine sand and silt entering any of these gravel or other packs will 
drastically reduce porosity and permeability of the filtering system. When 
fine particles occupy openings between the coarser particles of the 
artificial gravel pack, i.e., when porosity and permeability are reduced, 
the velocity of the fluid flowing through the screen-filter will be 
drastically increased, causing abrasion and incrustation of the screen, 
reduction of the yield, with increased possibility to the collapse of the 
screen or casing. 
Cleaning and maintenance of such well screen-filters are very costly and 
hazardous. Therefore, the key to good well efficiency is effective 
screen-filter operation which depends in turn upon the screen design. 
DISCLOSURE OF INVENTION 
The present invention provides a well screen-filter having substantially 
concentric and cylindrical, perforated or punched or slotted or wrapped 
screen members positioned to define an annular filtrating space 
therebetween. The double walled screen filter has a lower perforated 
joint-ring connected to the lower ends of the screen members and 
supporting filter material in the annulus between the two screens and 
below an upper cap. The double walled screen-filter is capable of being 
installed in a hole of a diameter just slightly larger than the outer 
diameter of the screen-filter. This is important because a larger diameter 
hole substantially increases drilling costs without significantly 
improving water yield. 
The inner and outer screen members can be made of different materials, as 
for example, different types of polymer (PVC or other), fiber glass, 
carbon steel, stainless steel or alloy. 
The outer screen is made of a stronger material than the inner screen 
because the outer screen is exposed to all pressure and tension whereas 
the inner supports only the filter material. 
The inner and outer screens may be manufactured from pipes or metal sheets, 
by perforation, punching or slotting or formed from metal bars and wires 
wrapped around the bars. The collapse strength of the double walled screen 
is increased over that of a single screen filter by the two jointed 
screens and therefore the total open area of both outer and inner screens 
can be maximized to increase the screen transmitting capacity of the 
double walled screen filter. 
Manufacturing the inner and outer screen from noncorrosive and inexpensive 
plastic materials, such as PVC, with high total open area, i.e., with a 
high transmitting capacity, according to the invention, represents a 
substantial improvement in both the design of the screen and economy. 
The transmitting capacity is the most important property of any screen and 
depends upon the total open area per foot (or per meter) of the screen, 
i.e., of the total open area per total length of the screen, quality of 
the screen and the hydraulic properties of aquifer and filtrating 
materials such as a gravel or polymer balls pack. The maximum transmitting 
capacity of the screen is expressed as gallons per minute per foot, or 
liters per minute per meter, at an entrance velocity of 0.1 ft/sec or 0.03 
m/sec (laminar flow), and therefore can be easily calculated from the 
total open area of the screen by multiplying the number of square inches 
of open area per foot by a factor of 0.31. Thus, for the inner and outer 
screens, total open area (over 10 percent or more) can be very easily 
adjusted, even for small diameter of outer and inner screens, in order to 
obtain the maximum transmitting capacity. 
Thus, according to the invention, the total length and diameter of the 
double walled screen-filter can be considerably reduced, because of the 
possibility of obtaining the same quantity of fluid as for a larger filter 
under favorable hydraulic conditions. This represents reduced cost for the 
screen-filter, as compared to any other existing type of screen-filter, an 
important issue since the construction of the screen-filter is always the 
most expensive component of any well construction. 
Vertically perforated lower joints and upper solid cover cap can also be 
made from the same or different materials as the inner and outer screen 
members. The vertically perforated lower joint is used to join the inner 
and outer screen and to support the gravel or polymer balls pack material. 
The lower joint also enables proper development of the well and transfers 
fine particulate material into a sedimentation tube (sand trap), as the 
material flows from the acquifer through the outer screen to the gravel or 
polymer balls pack. 
The strength of the vertically perforated lower joint depends on the 
materials to be made, the length and diameter of the screen segment, the 
annular space between two screens (inner and outer), the type of pack 
materials, i.e., its total weight, depth and other conditions of the well. 
The vertically perforated lower joint can be attached to the screens by 
inside and outside threads, or by outside threads only, by welding or by 
cementing (for screen members made of PVC materials), or by other means. 
