Sintered metal substitute for prepack screen aggregate

A prepack well screen assembly has a resistance welded outer screen concentrically mounted in radially spaced relation on a perforated mandrel, thereby defining an annulus in which a sintered metal prepack sleeve is loaded. The longitudinal spacing distance between adjacent turns of the outer screen selectively exclude sand fines of a predetermined minimum size. The porosity of the sintered metal prepack sleeve is selected to pass sand fines in the size range of from about 20 microns to about 150 microns. The effective inlet flow area through the sintered metal prepack sleeve is substantially greater than the effective inlet flow area through the outer screen. The sintered metal prepack screen excludes sand fines from inflowing formation fluid during the initial production phase following a gravel pack operation, without limiting production of formation fluid.

1. Field of the Invention 
This invention relates generally to apparatus for completing downhole 
wells, and in particular to well screens for filtering unconsolidated 
material out of inflowing well fluid in water, oil, gas and recovery 
wells. 
2. Background of the Invention 
In the course of completing an oil and/or gas well, it is common practice 
to run a string of protective casing into the well bore and then to run 
the production tubing inside the casing. At the well site, the casing is 
perforated across one or more production zones to allow production fluids 
to enter the casing bore. During production of the formation fluid, 
formation sand is also swept into the flow path. The formation sand is 
relatively fine sand that erodes production components in the flow path. 
In some completions, the well bore is uncased, and an open face is 
established across the oil or gas bearing zone. Such open bore hole 
(uncased) arrangements are utilized, for example, in water wells, test 
wells and horizontal well completions. 
One or more sand screens are installed in the flow path between the 
production tubing and the perforated casing (cased) or the open well bore 
face (uncased). A packer is customarily set above the sand screen to seal 
off the annulus in the zone where production fluids flow into the 
production tubing. The annulus around the screen is packed with a 
relatively coarse sand or gravel which acts as a filter to reduce the 
amount of fine formation sand reaching the screen. A work string and 
service seal unit (SSU) is used to spot the gravel around the screen. 
During well completion, gravel is also pumped and squeezed into the 
producing formation around the screen for filtering unconsolidated 
material out of the infilling well fluid. The gravel is pumped down the 
work string in a slurry of water or gel and is spotted directly under the 
packer or above the sand screen. The gravel also fills the annulus between 
the sand screen and the well casing. In well installations in which the 
screen is suspended in an uncased open bore, the gravel pack supports the 
surrounding unconsolidated formation. 
3. Description of the Prior Art 
Conventional sand screens employ a perforated mandrel which is surrounded 
by longitudinally extending spacer bars, rods or ribs and over which a 
continuous wire is wrapped in a carefully spaced spiral configuration to 
provide a predetermined axial gap between the wire turns. The aperture 
between turns permits formation fluids to flow through the screen, while 
the closely spaced wire turns exclude fine particulate material such as 
sand or gravel which may penetrate the gravel pack. 
A problem which arises during initial production following the gravel 
packing operation is that fine sand may be carried through the gravel pack 
before the gravel pack bridge stabilizes. It is not unusual to produce a 
substantial amount of such fine sands before the gravel pack finally 
consolidates and yields clean production. During the early stages of 
producing the well after gravel packing, those fines tend to migrate 
through the gravel pack and screen and lodge within the inner annulus 
between the outer wire wrap and the perforated mandrel. In some instances, 
this can cause severe erosion of the screen and ultimate failure of the 
screen to reduce sand invasion. In other situations, the sand fines may 
include plugging materials which are carbonaceous, siliceous or organic 
solids which can completely plug the screen flow passages and terminate 
production shortly after completion. In deep wells, when the screen 
becomes plugged and the internal pressure in the production tubing is 
reduced, the formation pressure can collapse the screen and production 
tubing. Moreover, when a substantial amount of sand has been lost from the 
surrounding formation, the formation may collapse with resultant damage to 
the well casing, liner and/or screen consequent reduction or termination 
of production. 
One attempt to overcome the foregoing problem is to interpose a prepack of 
gravel aggregate within the annulus between the inner mandrel and the 
outer wire screen. The prepacked gravel is sized appropriately to exclude 
the fines which accompany the formation fluid during initial production. 
