Installation of PVC Wells and Grouting
It is critical to understand the impact grouts have on the installation of a well.
By Thom Hanna, PG
We discussed the physical attributes of PVC pipe in this column in the Water Well Journal October 2024 issue, but most of the questions I get are about how deep a contractor can complete a well using PVC.
Figure 1. Shur-Grip® Spline connection to PVC pipe.
Typically, that comes down to a collapse issue. The screen has holes, and the pressure should equilibrate inside and outside the well if the drilling fluids have been thinned.
So, the question is really what are the forces applied to the casing during completion? The biggest one is the differential pressure created when grouting the well. With that, we will discuss the use of cement grouts and touch on the use of bentonite grouts in this column.
One of the better completion designs for a water supply well would be using PVC casing with a stainless steel screen. This type of completion will provide a corrosion-resistant casing that will not shed iron, which can be a food source for iron-related bacteria and cause red water issues. The stainless steel screen will allow for the construction of a high-capacity well because of its large open area.
Once the screen is selected, the connection needs to be specified if the screen is directly attached to the casing. Since there can be slight variations in threads, it is best to use the same screen and casing supplier. However, an adaptor can be used if the casing connector method is different (Figure 1).
PVC pipe is more flexible than steel, and it should be centered in the borehole using centralizers connected to that completion string before any grouting or filter packing is completed. Any voids in the grout or filter pack material could lead to a collapse of formation materials against the casing, compromising the integrity of the grout seal or in the screen section, allowing for the pumping of sands and fines.
As in any well completion, the tremie pipe must be installed in the well prior to placing the casing materials in the borehole to ensure the tremie pipe is installed to the desired depth.
When casing protrudes above ground level, it must be protected as PVC is not as strong as steel. When handling the material in cold weather, care must be used to prevent shattering the material, especially when the temperature is below 32ºF (0ºC).
Tensile strength is usually less important for PVC, as it generally is used in shallow wells. Because the weight of PVC is about one-fifth the weight of steel, it occasionally floats during installation and creates special handling problems.
Grouting PVC
Figure 2. Typical grouting methods for PVC casing (Minnesota Administrative Rules for Water Wells).
Grouting PVC casing is typically done by using Portland cement or bentonite grout. It can be accomplished by pouring the grout down the anulus, using a tremie pipe to pump the grout to depth or pumping the grout using the Haliburton method.
When first installed, grout is a liquid slurry mixture that can create potentially excessive external hydraulic collapse pressures, especially if there is not fluid inside the casing. However, when grout is cured or set to a semi-rigid state, the collapse pressure caused by the slurry is eliminated. Cured grout provides lateral support and holds the casing firmly in place and seals the borehole against water infiltration from the surface or from undesirable aquifers.
Cement Grout
Cement grout is widely used in the construction of water wells to seal the annular space between the well casing and the borehole. Various types of cement grout—primarily classified by their composition and properties—are used based on specific well conditions and regulatory requirements. The main types include (Sterrett 2007):
- Portland Cement (Type I)
Composition: Standard Portland cement
Applications: General use where no special properties are needed - Portland Cement (Type II)
Composition: Portland cement with moderate sulfate resistance
Applications: Used in environments with moderate sulfate exposure - High Early Strength Cement (Type III)
Composition: Portland cement formulated for high early strength
Applications: Suitable for situations requiring quick setting and early strength gain - API Class Cement (Class A, B, C, G, H)
Class A and B: Applications: General purpose, moderate sulfate resistance
Class C: Applications: High early strength
Class G and H: Applications: Can be used with accelerators and retarders for deep wells
Care must be taken when using cement grouts as the heat of hydration will reduce the strength of the pipe. Accelerators tend to cause more heat and can increase the potential for a collapse if not taken into account as discussed later. If accelerators are used, care must be taken as the heat of hydration will increase and is generally not recommended for a PVC completion.
In addition to these types, cement grout can be modified with various additives to enhance its properties:
- Bentonite
• Effect: Improves fluidity, reduces shrinkage, holds cement particles in suspension
• Proportion: 3 to 5 pounds (1.4-2.3 kg) per 94 pounds (42.64 kg) of cement
• Usage: Mix bentonite and water before adding cement. - Other Additives
• Diatomaceous Earth: Reduces density
• Pozzolan: Reduces density and water requirement
• Sand: Increases density and reduces cost
• Accelerators (e.g., calcium chloride): Accelerate setting time. Typically not recommended for PVC
well completions due to increased heat of hydration.