The vertically perforated lower joint, even with threads, can be 
incorporated very easily between the threads of ordinary threaded pipes 
and the screen threads, without any changes of the threads of the joint 
system on existing pipes and screens. 
Vertical openings on the lower perforated joints comprise numerous holes. 
The size of the holes on the vertically perforated lower joints is the 
same as that of the slot openings of the outer screen, and the number 
depends upon the quality of the materials and space, although it is 
preferable to have as many as possible. 
The filtering materials, gravel or polymer balls pack, must be uniformly 
filled in the annular space between inner and outer screen members. The 
grannular size of the material depends upon aquifer conditions (determined 
on the base of sieve analyses of the aquifer grain sizes), and must be 
bigger than the slot sizes of the screen members. The filtering materials 
(gravel or polymer balls pack) may be filled on the spot into the annular 
space between the screens during installation of the double walled screen, 
with the grain sizes as required by the bearing formation, thus making it 
possible to also fill in segment filtrating materials of different size in 
accordance to the changes of the granulation of the bearing formation 
column. 
For example, the lowest part can be filled up with largest size of the 
gravel or polymer balls, and above this column pack, smaller grain sizes 
can be filled up, or vice versa. 
Such loose filtrating materials (gravel or polymer balls), particularly 
when they are well rounded grains and uniform, will possess more than 
double the free flow area or transmitting capacity of any densely packed 
filtrating material or almost three times the free flow area or 
transmitting capacity than a bonded-on screen of gravel or polymer balls 
granules. Laboratory and field tests prove that the thickness of the 
filtrating materials (gravel or polymer ball pack) filled up into the 
annular space between inner and outer screens could be very modest, i.e., 
about 1" (25.4 mm) and successfully retains the finest formation particles 
regardless of the velocity of fluid tending to carry these fine particles 
through filtrating materials. 
The upper joint, or solid cover cap on the top of the double walled 
screen-filter, is used only to seal filtrating materials in the annular 
filtrating space. It can be made from the same materials as the other 
members of the double walled screen, and connected only with threads or by 
welding or by cementing or by other means to the inner screen. 
Several field tests performed in water wells with double walled 
screen-filter (with gravel pack as filtrating materials), but without 
lower perforated joints did not give satisfactory results, either during 
well development or during pumping water, because of clogging by 
filtrating materials carrying particles from the water bearing formation. 
These lower perforated joints further enable proper cleaning and 
development of the bearing formation and cleaning of filtrating materials 
(gravel or polymer ball pack) from the fine particles entering the 
filtrating materials from the bearing formation during development of the 
well. In addition, the holes of the lower perforated joint make possible 
easy and free passage of the finest particles into a sedimentation part of 
the well (sedimentation pipe or sand trap) during pumping or lifting of 
fluids, thereby maintaining the effectiveness of the filtrating material 
for reliable double screen-filter operation. 
Such double walled screen-filter with perforated lower joint, filtrating 
material and upper cover cap, according to the invention, can be 
constructed on the spot, during installation into the borehole, 
representing a considerable savings with respect to transport expenses, 
installation and maintenance. 
The construction of the screen-filter of the invention also fully satisfies 
all technical requirements related to mechanical strength, as for example: 
resistance to pressure, tension and stress during installation into 
borehole, including resistance to denting from the effect of inside and 
outside pressure caused by formation conditions or by pressure and flow 
from the aquifer through the screen-filter segments; or by differential 
pressure into the well of any case. 
When the screen members of the double walled screen-filter are made of 
polymer materials, they are fully resistant to corrosion and destruction 
by bacteria and considerably resistant to incrustation. 
The members of the double walled screen-filter with lower perforated joints 
to be installed in the deep wells, according to the invention, can be 
constructed from different materials. For example, the outside screen and 
lower perforated joint (particularly when the double walled screen is 
composed from only one segment) may be made from metal materials of a 
corresponding strength (preferably non-corrosive) as that of the 
sedimentation pipe and protective casing; the inner screen and upper cover 
caps may be made from polymer material (such as PVC). Such inner screen 
and upper cover cap, made of polymer material, can be easily destroyed if 
necessary (after inproperly functioning) and replaced together with 
filtrating material without any damage to the metal outside screen and 
lower perforated joint or other parts of the well construction. 
