Understanding the factors that go into selecting the proper screen.
By Thom Hanna, PG
There are two important factors that need to be looked at for selecting a screening device for a well:
- The open area needs to be considered for proper well development and the completion of an efficient well.
- The strength of the screen needs to be considered for the depth, type of completion, and borehole conditions.
What do we need to think about when we design a well, and looking at what is the proper amount of strength needed for a screen design? I have designed and overseen the completion of wells that are more than 1000 feet in depth with screens that have a collapse strength of 160 psi.
Do I recommend this? No, but it shows you can complete wells with relatively low strength to a significant depth if you are careful.
In theory, hydraulic collapse strength is not an issue as the screen has holes in it and there will be good hydraulic connections between the inside of the screen and the formation. However, there are forces due to the loading of the filter pack and formation until the well is developed. After development in an unconsolidated aquifer or filter-packed completion, the gravels settle against the screen and lock it into place.
We will discuss screening devices and how they differ.
Well Screen Types
The intake section of a well or screen is where groundwater enters the well. If you think about it, the well screen’s main purpose is to keep the filter pack in place. The screen itself is not a filtering device, although it acts as one in the development of a naturally developed well. The well screen allows water inflow, retains select material, and is the access to the aquifer for well development and future well maintenance while keeping the well open.
Figure 1. Continuous-slot screens are constructed by welding cold-rolled, triangular-shaped wire onto a circular array of longitudinal rods. The slots can be varied.
Maximizing each of the criteria listed in Table 1 is not always possible, depending on the type of screen required. Open areas of more than 30% are common for continuous-slot screens with no loss of column strength. PVC is desirable in highly corrosive waters, but the low strength of PVC might make it impractical for use in deep wells.
The type of aquifer and the chemistry of aquifer waters can be different from location to location. Screen design should accommodate the varying characteristics. Experience shows that screens having the features described as follow provide the best service in most geologic conditions.
- Continuous slot openings around the screen circumference permit maximum accessibility to the aquifer for more efficient development.
- Slot openings that provide maximum open area consistent with strength requirements take advantage of the aquifer hydraulic conductivity.
- V-shaped slot or pinch-point openings reduce clogging and optimize sand control.
- Metal screens must be selected for the intended service life of the well with the water chemistry in mind.
- Screens must withstand stresses normally encountered during and after installation.
- Using a wide variety of slot openings (from 0.005-inch to 0.250-inch) and appropriate filter pack size can be effective.
Common Types of Steel Well Screens
The most common well screen materials are stainless steel, low-carbon steel, galvanized steel, plastic, and fiberglass. As more wells are completed in more aggressive water environments, duplex 2205 is being used in wells used along coastal aquifers and lithium mining.
Figure 2. Continuous-slot screens have V-shaped openings, and the slots are non-clogging because their openings widen inwardly. Particles that can pass through the narrow outside opening can enter the screen (Sterrett 2007).
As with casing, strength and corrosion-resistance requirements must be satisfactory for the well’s depth and water chemistry. The material selected should enable the screen to have the desired flow properties. The most common types of steel well screening devices are wire-wrap, slotted steel, louver, bridge slot, and pipe-based screens.
Continuous-Slot Wire-Wrap Screen
Continuous-slot wire-wrap screens are used throughout the world for water, oil, and gas wells, and for environmental well completions. It is the most common screen type used in the water well industry.
Wire-wrap screens are produced by wrapping and welding cold-rolled triangular-shaped wire around a circular array of longitudinal rods. This produces a rigid unit that has high strength and minimum weight.
Slot openings result from the spacing of successive wraps of the outer layers of wire. Slot sizes typically range from 0.006-inch to 0.250-inch, and manufacturers can keep the sizes to close tolerance. Slots can be varied on a single screen (Figure 1) and this is often used as tight-wind casing for a sump or riser pipe on telescoping completions.
Figure 3. Bridge-slot screen.
The slot openings number designates the corresponding width of the openings in thousandths of an inch. For example, a 10 slot is an opening of 0.010-inch.
The V-shaped openings—designed to be non-clogging—are narrowest at the outer face and widen inwardly. Thus, oversized particles are retained outside the screen, and sand grains that pass through the opening enter the screen without becoming wedged in the slot. In screens with cut slots, the entering particles can turn or twist and become lodged in the slots, which can reduce the available intake area considerably and cause either lower yield or greater drawdown (Figure 2).
Shutter screen, also known as louver screen, is manufactured by forming downward facing louver-shaped openings into pipe (Roscoe Moss Co. 1990). Shutter screens have slots that are perpendicular to the pipe axis and are arranged in rows.
The slots are mechanically punched in the wall of the pipe. The punching process could harden the metal around the perforations, making it more susceptible to stress corrosion cracking. The slotting process (a hydraulic punch against a die) limits the number of slot sizes. In practice, slots finer than 0.040-inch are not practical or easy to consistently produce.
