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Slotted liners and wire wrapped screens
The slotted liner or screen is the mechanical device that contains the gravel-pack sand in an annular ring between it and the casing wall or open hole.
Function of slotted liner or screen
Fig. 1 shows a schematic of the function of a slotted liner or screen in an openhole gravel pack.
Fig. 1—Openhole gravel-pack schematic.
Slotted liners are made from tubulars by saw-cutting slot configurations, as shown in Fig. 2. Slot widths are often referred to in terms of gauge. Slot or screen gauge is simply the width of the opening in inches multiplied by 1,000. For instance, a 12-gauge screen has openings of 0.012 in.
The machining consists of cutting rectangular openings with small rotary saws. Routine slot widths are 0.030 in. or larger. The minimum slot width that can be achieved is about 0.012 in. Slots that cut less than 0.020 in. in width involve high costs because of excessive machine downtime to replace broken saw blades that overheat, warp, and break.
The single-slot staggered, longitudinal pattern is generally preferred because the strength of the unslotted pipe is preserved. The staggered pattern also gives a more uniform distribution of slots over the surface area of the pipe. The single-slot staggered pattern is slotted with an even number of rows around the pipe with a typical 6-in. longitudinal spacing of slot rows.
The slots can be straight or keystone shaped, as illustrated in Fig. 3. The keystone slot is narrower on the outside surface of the pipe than on the inside. Slots formed in this way have an inverted “V” cross-sectional area and are less prone to plugging because any particle passing through the slot at the outside diameter (OD) of the pipe will continue to flow through, rather than lodging within the slot. While the slotted liners are usually less costly than wire-wrapped screens, they have smaller inflow areas and experience higher pressure drops during production. Slotted liners also plug more readily than screens; they are used where well productivity is small and economics cannot support the use of screens.
The length of the individual slots is measured on the inside diameter (ID) of the pipe. Usual practice dictates 1½-in. long slots for slot widths of 0.030 in. and under, 2-in. long slots for slot widths between 0.030 to 0.060 in., and 2½-in. long slots for slot widths of 0.060 in. and larger. Slot width tolerance is generally ± 0.003 in. for widths of 0.040 in. and wider and ± 0.002 in. for widths less than 0.040 in.
The primary advantage of a slotted liner over wire-wrapped screens is usually cost; however, small gauge, high-density slot patterns may cost as much as wire-wrapped screens. The disadvantages of the slotted liner are:
- limited flow area (2 to 3%, creating a low tolerance to plugging)
- minimum available slot size (approximately 0.012 in.).
Slot widths that are less than 0.020 in. and cut in standard carbon steel-pipe grades can rust and will either close or reconfigure the slot opening so that they do not function properly unless they are coated, protected, or stored indoors before use.
Wire-wrapped screens offer another alternative for retaining the gravel in an annular ring between the screen and the formation. Wire-wrapped screens have substantially more inflow area than a slotted liner, as Fig. 4 illustrates. The screen consists of an outer jacket that is fabricated on special wrapping machines that resemble a lathe. The shaped wire is simultaneously wrapped and welded to longitudinal rods to form a single helical slot with any desired width. The jacket is subsequently placed over and welded at each end to a supporting pipe base (containing drilled holes) to provide structural support. This is a standard-commodity design manufactured by several companies. A schematic of the screen construction is shown in Fig. 5. Screen tolerances are typically plus 0.001 and minus 0.002 in.; hence, a specified 0.006-in. slot could vary in slot width from 0.004 to 0.007 in.
Fig. 4—Comparison of effective inlet areas (20-gauge screen).
Because these designs have been used for more than 40 years in worldwide oilfield operations, a great deal is known about the performance of wire-wrapped screens. The typical pipe-base screen fabrication consists of a grade 316L stainless steel jacket placed over a N-80 pipe base; however, other metals can be specified as required for site-specific applications. The inflow area of screens varies from about 6 to 12% (or higher), depending on the slot opening. Screens with the smallest slot openings are typically 6 gauge (0.006 in.). For large gravel, 10 to 20 mesh, screen slot openings are about 18 gauge (0.018 in.).