The members of the double walled screen-filter and filtrating material 
(either from gravel or polymer ball pack) made from polymer can also be 
very easily destroyed and replaced when it is installed into a telescopic 
type of well and particularly when the upper part of the well is cased 
with a metal casing. 
Another very important aspect of the invention is mixing concentrated dry 
chlorine pellets with filtrating materials to disinfect the well, 
chlorinate the water, destroy iron bacteria and inhibit incrustation. 
This further contributes essentially to a prolongation of the life 
expectancy of the screen-filter and well, and improves performance of the 
well. 
It is therefore an object of the invention to provide a well strainer, 
i.e., screen-filter, so constructed as to enable proper well development, 
highest transmitting capacity under laminar flow condition, eliminatimg 
sanding and silting in, either by pressure flow or pumping conditions. 
Further benefits, features and aspects of the invention will be apparent 
from the following description of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1 to 4, the numeral 1 represents an outer screen having 
a slot opening; 2 represents an inner screen having slot openings; 3 is a 
loosely packed filtrating material (gravel or polymer balls pack); 4 a 
lower perforated joint; 5 an upper joint cover cap; 6 are slots (holes); 7 
are threads of joints; A is free space between upper part of the screen 
threads and upper part of sedimentation tube when lower perforated joint 
is incorporated into existing threads between screen and sedimentation 
pipe; and B is same as A, but for upper joint 5 and between screen and 
pipe. 
Referring to FIGS. 5 to 8, the numeral 8 represents a sedimentation tube, 
or cone (sand trap); 9 is a bottom cap of the well construction with 
threads or other connection on the side; 10 is a hole with left hand 
threads in the middle of bottom cap; 11 represents a protective 
pipe-casing above the double walled screen-filters; 12 is a cement ring 
(seal); J is an upper part of the double walled screen-filter construction 
for installation into the hole by hook; 13 is the hole; 14 is a previously 
installed sedimentation tube; 15 is a previously installed screen; 16 is a 
previously installed or formed gravel pack; 17 is a previously installed 
protection pipe-casing; 18 is a new sedimentation tube; 19 is a lower 
packer; 20 is an upper packer; 21 is a newly installed double walled 
screen-filter construction; 22 are driven pipes; 23 is a drive cone for 
well point; 24 is a jetting nozzle; 25 is a threaded joint; and, 26 is a 
spire. 
The embodiment, as illustrated in FIG. 4, shows that the lower perforated 
joint(s) is threaded outside and inside and incorporated into existing 
threads of a screen-pipe in order to couple together the inner and outer 
screens, although it can be threaded on only the outside for connection 
with the outer screen, on the inside for connection with the inner screen 
to be adjusted accordingly so that the lower perforated joint will serve 
as a supporter only of the inner screen, or both sides of the lower 
perforated joint(s) to be connected with the inner and outer screens by 
welding or cementing, as already explained. The upper joint-ring or cover 
cap, shown in FIG. 4 is threaded only on the outside. The cap can also be 
connected to the inner and outer screens by welding or cementing, or by 
other means. 
The embodiment, as illustrated in FIG. 5, shows the double walled 
screen-filter construction directly connected with a protective 
pipe-casing, in a so-called straight well construction. The bottom cap 
with a hole in the middle having left hand threads is connected to a 
sedimentation tube 8. The purpose of such bottom cap construction is 
generally for the installation of the whole well construction and well 
development with direct circulation. 
Installation of the whole well construction (or only screen-filter 
construction in case of a telescopic type of well), is performed through 
the pipe with left hand threads connected to the bottom cap before final 
setting down of the well construction using any corresponding fluid, so 
that the double walled screen and the walls of the well, particularly in 
the bearing formation, are properly washed. Later on, when the whole well 
construction is properly lowered down at the borehole bottom, and left 
hand pipes unscrewed from the bottom cap, washing may also be performed 
inside, in order to ensure proper cleaning of the double walled 
screen-filter construction. 