Shutter screens have a pinch point at the louver opening that helps prevent clogging that can occur in a slotted pipe. Shutter screens, like pipe-base or mill-slot designs, offer high tensile and collapse strength as compared to wire-wrap screens. The percentage of open area in louvered screens is limited because considerable blank space must be left between openings. Open areas typically are less than 8%. Screens commonly are made in 20-foot to 40-foot lengths and threaded or welding-collar connections are common. Louvered screens are available in low-carbon and stainless steels.
Shutter screens are used primarily in the southwestern United States in fractured rock and coarse alluvial aquifers with filter pack or stabilizer gravel.
Figure 4. Mill-slot screen.
Bridge-slot screen openings are arranged in rows (Figure 3) running parallel to the screen axis. Like louvers, slots are punched in the pipe wall (or in a flat sheet which then is rolled into a pipe). The limits of the punch and die equipment determine the size and width of the openings created.
The shape of the bridge-slot screen limits its use in naturally developed wells if the aquifer material contains appreciable amounts of sand and other fine-grained materials. During well development the bridge-slot openings block easily; they also are not well suited for jetting procedures. The application of bridge-slot screens primarily is in filter-packed wells.
Open areas of bridge-slot screens typically are less than 10%. The open areas are limited because adequate blank spaces must be left between slot openings of the screens. Most bridge-slot screens are composed of mild steel and stainless steel, and commonly are made in 5-foot to 20-foot lengths that are welded together, but threaded connections are available.
Pipe-based screens are used in water wells in many parts of the world and are common in oil field work because of their strength and durability in non-vertical (deviated) borehole applications.
Pipe-based screens are made by perforating a base pipe and then mounting a continuous-slot screen over the base pipe. Commonly, the screen is slipped on over the pipe but there are methods for warping the screen on the base pipe as well.
The pipe-based screen has two sets of openings: the outer continuous slot and the holes drilled in the pipe base. The open area of the pipe base generally is less than that of the outer screen open area, and the inner pipe has a maximum open area of 10%. Hydraulic performance of a pipe-based screen depends on the open area in the base pipe, which usually is made of steel and has an outer screen made of stainless steel.
This bimetal contact typically supports some electrolytic (galvanic) action and causes corrosion of the base pipe. Pipe-based screens can be made with a stainless steel base pipe; this will reduce the corrosion issue and reduce the amount of iron that could be shed and introduced into the distribution system.
Slotted-steel pipe is produced using saws (Figure 4) or cutting torches. Mill-slot pipe has poor corrosion resistance, and the perforation methods tend to hasten corrosive attack on the metal when used in poor-quality water. Jagged edges and slot surfaces also are susceptible to selective corrosion.
In general, using slotted-steel pipe limits effective development, increases maintenance costs, and can significantly reduce the life of the well. The open area of mill-slot screens typically is less than 4%. Mill-slot screens are primarily used in agriculture wells.
Strength Requirements for Well Screens
There are three primary strength considerations for well screens. They are tensile strength or the amount of weight the top of the screen can hold; collapse strength or the differential pressure that can be places on the screen during completion and development; and column strength, which is important if the screen is set on the bottom of the borehole. Typically, casing and screen assemblies are held in tension during completion in deep-well applications.
When designing and completing wells, the screen must support the hanging weight of the casing and screen below that joint. The total forces on the screen are the weight of the screen, casing, and drag load from filter packing.
The tensional strength requirement for pipe and screens is a function of the hanging weight of the completion string, but there needs to be additional strength for forces due to filter packing and forces that might be applied to the screen in the event the screen is lowered, then stopped quickly, adding additional forces due to momentum on the string.
Besides the hanging weight, there are additional transient vertical stresses frequently imposed on the casing string. These can be caused by such things as successively lowering and stopping the casing string during installation, occasionally pulling upward on stuck pipe, and the downward frictional forces imparted to the completion string during filter packing and well development. These latter stresses are not readily quantifiable, and therefore, experience and empirical observation must be relied on to guide the design process.
Filter-packing exerts lateral (collapse) and axial (drag) forces on the screen. Field reports indicate that additional load factors—equal to 10% to 25% of the total casing and screen assembly weight—have been measured by weight indicators on the rig floor during filter pack placement.
The loads result from the weight of the pack being distributed by gravity and frictional resistance—among the pack grains—between the borehole wall and the casing or screen surface (Sterett 2007). These additional loads decline when well development is completed, and the filter pack is consolidated against the formation and well screen.
Unlike hydrostatic pressure (which increases proportionately with depth), the lateral grain-column stresses are not isotropic, and pressure is almost independent of grain-column length. It is influenced more by the hole-to-pipe ratio.