A version of the wire-wrapped screen is the rod-based screen that consists of the jacket only; however, rod-based screens may have additional heavier rods and a heavier wire wrap than the jackets used on pipe-base screens to provide additional strength. Rod-based screens are commonly used in shallow water-well completions that typically range from a few hundred to maybe a 1,000 ft in depth. Hence, they do not require the strength that is gained by installing the screen jacket over a pipe base. Screen diameters range from 1.5 to 7 in. in diameter (or larger). This is the diameter of the pipe base. The actual screen diameter is slightly larger (i.e., the actual OD of a 3.5-in. screen is about 4 in.).
Prepacked screens are a modification of wire-wrapped screens; they actually represent a modular gravel pack. They consist of a standard screen assembly with a layer of resin-coated gravel (consolidated) placed around it that is contained in an annular ring supported by a second screen (dual-screen prepack) or outer shroud (single-screen prepack). The resin coating is a partially cured phenolic plastic. Being dry, the resin-coated gravel can be handled like ordinary gravel. After prepacking the screen, the complete unit is heated to cure and harden the resin. The thickness of the gravel layer can be varied to meet special needs. The screens with the lowest profiles are those that contain an annular pack between the jacket and the pipe base. This screen has a thin lattice screen wrapped around it to prevent gravel from flowing through the drill holes in the pipe base before consolidation.
Examples of prepacked screens are in Fig. 6. Prepacked screens have been used with gravel packs instead of standard wire-wrapped screens and in stand-alone applications in horizontal wells. While the prepacked screens have been used in stand-alone service, experience has shown that they are highly prone to plugging, consequently restricting productivity. The inflow area of these screens is about 4 to 6% of the surface area. The exact amount depends on the slot opening and the size of the gravel.
Flow capacities of screens and slotted liners
Fig. 7 shows the pressure drop associated with commercial wire-wrapped screens. Because all have similar designs, there is little difference in performance from one manufacturer to another. These flow capacity tests were performed using water containing no plugging material. The data indicate that all screens have exceptionally high flow capacities. Flow testing with slotted liners revealed that their flow capacity was related to the slot density rather than the screen diameter. Their flow capacities are typically less than half that of wire-wrapped screens with the same diameter. Note that the flow rates were measured in increments of B/D/ft of screen. For flow rates that are typical of most wells, the pressure loss through the screen is negligible, provided that they are not plugged. Slotted liners are more easily plugged than wire-wrapped screens because the slots are usually cut parallel to each other. On the other hand, wire-wrapped screens are fabricated with keystone-shaped wire that allows a particle to pass through the screen if it can traverse the minimum restriction at the OD of the screen. The keystone design can be observed in Fig. 3.
Fig. 7—Flow capacity of 12-gauge screens with 20/40 U.S. mesh gravel.
Tensile collapse strengths of wire wrapped and prepacked screens
Tensile strength test results performed on screens and slotted liners in standard testing equipment showed that standard pipe-base screens have higher tensile ratings than rod-base screens. Testing demonstrated that yielding occurred in the pipe body as well as the coupling. As a consequence, when yielding in the connection caused a thread to separate, the test was terminated. The tensile strength of standard pipe-base screens was about twice that of the rod-base screens. For conservative designs, the tensile strength should be the lesser of 65% of the pipe body or the published joint pull-out strength.
Individual tests demonstrated collapse failures as high as 6,000 to 9,000 psi; however, this represented the simultaneous failure of the screen and the pipe base; screens have a collapse rating of about 3,500 psi, which is the rating for the jacket.
Proprietary screen designs
Proprietary designs were originally developed for stand-alone installations in horizontal wells rather than a gravel-packed completion; however, gravel-pack screen applications should not be ruled out. They are also applicable in this service. Proprietary designs are premium designs that surpass the performance of either a standard wire-wrapped screen or a prepacked screen in their ability to resist plugging and erosion and are equipped with torque-shouldered connections to permit rotation.
