Every contractor needs to know how to install a critical part of a well system.
By Marvin F. Glotfelty, RG
A good annular seal around the casing of a well is one of the most important components of a properly constructed water well.
The annular seal is typically composed of bentonite or cement, which is placed around the well casing to provide a barrier to migration of water between poor-quality intervals of the aquifer and the well’s screened interval.
If the annular seal does not have the proper chemical and physical characteristics, or if it was not properly installed, the seal may leak and allow cross-contamination to occur within the well.
The Nebraska Grout Study represents a good source of information on the nature and characteristics of these annular sealing materials. Tom Christopherson’s McEllhiney Lecture in 2011 discusses that study and can be viewed on The Groundwater Foundation’s website.
Well Design Impacts on the Cement Annular Seal
The surface seal of a well is located in the upper 20 feet to 100 feet, as required by most states (Figure 1). While the surface seal is an important well design component, the well seal shown lower in the well does the actual work to prevent cross-contamination in most cases.
The effectiveness of the well seal to isolate different portions of the aquifer is reliant on an appropriate overall well design that accommodates the intended purpose of the well and the local hydrogeology.
We almost always include casing centralizers in the well’s design to provide for an equally spaced annulus on all sides of the casing, so that the annular seal will be composed of a complete envelope of cement surrounding the casing.
Many well owners wish to have permanently installed sounding tubes or gravel feed tubes extending through that annular seal (Figure 1). Annular tubing strings provide access to the well owner for future measurement of groundwater levels (via the sounding tube), or installation of additional filter pack sand (via the gravel feed tube).
Despite the utility and convenience that annular tubing strings provide to the well owner, I am not a fan of these annular tubing strings because they may compromise the integrity of the annular seal. Nonetheless, well owners regularly request annular tubing strings, so they are regularly included in our well designs. We just have to account for these tubing strings in our overall well design so the well owner can have their desired operational access, while avoiding disruptions to the integrity of the annular seal, to the extent possible.
The primary concern with annular tubing strings relates to plumbness and alignment of the borehole, and thus, the positioning of the casing and tubing strings within that borehole. Water well contractors typically drill straight boreholes, to within about ½° of vertical as is required by AWWA Standard A100. However, even a slight deviation of the borehole will cause the tubing strings to shift to the low side of the borehole at various depths.
Unlike the centralized well casing, the annular tubing strings and the temporarily installed tremie pipe will not be centralized. Therefore, although we draw diagrams with perfectly straight boreholes and perfectly centralized annular tubing strings (Figure 1), the actual subsurface conditions may include annular tubing strings that are lying against the borehole wall at some depths and lying against the side of the casing at other depths (Figure 2).
This raises the concern that flow paths may exist to allow corrosive or contaminated water to find its way from the aquifer to the exterior of the well casing. If shrinkage of the cement seal causes it to separate slightly from the well casing (Figure 3), the flow path for poor-quality water migration could extend all the way down to the well screen.
Cement Composition for the Annular Seal
There are three general categories of cement used for annular seals in the water well industry (Figure 4). The common cement types are neat cement (pure Portland cement and water), pozzolan cement (with a fly ash additive), and sand cement (with a sand additive). Each cement type has advantages and disadvantages, so the particular type of cement is selected on the basis of its specific application in the well construction.
Neat cement provides a good annular seal in accordance with the chemistry of the cement being used. Pozzolan cement improves the chemical stability of the cement slurry and improves the flow characteristics of the slurry as well. Sand cement provides an extension of the slurry volume at a somewhat decreased cost. These cement types have variable densities (Figure 4), so the well design should consider the
measured weight of the actual cement slurry being used.
The American Society for Testing and Materials (ASTM) has designated various cement types as being appropriate for particular applications. Several of the ASTM categories are equivalent to American Petroleum Institute (API) classes of cement. The ASTM-designated cement types and their respective properties are:
- ASTM Type I: General purpose cement (same as API Class A)
- ASTM Type II: Moderate sulfate resistance (same as API Class B)
- ASTM Type III: High early strength, and shorter curing time (same as API Class C)
- ASTM Type IV: Low heat of hydration (long curing time, not commonly used in water wells)
- ASTM Type V: High sulfate resistance.
The reactions of cement slurry to the common additives are listed in the following section. While all the reactions to cement additives are important, one of the most critical attributes of a cement additive is how it will affect the cement shrinkage.
Excessive shrinkage of the cement envelope can result in a micro-annulus adjacent to the cement envelope (Figure 3), which can lead to migration of poor-quality groundwater. If a small void space occurs along the cement annular seal at the time of its installation, that void area may become enlarged over time due to erosion of the borehole wall or from corrosion of the well casing as the poor-quality water flows against those surfaces.
Common Additives to the Cement Annular Seal
Here are general descriptions of the most common cement additives used in the water well industry:
Bentonite acts as an extender, which increases the cement slurry volume. Bentonite also slightly increases the slurry viscosity, decreases the slurry density, and reduces the amount of water loss (bleed water). Addition of bentonite is normally limited to about 4% by volume, and it may increase the requirement for makeup water.
Pozzolan is a siliceous or silico-aluminous powder that chemically reacts with the cement to provide several benefits. Pozzolan (also called fly ash) reduces the permeability of the cement seal and adds strength and chemical resistance to the cement. It also improves the flow properties of the cement slurry as it is pumped, decreases the cement shrinkage and density, and reduces the heat of hydration while the cement slurry is setting up (hardening).
Sand is used as an extender to increase the slurry volume and reduce its overall cost. Sand increases the viscosity of a cement slurry and the requirement for makeup water and significantly increases the cement slurry density (Figure 4). Addition of a dirty sand (containing organics or loam) to a cement slurry will increase the water loss volume and cement shrinkage. However, the addition of clean sand to a cement slurry will actually reduce the cement shrinkage.
