General Well Design Considerations – Part 2
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 will discuss casing diameter, intake structure length, and gravel pack placement design considerations.
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 several design considerations that come with casing diameter selection, 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 needs to 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 be the need to 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, as follows:
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/or microbiological plugging of the intake structure.
Intake structure length can and will always be a point of contention. Too long, you may encounter some of the issues mentioned above, too short and you may encounter fear and resistance. 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”. This has some acceptable applications when constructing shallow monitoring wells, and most of it has to do with the complexity of setting up tremie pipe and the associated systems for pumping gravel pack, when building wells at greater depths.
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 a word we use to describe a situation where the gravel pack accumulates higher in the annular space, for several possible reasons, and has not reached 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, among many possible issues.
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, if it occurs, is that at the top of the intake structure which is closest to the pump and experiences the greatest hydraulic pressure during pumping, the finer segment of the gravel pack is present. This may 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 is scouring the borehole wall, in other words scraping off material from the borehole wall as the gravel falls. This can contaminate the gravel pack with fine sediment, biasing the gravel tally and increasing development time.
Free falling is often thought to be a time (and therefore a cost) savings, but rarely is this the case.
The second method involves pumping a slurry of gravel and usually water, via a (generally) series of flush-threaded smaller diameter “tremie” pipe, which act 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 and then 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, and 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 3,000 pounds of gravel pack. From all of this comes a schedule, or gravel pack tally, of the number of super sacks per (usually) ten (10) foot section of annular space.
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.