A properly designed well system will have an annulus width that addresses all critical issues.
By Marvin F. Glotfelty, RG
The annulus of a well is the area between the exterior of the casing or screen, and the interior of the borehole (Figure 1). The casing may be driven or drilled directly into the earth using drilling techniques such as cable tool or dual rotary, but most wells have an annular void that is filled with various materials after the casing and screen have been installed. Various annular materials provide three types of benefits: filtration, sealing, or well stabilization. Filter pack sand is commonly installed adjacent to the well screen to facilitate water production without invasion of native sediments (fine sand and silt) that will damage the pump equipment and clog water distribution systems. Cement or bentonite can be placed at strategic depths of the annulus to seal off flow pathways that could allow comingling of groundwater from different sources in the subsurface. Groundwater chemistry is often layered, just as the sediments in many aquifers are stratified, and blending of different water chemistries within a well can cause degradation of pumped water quality and scale accumulation problems in the completed well. In portions of the well where a filter envelope or annular seal are not needed, less costly fill material (such as pea gravel or drilled cuttings) can be used to cost-effectively fill the annulus with a formation stabilizer. The width of an annulus should be at least 2 inches on each side, to accommodate an open-ended tremie pipe (as shown in Figure 1) through which annular materials are installed. A 2-inch annular width is generally adequate for lower-production residential water wells, monitoring wells, or piezometers that will not be pumped at high flow rates. Pumping these wells only at relatively low rates (or not at all) will avoid high entrance velocities as water flows into the well, so sand invasion will not be a problem. Larger water production wells, however, may produce several hundred to several thousand gallons per minute, so larger wells typically require a thicker filter pack envelope to prevent the invasion of native sands during pumping. For larger wells, a 3-inch-wide to 6-inch-wide annulus may be needed to provide adequate filtration. The annular width of a particular well will be selected by the well designer to address the well’s site-specific aquifer characteristics and anticipated pumping rate. It is not uncommon for aquifers to be comprised of an alternating sequence of productive and nonproductive strata. Localized pay zones from which most of the water production occurs are susceptible to sand production due to higher entrance velocities that result from the greater flow rates in these restricted, but productive intervals. Many years ago, I was involved with a well installation project where we used lithologic and geophysical logs to characterize the local aquifer with alternating layers of coarse-grained and medium-grained sand. Based on the geologic data, we thought this well would have somewhat consistent water production throughout its 300-foot screened interval. After a 16-inch-diameter well had been installed, we conducted aquifer testing and spinner logging at a pumping rate of 1700 gallons per minute. Our post-construction flow profile of the well revealed that approximately 1360 gpm of the well’s total flow was being produced by only 30 feet of the total screened interval. So, 80% of this well’s water production was entering the well through only 10% of the well screen! The lesson we learned is to be mindful about the possibility of localized water-producing intervals, which could also potentially become sand-producing zones. Fortunately, this well was sand-free. Thicker filter pack envelopes provide the benefit of improved filtration, but this is a double-edged sword. If the filter pack thickness is too great, it may impede our ability to fully develop the well. There has been a lot of consideration and discussion about the characteristics of different well screen types and filter pack grain sizes, but the effective permeability of a well’s filter pack pores and screen apertures will be much greater than the permeability of the aquifer formation itself. Thus, the filter pack and screen slots do not constitute an actual limitation to the well’s groundwater production. It is the wall cake—the thin layer of fine-grained clay material that coats the face of the borehole—that constitutes the principal limitation to groundwater production and well efficiency. Accordingly, well development activities should be focused on breaking down and removing the wall cake. And that brings us back to the annular thickness. If the filter pack envelope is so thick that energy cannot be effectively conveyed through it during well development, then the well will be incompletely developed, and the water production will be reduced (Figure 2A). Incomplete well development may also exacerbate biochemical clogging during the future operation of the well because any residue from the wall cake will likely contain organic polymers from the drilling fluid, which will become food for endemic bacteria. If a well is designed with an excessively thick filter pack to prevent sand production while the well is being pumped, that design may backfire, and the thick annulus may actually result in increased sand invasion instead of decreased sand invasion. If the wall cake on the borehole side of the annulus is not removed, the remaining clogged pores will limit the pathways through which water can flow into the well. A more moderate annular thickness will allow well development activities (e.g., swabbing, jetting, airlifting, etc.) to convey adequate energy to break down and remove much of the wall cake. A thinner filter pack envelope will also facilitate chemical treatments to be more effectively applied to break down the wall cake. More effective mechanical and chemical well development will result in removal of the wall cake to optimize water production and well efficiency (Figure 2B). Depending on the local geology, some wells can be developed without the introduction of an artificial filter pack. If the native formation material is coarse enough to be retained by the well screen slots, the driller can develop the well without adding filter pack sand. Development of a natural filter pack will break down and remove the finer-gained portion of the formation (including the wall cake) and bring the native sediments adjacent to the well screen (Figure 3). Regardless of the specific well design being implemented or the particular well development technique being used, it is always important to monitor the progress of the well development so an informed decision can be made as to when the well development is complete. Field measurements of sand content in the discharged fluids should be periodically conducted with an Imhoff cone (Figure 4), and other parameters such as fluid temperature, pH, electrical conductivity, odor, and color should also be monitored. A limited annular thickness along the screened interval of a well makes sense. However, in some cases, the well’s other structural attributes can be in conflict with a thin annulus. Some well owners prefer to have annular tubing strings (sounding tubes or gravel feed tubes) in the annulus outside the well casing. These annular tubing strings may have an outside diameter of 2 to 4 inches, so a thin annulus would create a tight space for the annular tubing strings, and make it virtually impossible to emplace a good annular cement seal to prevent cross-contamination. If different steel types are used in the casing string (such as stainless steel and mild steel), a dielectric coupling may be needed to prevent galvanic corrosion at the connection point of the dissimilar metals. Dielectric couplings have a flanged shoulder that protrudes out into the annulus by over an inch on each side, so this is another situation where a larger annulus would be required. Remember that although we draw well diagrams that are perfectly plumb, real-world annular features will be offset slightly in the less than perfectly aligned borehole, so annular seals can be compromised. A well designer can address these competing issues by considering a well design with a telescoping borehole diameter (Figure 5). Such a well design will provide a large enough annulus in the upper portion of the well to emplace the cement seal, and yet this well design also includes a small enough annulus adjacent to the screen to facilitate complete well development.
Alternating Sequences
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drilling issue that you have wondered about for a long time? A question you have wanted a second opinion on for a while? Send them to The Art of Water Wells column author Marvin F. Glotfelty, RG, and he will utilize his more than 35 years of experience to tackle the question for you. Email Glotfelty at mglotfelty@geo-logic.com, and the answer will appear in an upcoming NGWA: Industry Connected Video.
Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was The Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at mglotfelty@geo-logic.com.