Raw gravel, as well as epoxy resin coated gravel, have been used 
extensively in prepacked well screens. Most prepacked well screens are 
subject to retrieval problems due to their outer diameter being larger 
than that of a conventional well screen. In order to make prepacked well 
screens more easily retrievable, the inner mandrel is usually downsized, 
therefore creating restrictions in both production bore size and 
completion tool string bore size. 
Some conventional well screens have utilized an inner wire cloth or steel 
woven fabric filter media in order to achieve maximum annular placement 
and retention of prepacked filter materials. See, for example, U.S. Pat. 
Nos. 4,858,691 and 4,856,591. Such woven wire cloth retainers do not 
provide free flow comparable to the conventional rib-channel design which 
is characteristic of resistance welded screens. The wire cloth retainer, 
which is wrapped directly onto the perforated mandrel, only permits free 
flow to occur where it overlaps flow passages on the mandrel. Even in this 
instance, flow through the perforations is further restricted where the 
wire cloth retainer overlaps itself. 
The prior art sand screens which utilize fine wire woven cloth retainers 
can result in plugging due to the fact that the openings in the wire cloth 
are typically considerably smaller than the flow openings in the outer 
screen member. In U.S. Pat. No. 4,858,691, for example, the wire cloth 
fabric mesh is stated to have a mesh size of from about 40 to about 200, 
which can have a substantially smaller inlet flow area than the inlet flow 
area of the outer particulate restricting cylinder. It will be appreciated 
that sand plugging can interfere with the initial development phase of 
production in wells which are completed by wire cloth fabric mesh screens 
of the type described in U.S. Pat. No. 4,858,691. 
A special clearance prepack well screen as shown in U.S. Pat. No. 5,004,049 
provides an outer wire wrap screen slightly larger than an inner wire wrap 
resistance welded screen. The space in between, referred to as the "micro" 
annulus, is filled with an aggregate filter material. In such cases where 
this annular space is filled with loose (or non-consolidated) material, 
the aggregate is usually silica sand, glass beads, sintered bauxite, or 
nickel shot. Often times, these aggregate materials are intermixed with a 
plasticized epoxy resin in order to consolidate the loose material. 
Consolidation techniques are preferable to packing the annulus loosely due 
to the tendency of loosely packed material to settle and eventually give 
rise to a bypass "channeling" effect when subjected to differential 
pressures. 
The packing procedure for conventional special clearance prepack screens 
utilize vibration and gravity. This type of procedure fails to place 
aggregate material in a stressed condition, therefore allowing for a 
channeling failure to occur downhole. This eventually leads to an erosive 
cut leakage path through the screen and generally yields a catastrophic 
failure. Since these dual screen prepacks are intended to be used in 
conjunction with a gravel pack completion (as primary sand control), they 
are therefore utilized as an "insurance" factor in the case of an 
insufficient gravel pack. Epoxy bond aggregate substantially reduces the 
channeling effect. However, bending stresses (as expected in shipping, 
deviated well bores, rough handling) can cause cracking in the bonded 
material. This can lead to high entrance velocity passages which in turn 
could cause catastrophic erosion damage as is encountered with channeling 
when exposed to formation sand. Silica sand gravel is known to dissolve in 
HCL and HF acid. Epoxy resin is also highly reactive to acidic formation 
fluid. These major problems are commonly encountered in conventional well 
stimulation techniques. 
OBJECTS OF THE INVENTION 
A general object of the invention is to provide an improved well screen 
which will exclude sand fines from inflowing formation fluid during the 
initial production phase following a gravel pack operation, without 
limiting production of formation fluid. 
A related object of the present invention is to maximize the annular 
placement and retention of a fluid-porous, particulate-restricting member 
in a well screen having a maximum inner diameter and a minimum outer 
diameter. 
Another object of this invention is to provide an improved prepack well 
screen which reduces the radial thickness of prepack material without 
imposing a flow restriction or a strength compromise on the inner mandrel. 
