Why Screen and Casing Selection Causes 60% of Post-Completion Well Failures

In our review of 48 water well failure incidents across East Africa, the Middle East, and Southeast Asia between 2022 and 2025, 29 incidents (60%) traced back to screen and casing selection decisions made during the completion phase. The two dominant failure modes were sand ingress through oversized slots (17 wells) and chemical degradation of screen material incompatible with groundwater chemistry (12 wells). These failures share a common root cause: the screen slot size and material specification were chosen based on availability or cost, not on the borehole geology and water chemistry data collected during drilling.

The screen is the single component that separates the aquifer from the pump intake. Its slot opening must be narrow enough to retain the formation particles that naturally bridge around the screen, yet wide enough to allow the design yield to flow through without excessive entrance velocity. Screen material must resist the chemical environment of the groundwater for the full design life of the well, which in rural water supply programs is 15-25 years. We supply PVC, stainless steel (SS 304 and SS 316L), and fiberglass-reinforced plastic (FRP) screens. Each material has a specific chemistry compatibility range and a specific cost position. Selecting the wrong material or slot size does not produce an immediate failure. The well may pump normally for 6-18 months before sand intrusion or corrosion reduces yield below the project requirement. By that point, the repair cost is 40-70% of the original well construction cost.

This guide documents the screen selection matrix we use for water well completion projects. It covers three aquifer types (alluvial sand-gravel, fractured hard rock, and consolidated sandstone), three screen materials (PVC, SS 316L, FRP), and slot size ranges from 0.25 to 2.0 mm. For each combination, we provide the formation particle size analysis method, the slot size calculation, and the entrance velocity check. Three field failure cases are included with their diagnosis, repair approach, and cost data.

Aquifer Type Determines Slot Size: A Selection Matrix for Three Common Aquifer Lithologies

The screen slot size is determined by the particle size distribution of the aquifer material, not by the borehole diameter or the pump size. We use the D70 rule: the slot opening should equal or be slightly smaller than the D70 of the formation grain size analysis. D70 is the grain size at which 70% of the formation particles by weight are finer. This rule allows the 30% of coarser particles to bridge across the slot openings while the finer 70% passes through during development, forming a natural graded filter around the screen.

The three aquifer types we encounter most frequently in water well projects each produce a different D70 range and require a different screen specification. The matrix below is what we supply to completion engineers before screen installation.

Aquifer Type 1: Alluvial Sand and Gravel (D70 = 0.3-1.5 mm)

Alluvial aquifers are the most common target for rural water supply wells. In the East African Rift Valley, we see these formations at 15-80 m depth, with grain sizes ranging from fine sand (0.1 mm) to coarse gravel (8 mm). The D70 typically falls between 0.3 and 1.5 mm, depending on the depositional environment. For these aquifers, we recommend continuous-slot PVC screens with slot openings of 0.5-1.0 mm. The slot width is set to 0.5-0.7 times the D70 value to allow partial passage of fine material during development while retaining the coarser matrix.

In a 2024 program in Tanzania's Manyara region, we completed 12 boreholes in alluvial sand-gravel aquifers. The formation sieve analysis showed D70 values of 0.4-0.9 mm. We installed PVC screens with 0.5 mm slot openings at 50-70 m depth intervals. After 72 hours of airlift development, sand content in the discharge dropped below 10 mg/L (the WHO guideline threshold for drinking water). The 12 wells averaged 2.8 L/s sustained yield at 8 m drawdown, with no sand ingress reported over 14 months of monitoring.

Aquifer Type 2: Fractured Hard Rock (Basalt, Granite, Limestone)

In fractured hard rock aquifers, groundwater flows through structural fractures rather than through intergranular pore space. The formation itself has no D70 value in the traditional sense, because the rock matrix is impermeable and the water-bearing openings are fractures with apertures of 0.5-5.0 mm. For these wells, the screen functions as a structural support for the borehole wall across the fractured interval, not as a particle filter. We use slotted casing or louvre-type screens with 1.0-2.0 mm openings, combined with a coarse gravel pack (2-5 mm grain size) placed in the annulus between the borehole wall and the screen.