Accelerators are added to the cement slurry to speed up the hardening process, and thus reduce the waiting on cement (WOC) standby time for the drill crew. This additive also increases the water loss volume (bleed water) and the cement shrinkage, along with the heat of hydration as the cement cures. The most common accelerator additive for water wells is calcium chloride (CaCl2) which is normally limited to about 2% by weight.
Retardants are added to slow down the cement hardening, thus this additive will extend the waiting on cement (WOC) standby time. Retardants will also decrease the heat of hydration as the cement cures. Cement retardants include such things as lignosulfonates, cellulose derivatives, hydroxycarboxylic acids (such as citric acid and glycolic acid), or sugar (although pure sugar is not commonly used as a cement additive).
Dispersants are sometimes added as a friction retarder to improve the flow properties of the cement slurry as it is pumped.
Fluid Loss Agents can be added to reduce the water loss volume (bleed water) as the cement slurry cures.
Lost Circulation Materials can be used when cementing porous formations such as coarse gravel or fractured rock. The addition of lost circulation materials to a cement slurry serves essentially the same purpose as when these materials are added to drilling fluid—to prevent excessive loss of the fluid into fractures or pore spaces of the adjacent formation.
Cement Annular Seal Installation Techniques
The most common technique for installation of a cement grout annular seal is via a tremie pipe, where a relatively small-diameter open-ended pipe is suspended down the annulus to facilitate the installation of annular materials.
The tremie pipe is commonly flush-threaded and installed prior to the well casing and screen, so that it won’t be obstructed by casing centralizers or other tight spots in the annulus. While the cement slurry is being pumped through the tremie pipe, the bottom of the pipe should always remain submerged below the cement fill level to prevent entrainment of air or water in the cement slurry.
After the cement fill level has risen to an adequate height above the base of the tremie pipe, a joint of tremie pipe can be laid down, so that the tremie pipe is gradually removed from the annulus as the cement placement progresses.
The cement column in the well annulus will exert hydrostatic pressure against the casing, and if that hydraulic pressure exceeds the casing’s strength, it will cause casing collapse.
One method of avoiding excessive hydraulic pressures when a tall cement column is being installed is to pour the cement in stages. This means a portion of the cement seal height is installed and allowed to cure, and then additional pours of cement are added after the previous pours have hardened until the total height of the intended cement seal is in place.
Cementing in stages provides the safety factor of avoiding excessive hydraulic pressures against the well casing, but this technique also introduces the risk that sediment may slough off the borehole wall during the waiting periods between cement pours. I encountered a good example of the potential for sediment sloughing while cementing in stages several years ago when
I was reviewing a well installation report that had been prepared by another groundwater professional. The report I was reviewing indicated the annular seal of the well’s intermediate casing (which was installed with four sequential cement pours) had been filled with a volume of cement that totaled more than 11 cubic yards less than the calculated volume of the annulus. That 11-cubic yard discrepancy had clearly resulted from sediment sloughing in on top of each stage of cement during the waiting periods between respective cement pours.
A good approach for installation of a watertight annular seal in wells with a telescoping design (larger-diameter upper casing above a smaller-diameter screened interval in the lower well) is to install the cement annular seal by pressure grouting (also called inner-string grouting).
This technique is commonly done in the oil and gas industry and is also appropriate for water well construction. There are several variations of the pressure grouting technique, but the most common approach involves use of a float shoe, which is a drillable check valve that is placed near the base of the well casing.
The float shoe is often used in conjunction with a Braden head, which is used to seal off the top of the well casing while still allowing the drill pipe or other tubing string to pass through it down into the well (similar to a sanitary well seal). The float shoe is connected to the base of the well casing with a welded or threaded connection, and after the casing has been installed to the prescribed depth in the borehole and is hanging in tension, the drill pipe or tubing is lowered down to the float shoe and coupled into it by use of a stinger device that is designed for that purpose.
A diagram of the pressure grouting technique is shown in Figure 5, and photographs of a float shoe and stinger device are shown in Figure 6.
Once the float shoe is coupled to the drill pipe via the stinger device, the driller generally circulates drilling fluid down through the drill string and up the annulus to confirm there is an open pathway for cement placement. The calculated volume of cement is then pumped down through the drill string and into the annulus after which the cement is displaced from the interior of the drill pipe.
If the drill pipe were to clog for some reason during cementing, the driller can simply pull the pipe or tubing string up out of the float shoe, and the cement in the annulus will not U-tube back into the well casing because the float shoe acts as a check valve. There are several variations of the setup for pressure grouting, which can be done with or without a float shoe, and with or without a Braden head.
A variation on the pressure grouting approach, but with similar considerations and similar results, is the displacement method. With this cementing method, the volume of the cement seal is installed into the well casing and then a drillable disc-shaped or cork-shaped wipedown plug (also called a displacement plug) is pumped with water pressure down through the casing to its base, which displaces the cement and causes it to flow out from the bottom of the casing into the annulus.
Regardless of which pressure grouting technique is used to emplace the cement, the float shoe or displacement plug at the bottom of the well casing will be drilled out after the cement has cured. Thus, this approach works well for an intermediate casing that is part of a well design with telescoping casing diameters.
And also, regardless of which pressure grouting technique is used, the well casing could be susceptible to buoyant movement (floating) or collapse (if the hydraulic stresses become excessive), so a buoyancy calculation and a hydraulic stress calculation should always be conducted before this type of cementing is conducted.
Formulas for those calculations are available in the book, The Art of Water Wells, and will also be presented in a future column.
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 firstname.lastname@example.org.