Because horizontal completions typically consist of a thousand to several thousand feet of completion interval, the main issue is the susceptibility of a particular design to plug with time rather than the flow capacity. These new designs have increased inflow areas to as much as 30% of the surface area of the screens. The materials used and the designs differ from conventional wire-wrapped screens. They consist of designs with the following materials:
- Dutch weave
- Porous membrane
- Sintered metal
- Corrugated weave
The logic used in these designs was that because these screens have inflow areas of 30% compared to about 5% with prepacked screens, their longevity should be extended by about a factor of six when operating under similar downhole conditions. Other issues involve the ability to run the screen without creating damage that would either prevent sand control or restrict productivity. To address this concern, most of the proprietary designs have an outer shroud to protect the screen during installation. Proprietary connections are typically used for horizontal service because of their high strength and the ability to rotate if necessary.
Sintered metal screens
The sintered metal screen design was initiated in gravel-pack use in about 1990. The design consists of placing a sintered metal sleeve that is 0.15 to 0.25 in. thick over a drilled pipe base. The sintered metal sleeve contains approximately 30% flow area. The sleeve acts as the filtration medium, while the pipe base provides tensile strength and collapse resistance. Fig. 8 is a schematic of the screen design.
Tensile strength and collapse resistance of this design should be about the same as that for wire-wrapped screens. For conservative designs, the tensile strength capabilities should be the about 65% or the lesser of either the published pipe strength or the joint pull-out of the coupling. The collapse rating should be similar to published values for wire-wrapped screens of about 3,500 psi.
Porous metal membrane screens
This screen design consists of multiple layers (3 or 4) of porous metal membrane (PMM), which contains about 30% open area through variable-sized pore openings. These are between an underlying drainage and overlying protecting mesh screen. They are placed concentrically between a drilled pipe base and a perforated outer shroud. The filter medium for the screen is sintered metal powder that is pressed against a stainless steel lattice screen to provide structural support for the filtration medium. A schematic of the screen’s construction is illustrated in Fig. 9.
Test data from the manufacturer show tensile strength testing performed to 110k lbf and a collapse test to about 7,000 psi performed on 2 7/8-in. screens, both of which reflect the strength of the pipe base. These data are similar to values for commodity wire-wrapped screens. The tensile strength rating should be less than 65% of the pipe body or connection because physical properties of the screen jacket and perforated shroud should not contribute to these properties significantly.
Shrouded multilayer screens
This screen design consists of three layers of media that form the jacket, which are placed concentrically around a drilled pipe base. The base wrap for the jacket consists of a round stainless steel wire-wrapped support that serves as a drainage layer for the overlying filtration medium. The shroud is placed concentrically over the filtration medium. See Fig. 10 for a schematic of the design.
The purpose of the base wrap or inner jacket is for support for the overlying filtration medium against high differential pressure. The wrap also promotes using the entire surface area of the filtration medium that optimizes plugging resistance. The openings in the base wrap are typically about 25 microns or larger than the filtration medium to provide secondary sand control. The filtration medium provides pore throat openings that assist in maximizing the inflow area that develops a more permeable filter cake. The design of the filtration medium, a Dutch weave, redirects the flow through it to minimize erosion and extend screen life. The design being offered is rated at a uniform pore-throat opening sizes from 110 to 230 microns. The inflow area for this design is also about 30% of the surface area of the screen. The outer shroud protects the inner filtration section during installation in the well and assists in redirecting the flow stream during production so that erosion of the filtration section is minimized. The strength rating for this screen is a tensile rating of 65% of the pipe body or the published joint pull-out strength and a jacket collapse rating of 3,500 psi.
Plugging and erosion tests on screens
Prepacked screen designs are more susceptible to plugging than other designs. This stems from their depth filter design. Standard wire-wrapped screens are a surface filter, which are not as susceptible to plugging but are more prone to erosion. Certain proprietary designs are better at resisting plugging and erosion than others. The best designs have large inflow areas and redirected flow through the screen to minimize erosion.
- Penberthy, W.L. Jr. and Shaughnessy, C.M. 1992. Sand Control, 1, 11-17. Richardson, Texas: Monograph Series, SPE.
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