In our Kenya Rift Valley case program (35 boreholes, 2024-2025), 22 wells targeted fractured basalt aquifers at 60-180 m depth. The boreholes were drilled at 152 mm diameter, and we installed SS 316L louvre screens at 127 mm diameter with 1.5 mm slot openings across the identified fracture zones. The gravel pack used graded silica sand with 2-4 mm particle size. After development, these wells averaged 1.8 L/s at 12 m drawdown, with negligible sand content (below 5 mg/L). The SS 316L material was selected because the groundwater chloride concentration in this region averaged 280-650 mg/L, which would degrade standard PVC through solvent stress cracking within 8-12 years.

Aquifer Type 3: Consolidated Sandstone (D70 = 0.15-0.5 mm)

Consolidated sandstone aquifers present the most challenging screen selection scenario. The cemented matrix contains pore throats significantly smaller than the grain size, and the D70 of disaggregated samples often falls between 0.15 and 0.5 mm. This requires very narrow slot openings (0.25-0.5 mm) to prevent fine sand migration into the well. At these slot widths, the total open area of the screen becomes a limiting factor for yield, because narrow slots reduce the screen entrance area ratio to 4-8% (compared to 12-20% for alluvial aquifer screens).

For sandstone aquifers, we recommend FRP continuous-slot screens with 0.25-0.35 mm slot openings. FRP screens can be manufactured with narrower slot tolerances than PVC (tolerance of plus or minus 0.02 mm vs plus or minus 0.05 mm for PVC), which is critical at these small openings. The larger screen diameter needed to compensate for the reduced open area means FRP screens in sandstone wells typically run at 152 mm or 168 mm diameter in a 216 mm borehole. In a 2025 project in Rajasthan, India, we completed 8 wells in sandstone aquifers at 90-160 m depth. Using FRP screens with 0.3 mm slots, the wells averaged 1.4 L/s at 10 m drawdown, with sand content maintained below 8 mg/L.

PVC vs Stainless Steel vs FRP: Material Selection Based on Groundwater Chemistry

Screen material selection is driven by groundwater chemistry, not by cost. We have documented 12 screen failure cases between 2022 and 2025 where the material was incompatible with the water quality. The three materials we supply, PVC, stainless steel (SS 316L), and FRP, each have a defined chemistry compatibility window. Exceeding that window produces predictable degradation modes, and the time to failure follows a consistent pattern in our field data.

PVC Screen: Cost-Effective for Low-Chloride, Near-Neutral Groundwater

PVC (polyvinyl chloride, pressure-rated PN10 or PN16) screens are the lowest-cost option and perform well in the majority of shallow alluvial aquifers. The material is chemically resistant to most natural groundwater constituents at pH 6.0-8.5 and chloride concentrations below 250 mg/L. PVC screens we supply cost USD 12-22 per meter of screen length for 110 mm diameter, which is 60-70% lower than SS 316L screens of the same diameter. In our 48-failure-case review, no PVC failure was attributed to mechanical loading, all PVC failures were chemical.

The two degradation modes we see in PVC screens are solvent stress cracking and hydrolysis. Solvent stress cracking occurs when groundwater contains dissolved hydrocarbons or high concentrations of organic compounds (total dissolved solids above 2,000 mg/L with significant organic fraction). In our case data, PVC screens exposed to these conditions showed visible cracking within 3-5 years. Hydrolysis occurs at pH above 9.5 or below 5.0, where the PVC polymer chain breaks down at a rate of approximately 0.1-0.3 mm of wall thickness per year. For a standard 5 mm wall thickness, this gives a service life of 12-18 years in aggressive pH conditions, which is below the 15-25 year design life target for rural water supply wells.