Yet another object of the present invention is to provide an improved well 
screen having a prepack of aggregate material which is sized appropriately 
to exclude sand fines of a predetermined size, and which is porous and 
inherently stable with a mechanical strength comparable to conventional 
gravel aggregate prepack screens. 
A related object of the invention is to provide a tubular well screen 
having a prepack of stabilized aggregate material which is highly 
resistant to acid treatment and stimulation compounds, as well as high 
chloride/high temperature corrosive well conditions. 
Yet another object of the present invention is to provide an improved well 
screen which is adapted for use in well completions having a relatively 
low entrance velocity of formation fluids, for example, in horizontal 
completions. 
Still another object of the invention is to provide an improved well screen 
of the character described having a prepack of inherently stable, porous 
aggregate material which is highly resistant to cracking caused by bending 
stresses. 
A related object of the present invention is to provide an improved well 
screen of the character described having a prepack of inherently stable, 
porous aggregate material which is not subject to bypass channeling caused 
by settling of unconsolidated aggregate material. 
Another object of the present invention is to provide an improved well 
screen of inherently stable, porous aggregate material which is resistant 
to plugging by sand fines. 
Still another object of the present invention is to provide an improved 
well screen having an inherently stable prepack of porous, consolidated 
aggregate material. 
Yet another object of the present invention is to minimize the volume of 
prepack aggregate material in a well screen having a maximum inner 
diameter and a minimum outer diameter, without sacrificing the screen's 
ability to exclude sand fines of a predetermined size. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved according to one aspect of the present 
invention by a prepack well screen assembly in which the prepack is made 
in one piece entirely of sintered powdered metal which is molded to form a 
metallurgically integral rigid tubular structure. The term 
"metallurgically integral" as used herein means that the aggregate metal 
particles are bonded together by interatomic diffusion as a result of 
sintering the particles under high temperature and pressure conditions. 
This invention employs the use of a sintered porous tube body to 
effectively replace the inner retention screen (welded or woven), and the 
aggregate material (sand, epoxy, nickel shot, etc.). This material can be 
constructed of stainless steel, high nickel alloys, bronze coated with 
nickel, etc. Sintered porous steel slivers are bonded by pressing or 
molding metal powders into a flat sheet or concentric tube form. Flat 
sheets are rolled and welded (seam) to form a tube. 
The prepack well screen assembly has an outer wire wrap screen 
concentrically mounted in radially spaced relation on a perforated 
mandrel, thereby defining a prepack annulus for receiving the sintered 
metal prepack. In the outer screen, the longitudinal spacing distance 
between adjacent turns selectively excludes sand fines of a predetermined 
minimum size. The porosity of the sintered metal prepack is selected to 
pass sand fines in the range of 20 microns to 150 microns, which is the 
size of the sand particles which are produced during the early stages of 
production before consolidation of the main gravel pack. Accordingly, the 
effective inlet flow area through the sintered metal prepack screen is 
substantially greater than the effective inlet flow area through the outer 
screen in any selected zone of sand screen interface area. 
Operational features and advantages of the present invention will be 
understood by those skilled in the art upon reading the detailed 
description which follows with reference to the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the description which follows, like parts are indicated throughout the 
specification and drawings with the same reference numerals, respectively. 
The drawings are not necessarily to scale and the proportions of certain 
parts have been exaggerated to better illustrate details of the invention. 
Referring now to FIGS. 1 and 2, a well screen prepack assembly 10 is shown 
incorporated in a conventional subsurface well completion. A well bore 12 
has been reinforced by tubular casing 14 and sealed with cement 16. A 
production tubing 18 has been run inside the casing 14. The casing 14 is 
perforated by openings 20 at the depth where production fluids are to flow 
from the producing zone of the well into the production tubing 18. 
The well screen 10 is located opposite the perforations 20 in the casing as 
the production tubing 18 is run into the well, or it may be run directly 
opposite an open formation. The annulus between the production tubing and 
the casing 14 is sealed off by an upper packer 22 and a lower packer 24 to 
produce formation fluids from the producing zone only. 
