General Well Design Considerations: Part 1

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Considered are casing thickness, intake structure placement, and gravel pack thickness.

By Christopher S. Johnson, PG, CHg

I was talking about some general well design considerations at our last team meeting and thought I would share them here. I will present more in later articles in the coming months.

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Casing thickness

Well casing thickness is often thought of as a simple application of a number to resist the collapse of the casing during and after well construction. While correct, there are other considerations.

Thicker well casing reduces the risk of collapse from corrosion, assuming all conditions are equal, more than thin casing. In general, it will take longer for a thicker casing to develop a corrosion pinhole leak than a thinner casing, and it will take thick casing longer to experience a corrosion-induced breach or failure than a thinner casing.

Consideration must be given to well casing inside diameters (I.D.) when sizing tremie pipe and pump bowl diameters for construction purposes and pumping equipment sizing, respectively. Consideration must also be given to pipe-joining practices, as thicker material may affect welding, increases the weight of the casing, and therefore, the need for stronger couples (e.g. type and cut of thread), and affect pipe handling and construction practices.

Intake Structure Placement

I classify vertically and horizontally-slotted casing, louvered casing and wire-wrapped well screen as “intake structures,” which are part of the well string and the area designated as where water will enter the well on purpose.

Intake structure placement will vary between wells completed in hard rock/fractured aquifers, and alluvial basin aquifers, and some that are a bit of both. With respect to alluvial basin aquifers, this is often a point of contention, so we’ll turn our attention here first.

There is usually a design contest between the length of the intake structure, and the notion that “a longer intake structure” means more water.  The two primary considerations are the intake structure depth and lithology dictating intake structure length.

If the shallowest depth of the intake structure does not account for declining water levels and loss of well efficiency over time, it may be that the pump gets lowered into the well to maintain sufficient head, such that the pump intake is now within the intake structure.

This is not an ideal situation, as it can exacerbate sanding, pump failures, and possibly induce certain geochemical behaviors detrimental to well performance.

The ideal situation is to place the shallowest depth of the intake structure as deep as is feasible, creating more available drawdown in the well itself.  There is a good description of why you want to maintain sufficient available drawdown in well designs in Groundwater and Wells 2nd Edition in the section titled “Relationship of Drawdown to Yield.”

With respect to lithologic considerations and intake structure depth, far too often I see wells designed with a long intake structure length, sometimes on the order of 75% of the entire well. When compared to the lithologic or geophysical log, there is intake structure adjacent to silts and clays, which is an optimistic design in that these types of lithologies yield little water to the well. Furthermore, in our practice we’ve found that these lithologies can release higher concentrations of trace metals to a well, which may impair water quality.

When we see these long intake structure lengths, they are usually coupled with gravel packs that extend from nearly the top of the well to the bottom. These long lengths of intake structure and gravel pack may contribute a small quantity of water from the clays and silts they intersect, but more than likely what they are mostly contributing are water quality issues, either from the interstitial fluids entering the well or the exchange of water between aquifers as it moves up or down inside the well and gravel pack.

Many a successful well has been constructed with intake structures and gravel packs just in those lithologies that will yield sufficient water to the well. While this can, and almost always will, complicate the construction of the well, particularly if the well is constructed with craftsmanship, the advantages are real in equivalent performance, improved water quality, and a reduction in overall cost.

Gravel Pack Thickness

Numerous articles exist on the mathematical and theoretical minimum and maximum gravel pack thickness for various well designs. Considerations I try to keep in mind, are constructability and long-term management.

An ideal gravel pack will be as thin as feasible, to reduce turbulence and improve well yield, amongst other technical considerations. Gravel pack thickness is more likely a function of the tremie pipe diameter and the contractor’s need for shoulder room.

A small diameter borehole, even one that is quite straight and aligned, will give contractors pause, as they try to figure out if the tremie pipe, well casing, and all the stuff they want to put into the annulus between the casing and borehole will fit.

Keep in mind diameters, particularly when things have collars and connectors on them, as this will create numerous localized reductions in the available annular space, past which you will want to get tremie pipe, gravel pack and other assorted construction necessities.

A contest will occur between contractors wanting a larger diameter borehole (more shoulder room for “all the stuff”) and designers striving for a thin gravel pack. In general, and all things being equal, thicker gravel packs require more work up front and over time to remove residual drilling fluid (initial well development) and later removing physical or chemical obstructions that occur with time and operation.

Since wells must be cleaned and maintained from the inside out, thicker gravel packs make it that much more difficult to reach out and affect the boundary between the gravel pack and the water-bearing formation itself.  More difficult usually means more time and more money—and sometimes less improvement.

Thicker gravel packs will also impact long-term management of the well from the standpoint of keeping them (the gravel pack) free of obstruction. The cleaning methods employed on thicker gravel packs will generally require a higher-energy capacity to reach past the intake structure and adequately influence the furthermost reaches of the gravel pack at the borehole interface. These higher-energy capacity methods are usually less available on short notice, more expensive, and more time consuming.

So to wrap up my thoughts:

  • Thicker well casings are usually better.
  • Less intake structure length is sometimes better.
  • Thinner gravel packs are usually better.

Christopher S. Johnson, PG, CHg, is the president and principal hydrogeologist at Aegis Groundwater Consulting LLC in Fresno, California. Johnson works with well owners and operators on a variety groundwater-related projects, including locating new water resources, well design and construction management, aquifer testing, and well rehabilitation. He can be reached at chris@aegisgw.com.

 

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