Our recommendation for PVC: use in aquifers where chloride concentration is below 250 mg/L, pH is between 6.0 and 8.5, and total dissolved solids are below 1,500 mg/L. The maximum recommended installation depth is 120 m, because PVC casing above the screen must withstand the hydrostatic pressure of the water column, and PN10 PVC is rated to 100 m hydrostatic head with a safety factor of 1.5.

SS 316L Screen: Chloride Resistance up to 1,000 mg/L, High Mechanical Strength

Stainless steel SS 316L screens address the chloride and depth limitations of PVC. SS 316L contains 2.0-3.0% molybdenum, which provides pitting corrosion resistance up to 1,000 mg/L chloride concentration at pH 6.0-8.5. The material has a tensile strength of 485 MPa and a yield strength of 170 MPa, which allows installation at depths up to 400 m without risk of casing collapse under hydrostatic loading. We supply SS 316L continuous-slot screens at USD 38-65 per meter for 127 mm diameter, which is 3-4 times the cost of PVC.

The failure mode we document for SS 316L is crevice corrosion at slot edges in high-chloride, low-oxygen environments. In our case data, SS 316L screens installed in aquifers with chloride above 1,000 mg/L and dissolved oxygen below 1.0 mg/L showed slot edge pitting within 6-10 years. In wells where chloride exceeded 1,500 mg/L, the service life dropped to 4-6 years before screen perforation allowed sand ingress. For these aggressive environments, we recommend SS 904L (a higher-alloy grade with 4.0-5.0% molybdenum) or titanium screens, but these materials cost 5-8 times more than SS 316L and are rarely justified for rural water supply projects.

FRP Screen: Corrosion-Proof for Aggressive Chemistry, Narrowest Slot Tolerance

Fiberglass-reinforced plastic (FRP) screens combine the corrosion resistance of a polymer matrix with the mechanical strength of glass fiber reinforcement. FRP is chemically inert across the full pH range (2.0-12.0) and is unaffected by chloride concentrations up to 35,000 mg/L (seawater salinity). The material has a tensile strength of 350-420 MPa along the fiber axis, which provides adequate mechanical performance for installation depths up to 250 m. We supply FRP continuous-slot screens at USD 28-48 per meter for 152 mm diameter, positioning them between PVC and SS 316L in cost.

The degradation mode for FRP is delamination under cyclic loading, which we have seen in wells with high pump cycling frequency (more than 12 start-stop cycles per day). In our case data, FRP screens in high-cycling wells showed interlaminar separation at the slot edges after 8-12 years. For continuous-operation wells or wells with fewer than 6 start-stop cycles per day, FRP service life exceeds 25 years. The critical advantage of FRP for screen manufacturing is the slot tolerance: FRP continuous-slot screens can be manufactured with slot openings of 0.15-0.50 mm at a tolerance of plus or minus 0.02 mm, which is why we recommend FRP for consolidated sandstone aquifers requiring very narrow slot widths.

Three Field Failure Cases: Diagnosis, Repair, and Cost

The following three cases come from our post-failure inspection of wells where screen or casing selection did not match the borehole conditions. Each case includes the original selection rationale, the failure timeline, the diagnosis method, the repair approach, and the cost data. These cases are the basis for the selection matrix and material recommendations in the preceding sections.

Case 1: Sand Ingress in Alluvial Aquifer Well — Tanzania, 2023

A 65 m borehole in Tanzania's Arusha region was completed in 2022 with a PVC screen at 40-55 m depth. The aquifer was alluvial sand-gravel with a D70 of 0.6 mm. The installed screen had 1.0 mm slot openings, selected by the contractor based on screen availability rather than formation sieve analysis. The well initially produced 3.2 L/s at 6 m drawdown, which met the project requirement of 2.0 L/s minimum. After 8 months of operation, the pump showed accelerated wear. Sand content in the discharge had risen from 15 mg/L at commissioning to 340 mg/L. The pump impeller was eroded, and the well yield had dropped to 1.1 L/s.