FIG. 6 shows a telescopic type of well construction with installed double 
walled screen-filter in the lower part of a borehole with a hook and a 
pipe. The upper part of the double walled screen-filter construction is 
sealed with rubber packers 20 to prevent entrance of formation materials 
into the well through the annular space of the protective pipe-casing 11 
and outside of the upper part of the double walled screen-filter 
construction. Therefore, when the double walled screen-filter construction 
is made of polymer materials (such as PVC or similar) and installed into a 
cased borehole, particularly with metal pipe-casing (as upper part of the 
well construction), the filter may be easily demolished should any 
problems occur during or after installation and will, at the same time, 
preserve the water well from any damage. The lower part of the borehole 
need only be recleaned and a new double walled screen-filter construction 
system installed again. 
FIG. 7 shows how the double walled screen-filter construction may be 
installed in an existing well when sanding up represents a serious problem 
to either the production or wearing out of the well pump. The double 
walled screen-filter is installed inside the existing well screen with two 
packers 19 and 20. The lower 19 and upper 20 packers seal the annular 
space between the existing pipe-casing (sedimentation tube and protective 
pipe-casing) and the double walled screen-filter construction. 
Redevelopment of the well and cleaning of the double walled screen-filter 
may be performed easily through the lower perforated joint(s), with any 
corresponding fluid. 
FIGS. 8a, b shows how the double walled screen-filter is used in well point 
or in a small diameter hole. Therefore, for a well point, i.e., for driven 
wells, either the inner or outer screen can be made of perforated or 
slotted or punched steel tube of similar construction of different 
diameter, attached to the well point cone (which is used also as a driving 
tool and thereafter as a sand trap) and to a driving pipe on the top, and 
constructed with other screen-filter members as any other double walled 
screen-filter. 
For a drive well point according to the invention, as shown in FIG. 8a, an 
outer screen of 21/2" (63.5 mm) and inner screen of 1" (25.4 mm) nominal 
diameters, with a threaded joint of an approximately 3" (76.2 mm) outside 
diameter and a drive cone with jetting nozzles on the cone and different 
slot sizes (0.1-2 mm or over 2 mm) on the screens and proper granulus of 
the filtering materials (gravel or polymer loose balls pack) will prevent 
any sanding and silting in. 
Similar construction of the double walled screen-filter but with spire as 
shown in FIG. 8b can be used for a small diameter hole of any kind. 
The advantages of the double walled screen-filter construction are 
summarized as follows: 
(1) Production of fluids free of sand and silt from the wells under 
favorable hydraulic conditions; 
(2) Economy with regard to the drilling cost of wells; selection of the 
materials and manufacturing of the screen-filter; transportation, 
installation and maintenance of the screen filter; prolongation of the 
life of the screen-filter and the complete well construction; prolongation 
of the life expectancy of the pumping units and water distribution system; 
(3) Reduction of the screen length and diameter due to the possiblity of 
obtaining a high total open area and the optimum hydraulic characteristics 
of the screen-filter; reduction to the construction weight of the screen 
and resistance to corrosion and bacteria destruction, when using polymer 
materials; 
(4) Easy well development and maintenance of the double walled screen 
construction during production; 
(5) The possibility of replacing the entire lower part of the well 
construction (double walled screen-filter) when manufactured from polymer 
materials; 
(6) Using faster, more effective and cheaper well disinfection, water 
chlorination, and protection of well construction against bacteria and 
incrustation; 
(7) The possibility of manufacturing the same type of double walled 
screen-filter construction from considerably more resistant materials and 
under reasonable cost and using it in deep oil, gas or geothermal wells, 
where it is necessary to prevent sanding up. 
In this disclosure, there is shown and described only the preferred 
embodiments of the invention, but as aforementioned, it is to be 
understood that the invention is capable of use in various other 
combinations and environments and is capable of changes or modifications 
within the scope of the invention concept as expressed herein.