General Well Design Considerations – Part 3
By Chris Johnson
Originally Written for the NGWA toolkit
In Part 1 I addressed casing thickness, intake structure placement and gravel pack thickness, with respect to some basic design considerations. In Part 2 I discussed casing diameter, intake structure length, and gravel pack placement design considerations. In Part 3, regarding general well design considerations, I want to address casing composition, intake structure “type”, and gravel pack size.
Casing composition
Selecting well casing material can involve chemistry, mechanical engineering, metallurgy, materials science, corrosion engineering, and welding science. Or, it can be a simplistic, dogmatic approach that considers none of the possible variable involved with burying things underground with the expectation that nothing will happen to them.
Generally speaking, well casing is composed of either metal, plastic (polyvinyl chloride – PVC) or fiberglass. I going to focus on metal casings, which are most common, followed by PVC, then fiberglass, in general. That order of frequency can vary depending on locale, and the well type.
If money were no object, I suspect most of us would select the most durable, corrosion-resistant, structurally strong metal possible, to construct our well with, if possible. There are few titanium-cased water wells that I am aware of. However, the budget will always have a say in the material selection when it comes to casing selection.
On one end of the common metallic casing composition spectrum is low-carbon steel, also known as “mild” steel, although I prefer the “low-carbon” description because it more accurately describes the metallurgy of the material. This is generally the least expensive, and least durable of the metal casing materials. However, if the water chemistry is not aggressive towards such a material, then it may be an adequate selection. Keep in mind casing thickness when selecting this material, in other words increased casing thickness can sometimes improve corrosion resistance.
One the other end of the common metal casing spectrum, is stainless steel. Two typical types, 304 and 316L, are the common varieties used. The difference between the two is almost entirely metallurgical, and beyond the scope of this article to delve into. Designing with stainless steel has some limitations, one being cost, the other being the behavior of stainless steel in the presence of hydrochloric acid (a common well rehabilitation chemical) and high chloride water. Otherwise, stainless steel casing significantly increases the life span of a well, all else being equal.
The controversy that exists in well casing composition lies between these two extremes. Various opinions, articles and some scientific “research” suggests that varying the composition of low-carbon steel, can reduce the potential for corrosion. It is beyond this article to discuss, but a well designer should factor into that decision, enough actionable information to make a reasonable design choice.
Those factors should be the well design overall, and what the potential is for mixing different water qualities between multiple aquifers, via flow in the well, or in the gravel pack. Water chemistry, and the potential of the chemistry to be encrusting or corrosive. And lastly, the commitment of the client to maintenance. Regular and thorough monitoring and maintenance of a well may do a lot to mitigate casing metallurgical shortcomings.
Intake structure “type”
Here we have yet another controversy, that of the intake structure configuration, or to use a broader term “perforations”. Mills knife, vertical or horizontal machine slots, louvers, and wire-wrapped well screen are all possible choices. Each fabricator or manufacturer will present their case for why their product is superior, over that of any other type of intake structure configuration.
Well designers need to consider far more variables then the intake structure manufacturing folks. National standards are established, based mostly on the “entrance velocity” of water into a well, for the design of the perforations, louver opening, of wire spacing. However, they do not consider the local geologic and hydrogeologic conditions, groundwater geochemistry, materials selection and science, along with the potential operation and maintenance of a well.
It is not uncommon to encounter well issues, such as artificially expanded perforations, because of the well operator pumping the well at a flow rate which exceeds the entrance velocity for that type of perforation. This creates a condition where sand is pulled through the perforation, and over time it erodes the metal structure of the perforation, increasing the size, and exacerbating sand producing when pumping.
When designing then selecting an intake structure, consider the aquifer(s) lithology and grain size distribution, the planned well operation (desired flow rate, anticipated drawdown, etc.), groundwater chemistry and where the pump will be “set” within the well.
Gravel pack size
Gravel packs can have trade names, or mysterious designations, that are not standard to the industry, so I highly recommend asking the supplier what the designation signifies. Sometimes it the first and last sieve sizes (8 x 16 gravel pack) or it might denote a special blend (#30 Monterey Sand). In any case, you are going to want to know the size range and percent of total, of the particles that comprise your gravel pack.
“Ideal” gravel packs are non-existent, because as the designer, we need to attempt to match the gravel pack to the formations encountered. There is almost always more then one size or gradation of formations we encounter, and our standard is to design to the finest formation, in the hope of reducing the risk of pumping sand. So, ideal gravel packs are like unicorns, mythical.
Consider that if you are controlling via the gravel pack and the intake structure “type”, most of the finest gradation formation, then in theory coarser formations should not pass. In theory. In practice we must be more skeptical, and so try to design with not only sand control, but also water production in mind.
If we select our intake structure “type” opening, based on the gravel pack we pick, it is possible that we will end up with a small opening, that is restrictive and inefficient, and will reduce the overall production from the well. Herein science puts on the coat of art.
We have criteria for allowing somewhere between 10 and 30% of the gravel pack to pass through the intake structure, during development, with the belief that this will “settle” the gravel pack, and the sand content in the discharged water will reduce to acceptable concentrations. This is proven over and over, until it fails. As designers, we need to strive for reduced sand production potential, without sacrificing too much water production potential.
In summary:
With regards to casing composition, intake structure type, and gravel pack sizing, I can offer the following conclusions:
Pick the best metal composition you can afford, based on water chemistry, operational use and planned maintenance.
Intake structures should reduce sand, while maintaining water flow. Learning all you can about the different intake structure types, and making your own decisions, is best, i.e. avoid being “pamphlet smart”.
Understand how to select gravel pack, based on grain size analysis, and then how to select intake structure sizing, and this should lead to much better wells.
Best regards,
Chris