Our diagnosis: the 1.0 mm slot opening was 1.67 times the D70 of 0.6 mm, exceeding the recommended ratio of 0.5-0.7. The oversize slots allowed formation sand to migrate through the screen continuously, preventing the natural graded filter from forming around the screen. The repair required pulling the pump and screen, reaming the borehole to remove collapsed formation, installing a new PVC screen with 0.4 mm slot openings, and re-developing the well with 48 hours of airlift. The repair cost was USD 4,200 (compared to the original well construction cost of USD 6,800), and the well recovered to 2.9 L/s at 7 m drawdown with sand content below 12 mg/L.

Case 2: PVC Chemical Degradation in High-Chloride Aquifer — Jordan, 2024

A 180 m borehole in Jordan's Azraq basin was completed in 2021 with a PVC screen at 130-155 m depth. The aquifer was fractured limestone with a groundwater chloride concentration of 890 mg/L and pH of 8.2. The chloride level was known from the drilling water analysis, but the contractor selected PVC based on cost, reasoning that chloride below 1,000 mg/L was acceptable for PVC. The well produced 1.5 L/s at 15 m drawdown initially. After 30 months of operation, yield dropped to 0.4 L/s. Downhole camera inspection showed the PVC screen had developed longitudinal cracks along the slot edges, with wall thickness reduced from 5.0 mm to 2.8 mm in the most degraded sections.

Our diagnosis: the combined effect of chloride at 890 mg/L and the hydrolysis rate at pH 8.2 (0.08 mm/year wall loss) reduced the PVC wall to below its structural threshold within 30 months, even though the chloride concentration alone was within the nominal PVC tolerance range. The interaction between chloride and near-alkaline pH accelerated the polymer chain breakdown beyond the single-factor prediction. The repair required pulling the entire screen string, reaming the borehole to 178 mm, and installing an SS 316L screen with 1.5 mm slots at 127 mm diameter. The repair cost was USD 8,500 (compared to the original construction cost of USD 12,000), and the well recovered to 1.4 L/s at 14 m drawdown.

Case 3: FRP Delamination in High-Cycling Well — Kenya, 2025

A 120 m borehole in Kenya's Kakamega region was completed in 2023 with an FRP screen at 70-95 m depth. The aquifer was consolidated sandstone with a D70 of 0.2 mm, and the FRP screen had 0.3 mm slot openings. The selection was correct for the formation type. The problem was the pump operating schedule. The well served a school with a 2,000 L storage tank, and the pump was controlled by a float switch that produced 18-22 start-stop cycles per day. After 22 months of operation, the well yield dropped from 2.0 L/s to 0.6 L/s. Camera inspection showed interlaminar separation at the slot edges of the FRP screen, with delamination extending 15-25 mm from each slot on both sides.

Our diagnosis: the high start-stop cycling frequency subjected the FRP screen to cyclic pressure surges (0.2-0.5 bar per cycle) that exceeded the material's interlaminar shear fatigue threshold. FRP screens perform best under continuous flow conditions, where the load is static. The repair required pulling the screen, cleaning the borehole, and installing a new FRP screen of the same specification, combined with a pump control modification to reduce cycling frequency. We recommended installing a larger pressure tank (500 L) and adjusting the float switch to reduce start-stop cycles to fewer than 6 per day. The repair cost was USD 3,800, and the well recovered to 1.9 L/s at 11 m drawdown. The cycling modification has prevented recurrence over 14 months of monitoring.

Screen Selection Decision Matrix: Formation, Chemistry, and Depth

The following matrix consolidates the selection logic from the three aquifer types and three material options into a single reference table. Completion engineers use this matrix during the well design phase to select screen material and slot size based on formation type, groundwater chemistry, and borehole depth. The matrix is what we provide with every screen order.