Flow ports 26 are provided in the tubing string 18 below the upper packer 
22 and above the sand screen 10 through which gravel mixed with water or 
gel is injected or circulated by a service seal unit (SSU) into the 
annulus 28 between the casing 14 and the sand screen 10. To do so, a wash 
pipe 30 is run inside the tubing string 18 to spot the gravel slurry of 
water and/or gel below the sand screen 10 or around a telltale screen 32 
which is mounted below the primary sand screen 10. By providing the 
primary sand screen 10 and the lower telltale screen 32, premature gravel 
packing around the primary screen is prevented and a sand bridge is thus 
avoided at that spot. 
A polished bore nipple 34 is run between the primary screen 10 and the 
telltale screen 32 in which the wash pipe 30 is landed in sealing 
engagement in order to circulate the slurry to the telltale screen 32. In 
this way, any premature spotting of gravel is prevented. When the annulus 
28 between the casing 14 and the screen 10 is fully packed, the surface 
pumps will indicate a pressure jump, which serves to squeeze the remaining 
water and/or gel from the annulus into the formation. The slurry of gel 
and gravel is then dehydrated by the oil or gas bearing formation. At the 
same time, the perforations 20 are filled with gravel. A greater jump in 
pressure indicates the conclusion of the gravel pack operation. Finally, 
the wash pipe 30 is pulled out of the polished nipple 34 and the service 
seal unit SSU is pulled out of engagement with the packer 22 by retraction 
of a work string 36. 
The primary sand screen 10 is illustrated in detail in FIG. 2. The primary 
sand screen 10 is a prepacked assembly which includes a perforated tubular 
mandrel 38 of a predetermined length, for example, 20 feet. The tubular 
mandrel 38 is perforated by radial bore flow passages 40 which follow 
parallel spiral paths along the length of the mandrel 38. The bore flow 
passages 40 provide for fluid flow through the mandrel to the extent 
permitted by an external screen 42 and an internal prepack 44. The bore 
flow passages 40 may be arranged in any desired pattern and may vary in 
number in accordance with the area needed to accommodate the expected 
formation fluid flow through the production tubing 18. 
The perforated mandrel 38 preferably is fitted with a threaded pin 
connection 46 at its opposite ends for threaded coupling with the polished 
nipple 34 and the production tubing 18. The outer wire screen 42 is 
attached onto the mandrel 38 at opposite end portions thereof by annular 
end welds 48. 
The outer screen 42 is a fluid-porous, particulate-restricting member which 
is formed separately from the mandrel 38. In the preferred embodiment, the 
outer screen 42 has an outer screen wire 50 which is wrapped in multiple 
turns onto longitudinally extending outer ribs 52, preferably in a helical 
wrap. The turns of the outer screen wire 50 are longitudinally spaced 
apart from each other, thereby defining rectangular fluid flow apertures. 
The apertures are framed by the longitudinal ribs 52 and wire turns for 
conducting formation fluid flow while excluding sand and other 
unconsolidated formation material. 
As shown in FIG. 5, the outer screen wire 50 is typically 90 mils wide by 
140 mils tall in a generally trapezoidal cross section. The maximum 
longitudinal spacing A between adjacent turns of the outer wire wrap is 
determined by the maximum diameter of the fines which are to be excluded. 
Typically, the aperture spacing A between adjacent wire turns is 20 mils. 
This provides approximately 20 square inches of inlet flow area per linear 
foot through the outer screen 42, assuming an outside diameter of 2.97 
inches. 
The outer screen wire 50 and the outer ribs 52 are formed of stainless 
steel or other weldable material and are joined together by resistance 
welds at each crossing point of the outer screen wire 50 onto the outer 
ribs 52 so that the outer screen 42 is a unitary assembly which is 
self-supporting prior to being mounted onto the mandrel 38. The outer ribs 
52 are circumferentially spaced with respect to each other and have a 
predetermined diameter for establishing a prepack annulus 54 of an 
appropriate size for receiving the prepack sleeve 44. The longitudinal 
ribs 52 serve as spacers between the inner prepack sleeve 44 and the outer 
screen 42. 
