General Well Design Considerations: Part 2

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Casing diameter, intake structure length, and gravel pack placement design considerations.

By Christopher S. Johnson, PG, CHg

I addressed casing thickness, intake structure placement, and gravel pack thickness with respect to some basic design considerations in part one of this two-part series last month.  In part 2, I’ll wrap things up by discussing casing diameter, intake structure length, and gravel pack placement design considerations.

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Casing diameter

In general, larger-diameter casing does not equate to higher flow rates as a function of the larger diameter. The larger-diameter casing facilitates larger diameter pump bowls, which in turn can lead to higher flow rates.

However, there are design considerations that come with casing diameter selection and they come in two separate categories: constructability and well performance.

When collared casings are utilized, these create a projection into the borehole-casing annulus that must be accounted for when considering borehole-to-casing diameters. In other words, the inside diameter of the borehole needs to accommodate the outside diameter of the casing plus collars to allow both an adequate gravel pack thickness and the safe movement of tremie tubes within the annular space. This is a basic issue of constructability that must be considered.

In some cases, a larger diameter blank casing is constructed above a smaller diameter intake structure assembly. While this seems to facilitate room for a larger diameter set of pump bowls and reduces costs by using a smaller diameter intake structure, it creates a thicker gravel pack.

The same borehole-to-casing diameter design considerations are required, and added to that, will increase development time to address the thicker gravel pack. Finally, the thicker gravel pack increases the risk of reduced well performance because of energy loss into the well.

Intake structure length

It is common practice to “perforate the whole thing” in many instances in back of the envelope well design—in other words, to design and construct as much of the well with intake structure.

There are a few advantages, and several disadvantages:

  • Advantage: Reduces risks associated with well owner criticism regarding low flow rates.
  • Advantage: Generally, simplifies well design and construction.
  • Disadvantage: Attempts to extract water from low flow formations (e.g. silts and clays).
  • Disadvantage: Significant diffusion of hydraulic pressure over the long intake structure length, potentially reducing production from more prolific formations.
  • Disadvantage: Increases the possibility of entraining poorer quality water from clays and silts, and from aquifers with poor water quality groundwater.
  • Disadvantage: Potential for mixing differing groundwater chemistries, which may affect the water chemistry of the water pumped from the well and increasing the possibility of chemical and microbiological plugging of the intake structure.

Intake structure length can and will always be a point of contention. Too long and you’ll encounter some of the issues mentioned above; too short, and you may encounter fear of “missing out on some water” regardless of the potentially disadvantageous nature of long intake structure length.

Gravel pack placement

There are a couple of design considerations when placing gravel pack that should be considered, again from the standpoint of constructability and with respect to well performance.

There are two generally observed methods of gravel pack placement. First is to rely on gravity, essentially pouring gravel pack (or filter pack if you prefer) into the annulus between the borehole wall and the outside of the well casing. This method is commonly called free fall.

Free falling gravel pack when constructing a water supply well, whether for irrigation or potable consumption, generally involves drilling and construction below the water table. As such, the gravel pack must move downward, under the force of gravity, through the water column where it will (hopefully) accumulate around the intake structure of the well.

There are some inherent assumptions, and as such, risks with free falling gravel pack through a water column. First is the assumption that free fall placement is a controlled event. The risk of this method is the possibility of bridging, which is when the gravel pack accumulates higher in the annular space and does not reach the intended placement depth.

Unrecognized (the reason for gravel pack tallies) a bridging incident can expose the intake structure to formation fines and lead to a well pumping excess sand.

An additional concern of the free-fall method is the potential for segregating gravel pack into finer and courser fractions. This is a greater concern in a viscous-fluid filled annulus, such as a clay-based environment, as opposed to an entirely water-based drilling fluid environment.

The viscosity enhances the risk of causing finer gravel pack segments being suspended more readily then the larger segment of the gravel pack. It has been suggested that grain size, as opposed to weight, may reduce this risk, but it is none the less a consideration, particularly if you are free falling gravel pack through a great length of annulus.

The consequence of gravel pack segregation is at the top of the intake structure, which is closest to the pump and experiences the greatest hydraulic pressure during pumping, is the finer segment of the gravel pack is present.  This can exacerbate fine sand production.

The second most prevalent assumption is that free falling gravel pack is a benign process. In other words, it can do no harm to the annular space. The risk can come in scouring the borehole wall. This can contaminate the gravel pack with fine sediment, biasing the gravel tally and increasing development time.

The second method involves pumping a slurry of gravel and usually water, via a (generally) flush-threaded smaller diameter “tremie” pipe, which acts as a conduit from the surface where the slurry is prepared to the point at which the gravel pack needs to be placed.

Bridging is a concern with this method as well, but in this case, the risk is both bridging in the annulus and bridging inside the tremie pipe. If tremie pipe bridging happens, then the entire gravel packing process has to stop, the tremie pipe must be cleared, and then reinserted into the annulus so the gravel packing can resume.

I mentioned earlier “gravel pack tallies,” which is a calculated estimate of required gravel pack to be placed into the annulus. Essentially, the quantity of cubic feet (or more often cubic yards) representing the annular space is calculated, and then the quantity of gravel pack placed must at the minimum equal to, or preferably, exceed, the calculated volume.

Gravel pack tallies usually being with a mechanical survey of the borehole diameter, often after the borehole has been reamed to the full, finished size. Called a “caliper survey”, the borehole diameter is measured and recorded per foot, and reported in inches, which can then be used to calculate the volume per foot of borehole, and afterwards, the annular volume between the exterior of the well casing and the borehole.

Often, gravel is delivered to the drill pad in what are commonly referred to as super sacks bearing (authoritatively demonstrated on several occasions) at least 3000 pounds of gravel pack.  From all of this comes a schedule, or gravel pack tally, of the number of super sacks per (usually) 10-foot section of annular space.

In summary:

  • Casing diameter should be evaluated on desired flow rate, the pump bowl diameter to achieve that flow rate, and then on constructability.
  • Intake structure length should consider flow per foot of length, a function of formation performance, rather then simply total intake structure length and the risks of impaired water quality and reduced hydraulic performance.
  • Tremie gravel pack when possible and do so cautiously and patiently.

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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