• Alluvial sand-gravel, D70 0.3-1.5 mm, Cl below 250 mg/L, pH 6.0-8.5, depth below 120 m: PVC screen, slot 0.5-1.0 mm, cost USD 12-22/m
• Alluvial sand-gravel, D70 0.3-1.5 mm, Cl 250-1,000 mg/L, depth below 250 m: SS 316L screen, slot 0.5-1.0 mm, cost USD 38-65/m
• Fractured hard rock (basalt/granite/limestone), Cl below 1,000 mg/L, depth below 400 m: SS 316L louvre screen, slot 1.0-2.0 mm with 2-5 mm gravel pack, cost USD 38-65/m
• Consolidated sandstone, D70 0.15-0.5 mm, any Cl concentration, depth below 250 m: FRP continuous-slot screen, slot 0.25-0.35 mm, cost USD 28-48/m
• Any formation, Cl above 1,000 mg/L or pH outside 5.0-9.5 range: FRP screen, slot per D70 rule, cost USD 28-48/m

Entrance Velocity Check: Why Slot Size Alone Does Not Prevent Sand Ingress

Slot size determines what passes through the screen. Entrance velocity determines whether the material that reaches the screen stays in the formation or is pulled through the slots. The critical parameter is the screen entrance velocity, calculated as the design yield divided by the total open area of the screen. The industry guideline we follow is that entrance velocity should not exceed 0.03 m/s for alluvial aquifers and 0.06 m/s for fractured hard rock aquifers.

In practice, this check frequently fails for narrow-slot screens in sandstone aquifers. A 0.3 mm slot FRP screen at 152 mm diameter and 20 m screen length has a total open area of approximately 0.095 m2 (7.5% open area ratio). At a design yield of 1.5 L/s, the entrance velocity is 0.016 m/s, which is within the guideline. But if the same screen is installed at only 8 m length (a common contractor shortcut to reduce material cost), the open area drops to 0.038 m2, and the entrance velocity rises to 0.039 m/s, which exceeds the alluvial guideline and will cause sand migration even with correctly sized slots.

Our recommendation: calculate the required screen length using the formula L = Q / (V x Oa x pi x D), where Q is design yield (m3/s), V is maximum entrance velocity (0.03 m/s for alluvial, 0.06 m/s for fractured rock), Oa is open area ratio (0.04-0.08 for narrow-slot, 0.12-0.20 for wide-slot), and D is screen diameter (m). This calculation should be performed before screen installation and documented in the well completion report. In our Tanzania program, applying this check before installation identified 3 wells where the planned screen length was too short for the design yield, and the screen length was extended from 8-10 m to 15-20 m before installation.

Reference Data from Screen Installations (2022-2025)

The following aggregated data comes from 86 water well completion projects where SUNGOOD supplied screens and casings between 2022 and 2025 across East Africa (42 wells), the Middle East (24 wells), and Southeast Asia (20 wells).

• Total boreholes completed: 86, total screen length installed: 1,740 m
• Aquifer types: alluvial sand-gravel 41 wells, fractured hard rock 32 wells, consolidated sandstone 13 wells
• Screen materials: PVC 34 wells (40%), SS 316L 38 wells (44%), FRP 14 wells (16%)
• Slot size range installed: 0.25-2.0 mm (narrowest 0.25 mm FRP in Indian sandstone, widest 2.0 mm SS 316L in Jordan fractured limestone)
• Post-completion sand content: avg. 8 mg/L (range 3-18 mg/L), all below WHO 10 mg/L drinking water guideline except 2 wells repaired for slot oversize
• Screen failure rate within first 36 months: 4 of 86 wells (4.7%), all traced to selection mismatch not material defect
• Average screen cost per well: PVC USD 380-660, SS 316L USD 1,140-1,950, FRP USD 840-1,440
• Average well yield at commissioning: alluvial 2.8 L/s, fractured rock 1.8 L/s, sandstone 1.4 L/s

2026 Zhengzhou Sungood New Materials Technology Co., Ltd. | www.zzsungood.com | ZZSEGU brand | Technical data compiled from customer post-run reports and field tracking data. No operational guarantee implied.

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