It is advisable that the vertical yield strength of a given section of casing or pipe base well screen be at least two times the calculated hanging weight of the underlying materials. The ultimate tensile strength properties of construction materials provide some additional safety factor to prevent parting of the casing string under adverse conditions.
For Type 304 stainless steel, for example, the completion design is based on the yield strength of the material: 30,000 psi for fully annealed Type 304 stainless steel. This is the stress required to permanently deform the metal. The ultimate tensile strength of 60,000 psi provides an additional safety factor against separation failure of the metal.
Collapse or Horizontal Strength
In theory, water well screens do not need significant collapse strengths to prevent hydraulic collapse if the fluid pressure on the inside of the screen and outside of the screen are equal. Screens have holes in them, and they have been set to great depths by maintaining equal fluid pressure inside and outside the screen.
Some mining projects have used screens with less than 160 psi collapse to depths of more than 1500 feet. Well screens, especially those with a high percentage of open area, are not as susceptible to collapse because they leak or allow the fluid pressure to equilibrate.
The water well industry tends to focus on hydraulic collapse as these forces are well known and easy to calculate. However, there are other forces that can be transferred to the screen from the formation and gravel pack. There are both dynamic forces that can cause failure and static forces that can typically be mitigated.
Overburden pressure that can act on a screen can be a combination of the weight of the overburden and its fluid content (Roscoe Moss 1990). Generally, a formation will support itself in a borehole, but at times there can be zones within the formation that are not supported, and depending on the drilling fluids used to drill the borehole and the type of formation (e.g., gravels, fractured rock), the formation can deteriorate and unravel causing dynamic forces that can be exerted on the screen.
These situations are uncommon but are typically thought of as slippage of the gravel pack. Therefore, it is important to place the filter pack with a tremie pipe to prevent bridging and begin development slowly, as not to cause a large pursuer differential into the drilling fluids and are removed and the filter pack is settled around the screen.
A screen’s resistance to column loading is directly proportional to the yield strength of the material used to fabricate the screen. Column strength, like tensile strength, is supplied by the cross-sectional area of the screen wall. For louvered, bridge-slot, or mill-slot screens, the cross-sectional area of the non-slotted portion of the pipe supplies the column support. For continuous-slot screen, the cross-sectional area of the vertical rods supports the load.
If the screen is not aligned with the casing above it, a severe reduction in column strength occurs. This problem easily can occur where an oversized borehole is drilled to accommodate a filter pack.
To avoid this situation, the use of centering guides is recommended (if the screen is more than 20 feet long), and guides should be installed at least every 40 feet. Column strength is important only until the well is developed or gravel packed; thereafter the surrounding packing stabilizes the screen. In deeper applications, it is not recommended to land the screen assembly on the bottom of the borehole and maintained in tension during completion.
Forces can be encountered in unconsolidated aquifers due to compaction, and repacking of the formation adjacent to the borehole might result in the formation not supporting the overburden, and movement of the formation can result in a compressive force on the screens as was mentioned.
Empirical Method for Estimating Safe Depth of a Wire-Wrap Screen
An empirical method was developed many years ago to establish a depth rating for a given screen design, including wire size, diameter, and slot. The approach is illustrated by the following equation (Sterrett 2007).
where: SD = safe depth (ft or m);
The expression is considered applicable for D/t values greater than 35, which includes most large-diameter designs for high-capacity wells. Based on several thousand screen applications occurring over the past 15 years, a guideline table of minimum collapse values was developed (Table 2).
The values represented in the table provide a quick means to perform a practical estimation of minimum collapse to be considered for a given situation, and further evaluation of a final collapse design and costs can be simplified.
Table 3 provides representative open areas for different screening devices. One of the biggest benefits of continuous slot openings is improved development.
Water flows more freely through a high open area than a low open area because entrance velocity is reduced, and head loss is minimized. This reduces drawdown for a given pumping rate, lowering chemical precipitation tendencies by creating a reduced pressure differential around the screen intake.
There are many types of screening devices, and it can be important to select the proper screen for your project. Manufacturers are good resources to consult for screen design and technical information. Types of steels will be discussed in a future column, but make sure the type of screen and alloy you use is a good value for the well owner.
On a sad note, Mike Mehmert passed away last fall. He was a mentor and colleague with whom I enjoyed his company. He was a coauthor of Groundwater & Wells, Third Edition, and a co-author of some of these columns. I worked on several projects with him since he retired full time from Johnson Screens. He will truly be missed.
Roscoe Moss Co. 1990. Handbook of Groundwater Development. John Wiley & Sons: New York.
Schafer, D. 2002. Strength requirements and characteristics of pipe and well screen for deep water well applications. Report prepared for Los Alamos National Labs, Hydrogeologic Characterization Program. Los Alamos National Labs: Los Alamos New Mexico.
Sterrett, R.J. 2007. Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.
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Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at email@example.com.