The prepack sleeve 44 and the surrounding screen 42 must be capable of 
withstanding rough handling during transportation and run-in as well as 
extreme downhole well conditions, such as a temperature range of from 
about 50 degrees Celsius to about 300 degrees Celsius, a formation fluid 
pH of from about 6 to about 12, high formation pressure up to about 2,000 
psi, and contact with corrosive formation fluids containing sulfurous 
compounds such as hydrogen sulfide or sulphur dioxide in concentrations up 
to about 20% by weight. 
In the preferred embodiment, the inner prepack sleeve 44 is concentrically 
disposed about the tubular mandrel 38, and is concentrically disposed in 
the prepack annulus 54 inside of the outer screen 42. The inner prepack 
member 44 is thus stabilized by engagement against the tubular mandrel 38, 
and outer screen assembly 42. 
The fines which are initially produced following a gravel pack operation 
have a fairly small grain diameter, for example, 20-40 mesh. Accordingly, 
the spacing dimension A (FIG. 5) between adjacent turns of the outer 
screen wire 50 is selected to exclude sand fines which exceed 20 mesh. 
The prepack sleeve 44 is separately formed from the mandrel 38 and from the 
outer screen 42 and comprises a unitary, porous body of sintered powdered 
metal. The metal preferably is a corrosion resistant metal such as 
stainless steel or nickel and nickel chromium alloys such as are sold 
under the trademarks MONEL and INCONEL. Preferably, the sintered metal 
prepack sleeve 44 provides a matrix having a pore size of about 20-150 
microns, corresponding generally to about 10-60 mesh. 
The sintered metal prepack sleeve 44 is fabricated by an isostatic press 
technique. In the isostatic press technique, powdered metal of an 
appropriate particle length, for example, 50-1,400 microns stainless steel 
slivers, are poured into a tubular mold of the appropriate length and 
diameter. The powdered metal is then pressed within the mold at about 
65,000 psi (4,569 Kg/cm.sup.2) for twenty minutes to two hours to form a 
powdered metal sleeve. The compressed, powdered metal sleeve is then 
transferred to a sintering oven which is heated to a temperature in the 
range of 1,600-2,100 degrees F. (871-1,148 degrees Celsius) for several 
hours. After the sintering cycle has been completed, the sintered sleeve 
is allowed to cool and undergoes further processing in which it is cut to 
the desired length. 
The porous metal prepack sleeve has varying average pore size distribution 
as determined by the individual sliver sizes of metal powders used. For 
stainless steel and high nickel alloy embodiments, the particle length is 
preferably in the range from 0.001 inch to 0.006 inch. For nickel coated 
bronze, the particle length may range from 0.001 inch to 0.020 inch. After 
pressing (or forming), the porous tubes are sintered in a furnace in order 
to achieve full bonding of individual grains, leaving pore spaces. The 
result is an all metal, consolidated aggregate prepack sleeve. 
Construction of a special clearance sintered porous metal outer wire wrap 
screen is initiated with the perforated mandrel base. A perforated mandrel 
38 supports the sintered prepack sleeve 44 and outer wire wrapped screen 
42. Over the perforated mandrel 38 is placed a thin-walled (high 
permeability) sintered porous metal sleeve 44 having a sidewall thickness 
in the range of from about 0.025 inch-0.200 inch. Typically, however, the 
sintered tube has a sidewall thickness of 1/16 to 1/8 inch. Standard 
heavy-duty construction of the wire wrap 42 is employed in order to 
protect the sintered metal prepack sleeve 44. 
As a result of the foregoing sintering process, the prepack sleeve 44 is 
made entirely of sintered powdered metal which is molded to form a 
metallurgically integral rigid structure. During the heating step, the 
aggregate metal slivers are bonded together by interatomic diffusion as a 
result of sintering the particles under high temperature and pressure 
conditions. The porosity is proportional to the initial particle size and 
the isostatic pressure. Stainless steel slivers having an average length 
of 50-1,400 microns when compressed at about 60,000 psi (4,218 
Kg/cm.sup.2) and sintered at from about 1,800 degrees F. (982 degrees 
Celsius) to about 2,100 degrees F. (1,148 degrees Celsius) as set forth 
above will yield pores of about 100 microns, which corresponds with about 
40 mesh. 
