How Do Well Casing and Well Screens Work?
Quick Answer
Well casing is a pipe that holds the borehole open and seals out contaminants. A well screen at the bottom allows water in while keeping sand and sediment out.
Two components you’ll never see on a completed well do the most important structural and protective work: the casing and, in sandy formations, the screen. Understanding how they work explains why well construction quality matters so much — and why shortcuts in materials or installation create problems that can last the life of the well.
The Well Casing: Structure and Protection
The well casing is the pipe that lines the borehole. In North Texas, casing is either steel (most common in older wells and hard limestone counties) or PVC (increasingly standard for most new residential wells in sandy and mixed formations).
What the Casing Does
1. Structural support. Without casing, unstable formations — clay, sand, fractured limestone — would collapse into the borehole. The casing holds the hole open so the pump can be installed and maintained.
2. Contaminant seal. The casing creates a sealed tube from the surface to the producing aquifer. Shallow groundwater (which may be contaminated by septic systems, surface runoff, or agricultural chemicals) cannot migrate into the aquifer or enter the water column — provided the annular space is properly grouted.
3. Pump housing. The submersible pump and drop pipe hang inside the casing. The pump’s diameter must fit inside the casing — a 3.5-inch pump motor in a 4-inch casing, or a 4-inch pump in a 6-inch casing.
Casing Depth and the Casing Seat
In sandy or mixed formations, casing typically extends the full drilled depth. In hard limestone (Trinity wells in Hood, Parker, Erath counties), casing is set at the top of the rock — the casing seat — and the borehole continues below as open hole. The rock holds the open-hole section without casing, and water enters through natural fractures in the limestone.
| Formation Type | Casing Extent | Open Hole? |
|---|---|---|
| Sandy (Woodbine) | Full borehole depth | No |
| Mixed limestone/sand | To top of rock | Sometimes |
| Hard limestone (Trinity) | To casing seat | Yes — below the casing seat |
Cement Grouting: The Contamination Seal
The borehole is always larger than the casing — there’s a gap (the annular space) between the outside of the casing and the borehole wall. Left unfilled, this annular space creates a vertical pathway for surface water, shallow groundwater, and contaminants to travel directly down to the aquifer.
Cement grout is pumped from the bottom of the annular space upward to fill it completely. Texas requires grouting from land surface to a minimum depth of 20 feet for all domestic wells. Most licensed drillers grout the full length of the surface casing for complete protection.
Quality grouting is one of the most important long-term factors in a well’s water quality. A well with inadequate grouting can pass initial water tests and develop contamination years later as the unsealed annular space allows migration of bacteria, nitrates, or agricultural chemicals.
The Well Screen: Keeping Sand Out
A well screen is needed when the producing formation is unconsolidated — sand, gravel, or sandy limestone — and cannot hold itself open. Without a screen, formation material flows into the casing with the water, damaging the pump and reducing well yield over time.
How a Screen Is Made
The most common screen type for residential wells is the wire-wound continuous-slot screen: a stainless steel rod base wrapped with a V-shaped wire that creates uniform slots between each wrap. Water flows freely through the slots; particles larger than the slot opening cannot pass.
Slot size is matched to the formation grain size:
- Fine sand (Woodbine typical): 0.010–0.020 inch slots
- Coarser sand/gravel: 0.040–0.060 inch slots
- Mixed gravel: Larger slots with gravel pack outside the screen
Gravel Packing
In very fine or poorly sorted sands, the well screen alone may not filter adequately. A gravel pack — a layer of carefully graded gravel — is placed between the screen and the borehole wall. The gravel pack acts as a coarse filter that removes fine particles before they reach the screen, extends screen life, and improves well yield by reducing flow resistance at the screen.
When No Screen Is Needed
Consolidated rock formations — the hard limestone of the Trinity aquifer in Hood, Erath, Parker, and Palo Pinto counties — hold the borehole open naturally. Water enters through fractures in the rock. No screen is installed; the open-hole section from the casing seat to total depth functions as the producing interval. Drill cuttings and initial fines are cleared during well development, after which the production water is typically clear.
The Wellhead Assembly
Above grade, the casing terminates in a wellhead assembly that includes:
- Well cap: Threaded or bolted cover over the top of the casing; must be vermin-proof and water-tight
- Pitless adapter: Sanitary fitting through the casing wall below ground level; allows the water supply pipe to exit the casing horizontally underground without an open junction box
- Vent: Screened opening that allows air pressure equalization inside the casing as the water level rises and falls
The wellhead should extend at least 12 inches above finished grade to prevent surface flooding from entering the casing. Texas rule requires the wellhead to be maintained in good condition with no cracks, missing caps, or compromised seals.
Why These Components Matter for Maintenance
When a well develops problems, casing and screen issues are often the cause:
| Symptom | Likely Component | Diagnostic Step |
|---|---|---|
| Sand in water | Screen damaged or slot size wrong | Downhole camera + pump pull |
| Sudden drop in yield | Screen biofouled or encrusted | Camera inspection; consider acidizing |
| Coliform bacteria in water | Casing grout failure or cap breach | Inspect wellhead; camera for grout voids |
| Turbid water after rain | Grouting inadequate; surface water entering | Camera inspection; regrout if needed |
A downhole camera inspection — a small waterproof camera lowered into the casing — can directly visualize casing condition, screen integrity, and the presence of mineral deposits or biofouling. This is the standard diagnostic tool for well problems that can’t be explained by pump testing alone.
DFW Well Service installs steel and PVC casing, stainless wire-wound screens, and performs downhole camera inspections across the 19-county service area. Call (940) 536-8560 to schedule an inspection or discuss a new well project.