General Well Design Considerations – Part 1
By Chris Johnson
Article Originally Written for the NGWA Toolkit
At our last team meeting, I was talking about some general well design considerations, and thought I would share them here. These are a few of those considerations, and I will present more in later articles.
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, then does thinner casing. In general, it will take longer for a thicker casing to develop a corrosion pinhole leak then a thinner casing; and it will take a thicker casing longer to experience a corrosion-induced casing breach or failure, then will 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 therefor the need for stronger couples (e.g. type and cut of thread), and will also affect pipe handling and construction practices.
Intake Structure Placement
I classify vertically and horizontally-slotted casing, louvered casing and wire-wrapped well screen all as “intake structures”, which are part of the well string, and the area designated as the area 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, and 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, and there are more, that ought to be hashed out up front, is “shallowest intake structure depth” and “will lithology dictate 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, and pump failures, and possible induce certain geochemical behaviors that are 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, in the section titled “Relationship of Drawdown to Yield” (Groundwater and Wells, Driscoll, Fletcher G.).
With respect to lithologic considerations and intake structure depth, far too often I see wells designed with a very long intake structure length, sometimes on the order of 75% of the entire well. When compared to either (and preferably both) the lithologic or geophysical log, there is intake structure adjacent to silts and clays, which is a very optimistic design in that these types of lithologies yield very 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.
Usually, when we see these long intake structure lengths, they are 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 either 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. Practically, gravel pack thickness is more likely a function of the tremie pipe diameter and the Contractors 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 will fit, and the well casing, and all the stuff we want to put into the annulus between the casing and borehole. 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 the Contractor wanting a larger diameter borehole (more shoulder room for all the “junk in the hole”), and the designer striving for a thinner gravel pack. In general, all things being equal, thicker gravel packs require more work up front and over time to remove residual drilling fluid (initial well development) and then removing physical or chemical obstructions that occurs with time and well 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, sometimes for 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, wrapping those thoughts up:
Thicker well casings are usually better
Less intake structure length is sometimes better
Thinner gravel packs are usually better