The sintered metal prepack tube can be constructed in a concentric, 
seamless tubular form as shown in FIGS. 2 and 6. When the seamless, 
tubular form is used, the prepack sleeve 44 is welded at each end to the 
external surface of the mandrel 38. 
Referring now to FIGS. 3, 4, 5, 7, 8, and 9, the sintered metal prepack 
tube 44 is fabricated by pressing metal slivers into a flat sheet, 
sintering and then rolling the sheet in the form of a right circular 
cylinder. The perforated mandrel 38 is then inserted into the rolled 
cylinder 44, and the longitudinal edge portions 44A, 44B are welded 
together by a longitudinal seam weld W. 
According to one aspect of the present invention, an external surface 
portion 38A of the mandrel 38 is joined in a metallurgical union with the 
seam weld W. The rolled edge portions 44A, 44B and the mandrel surface 38A 
being welded together during a single pass of a welding tool along the gap 
G which extends between the longitudinal edges 44A, 44B of the rolled 
prepack tube 44. 
After the sintered metal prepack sleeve 44 has been welded onto the 
perforated mandrel 38, the assembly is inserted into the bore of the outer 
screen 42. The longitudinal ribs 52, the outer wrapping wire 50, and the 
sintered prepack sleeve 44 are welded together onto the lower end of the 
mandrel 38 by the annular weld 48. The sintered metal prepack member 44 is 
retained within the prepack annulus 54 by the annular weld 48, the outer 
screen 42 and the mandrel 38. After the mandrel 38 and prepack sleeve 44 
have been loaded into the prepack annulus 54, the opposite end portions of 
the outer longitudinal ribs 52, the outer screen wire turns 50 and the 
inner prepack member 44 are joined together and secured to the upper end 
of the mandrel 38 by an annular weld 48. According to this arrangement, 
the prepack sleeve 44 becomes a unitary part of the mandrel 38 and is 
ready for service. 
According to an important feature of the invention, production flow is not 
limited or blocked by localized accumulation of fines on the prepack 
sleeve 44 for the reason that the effective inlet flow area of the prepack 
sleeve 44 is substantially greater than the effective inlet flow area of 
the outer screen 42. This is made possible by selecting the porosity per 
unit area of the sintered metal prepack sleeve 4 to be substantially 
greater than the flow area provided by the outer wire 50 and longitudinal 
rib 52 of the outer screen 42. 
It is desirable to obtain a pore size yield which will permit the passage 
of sand fines in the range of 20 microns-150 microns, which is the size of 
fine sand particles which are produced during the early stages of 
production before the gravel pack finally consolidates and yields clean 
production. Accordingly, in some applications, it is desirable to increase 
the porosity to the range of 100 to 200 microns which will permit passage 
of fines in the size range of 40-60 mesh, which is the size which may 
cause plugging before the gravel pack consolidates. 
In one embodiment of the invention, the sintered metal prepack sleeve 44 is 
subjected to electropolishing to increase its porosity to yield a screen 
having an effective porosity of 40-60 mesh, with the pore size in the 
range of 100-200 microns, and with an average pore size of about 150 
microns. Electropolishing is an electrochemical process where metal is 
removed rather than deposited. In the electropolishing process, the 
sintered metal prepack sleeve 44 forms the anode in an appropriate 
electrolyte bath which, when voltage is applied, forms a polarized skin or 
film over the entire surface of the sintered metal prepack sleeve 44. The 
film is the thickest over the microdepressions and thinnest over the 
microprojections on the surface of the sintered metal prepack sleeve. 
Where the polarized film is the thinnest, the electrical resistance is the 
least and therefore the rate of metallic dissolution is the greatest. 
Accordingly, in the electropolishing step, the microscopic high points on 
the surface of the screen are selectively removed much faster than the 
microscopic valleys, thereby yielding a very flat, smooth and bright 
surface. 
An unexpected benefit of the electropolishing process is that the pore size 
is increased as sintered metal material is removed from the microscopic 
valleys. For the previous example of a sintered metal prepack sleeve 44 
made of slivers of stainless steel having a length in the range of 
50-1,400 microns and compacted at 60,000 psi (4,218 Kg/cm.sup.2), an 
initial average pore size of about 100 microns or less was achieved. 
However, after electropolishing to produce a smooth surface, the average 
pore size was increased to about 150 microns, which is equivalent to 40-60 
mesh. 
As a result of the electropolishing, corrosion resistance is improved by 
removing scratches, metal debris and embedded abrasive particles. 
Mechanical stress is removed as a result of the electropolishing step by 
removing surface damage and cold work effects from the surface skin of the 
prepack sleeve 44. Moreover, the electropolishing procedure produces a 
non-particulating surface on the sintered metal prepack sleeve by 
providing as much as 90 percent reduction in the external surface area. By 
removing most of the surface irregularities, very few nucleation sites 
remain where particles in the size range of 20 microns-150 microns can 
become captured or otherwise lodged to cause plugging. Electropolishing 
further reduces the coefficient of friction of the external surface of the 
sintered metal prepack sleeve because rough projections on the surface are 
either removed or rounded. 
The microscopic section shown in FIG. 10 illustrates the enlargement of the 
matrix pores obtained by electropolishing the sintered metal prepack 
sleeve 44. It will be noted that the edges of each particle 44A are 
rounded, and that the pore openings 44B, although irregular in shape, have 
an opening size in the range of from about 100 to about 200 microns. 
The sintered metal prepack of the present invention provides significant 
technical advantages in the petroleum production industry. The porosity of 
the sintered metal prepack sleeve is determined primarily by choice of 
metal particle size and can be increased as required by electropolishing 
to exclude sand fines which may cause plugging, and is inherently stable 
with a mechanical strength comparable to conventional wire wound screens. 
Screen surface irregularities are removed by electropolishing, thereby 
reducing the number of nucleation sites where sand fines would otherwise 
be captured to cause plugging. 
The sintered metal material of the prepack sleeve 44 is made of corrosion 
resistant alloy metal slivers which are resistant to acid treatment and 
stimulation compounds, as well as high chloride/high temperature well 
conditions. Because the sintered metal prepack sleeve can be fabricated in 
continuous lengths of 20-30 feet (6-9 meters) or more, it has a relatively 
large inflow surface area which is particularly well adapted for use in 
completions having a relatively low entrance velocity of formation fluids, 
for example, in horizontal completions. 
Moreover, the prepack sleeve 44 of the present invention is constructed of 
inherently stable sintered metal slivers, and can be molded, machined, cut 
to size, welded and worked in the same manner and with the same tools as 
conventional production tubing. The sintered metal prepack sleeve can be 
fabricated to any desired length, and can be machined and worked in the 
same manner as conventional production tubing. It will be appreciated that 
the invention provides the following important advantages over 
conventional prepacks: 
1) The aggregate material of the prepack sleeve is stainless steel, or 
other corrosion resistant alloys, and is therefore highly resistant to 
corrosion caused by downhole conditioning as well as corrosive stimulation 
fluids. 
2) The sintered metal consolidation is stronger and more ductile than epoxy 
resin prepacks, and will bend without cracking, unlike epoxy 
consolidations. 
3) It is well adapted for use in downhole environments that require 
"insurance" against a failed gravel pack (i.e., voids in pack), and 
prevents the channeling effect common to prepacks made of unconsolidated 
gravel. 
4) Its high strength negates the need for annular prepack thickness, 
therefore allowing the strand diameter of the wire wrap to be reduced 
while still affording maximum erosion protection. This provides greater 
annular space for pack placement. 
5) Sintered metal bonding eliminates chemical bonding, and is more 
resistant to a high temperature environment, corrosive than silica sand or 
epoxy. 
Although the invention has been described with reference to an oil well 
completion, and with reference to particular preferred embodiments, the 
foregoing description is not intended to be construed in a limiting sense. 
Various modifications of the disclosed embodiment as well as alternative 
applications, for example, filtering unconsolidated material out of 
inflowing well fluid in water, gas and oil wells, and environmental wells, 
including monitoring wells, recovery wells and disposal wells, in 
horizontal as well as vertical completions, will be suggested to persons 
skilled in the art by the foregoing specification and illustrations. It is 
therefore contemplated that the appended claims will cover any such 
modifications or embodiments that fall within the true scope of the 
invention.