• Retarders: Extend setting time.
Bentonite Grout
Bentonite grout is another popular choice for sealing water wells due to its unique properties:
- Composition:
• Primary Component: Bentonite clay
• Additives: Polymers to enhance pumpability - Advantages:
• Faster Setting Time: Sets faster than cement grout
• Lower Hydrostatic Pressure: Specific gravity of about 9.2
• Swelling Property: Swells to form a tight seal, unlike cement which can shrink
• Cost-Effective: Generally cheaper than cement - Preparation:
• Mixing: Typically, 1.5 pounds (0.7 kg) of bentonite per 1 gallon (3.8 L) of water
• Polymer Addition: About 1 quart (0.9 L) of polymer per 100 gallons (380 L) of water to delay immediate hydration and ensure pumpability.
Overall, the selection between cement grout and bentonite grout depends on the specific requirements of the well including the depth, environmental conditions, and regulatory standards.
Figure 3. Grout-induced heat rise around casings (Molz and Kurt 1979).
Both materials have distinct properties and benefits that make them suitable for different well construction scenarios. Cement grout is versatile and can be tailored with additives for various conditions, while bentonite grout offers rapid setting and excellent sealing capabilities, particularly in cost-sensitive projects. It is important to note that when Portland cement-based grouts cure, the hydration reaction produces heat that can be transferred to the casing.
Installation of Grout Seal
Portland cement and bentonite grout are most commonly installed using a tremie pipe (Figure 2). A tremie pipe, also referred to as a grout pipe, is a small-diameter pipe, hose, or tube used to convey grout. The pipe is commonly steel or plastic but may be a flexible hose.
The tremie pipe is usually installed between the casing and hole. The tremie pipe must be set to within 10 feet of the bottom of the annular space to be grouted. It is important to install the tremie pipe prior to installing the casing. Filter pack is first placed typically through the tremie pipe across the screen section and then grout is placed on top of the filter pack.
In addition to grouting through a tremie pipe, grouting may also be completed by use of the displacement method. The displacement method allows placement of grout in the borehole and then setting the casing with a bottom plug into the grout-filled borehole. The plug is either pushed to the bottom of the casing with a drill rod or pumped down using water and a sealed top on the well, which is known as the Haliburton method.
Cement grouts exert greater collapse pressure on casing than that exerted by water or drilling fluid. Table 1 shows the increase in pressure exerted on casing by cement slurry compared with that exerted by a typical drilling fluid.
Figure 4. Grout-induced heat rise around casings (Molz and Kurt 1979).
The potential pressure is greatest normally at the bottom of the casing. For example, the maximum pressure exerted by the cement grout at the bottom of a 500-foot (152-m) casing is 500 multiplied by 0.833, which equals 416 psi (2868 kPa).
A safety factor of 25% to 40% is recommended for use when selecting the wall thickness for the casing. For PVC casings with an upset joint a 25% safety factor is recommended and for flush joint casings a 40% safety factor is recommended. Fluid inside the casing reduces the cement column pressure, depending on the relative heights of the fluid and cement columns. If the inside of the casing is dry, then the casing must be strong enough to support the entire grout column.
The displacement or Halliburton method for grouting well casing ensures effective grout placement in the annular space between the casing and borehole wall, preventing contamination and enhancing well integrity. This process involves pumping grout, typically Portland cement, down the casing, using spacer plugs to separate the cement slurry from drilling fluid and water. Water is then pumped to push the slurry out, maintaining pressure to prevent backflow.
Pumping of grout in the annular space may be accomplished by using any type of pumping pressure including mechanical, air, or human powered pumps. Typical pumps used include mechanical pumps such as piston pumps or drill rig mud pumps.
Heat of Hydration in Portland Cements
Grout surrounding a well casing generates heat as the cement grout hardens and the heat temperatures can be significant. Because the strength of plastic casing materials decreases (Table 2) with the increasing temperature, it is important to have an estimate of the temperature increase that can be anticipated during well completion.
Figure 3 presents peak temperature versus grout thickness in inches that could be anticipated around a well casing. The type of cement will also factor into the heat of hydration grouts with a high solids content having lower heats of hydration.
When selecting a grout for PVC casings, the designer should think about using a high solids grout that would include the addition of fine sands or bentonite to the grout mixture. The addition of the solids will reduce the heat of hydration. The time required to reach peak temperature is a function of grout thickness or annular width.
Reaching maximum temperature can vary from seven to 22 hours depending upon the annular space (Molz and Kurt 1979), but in typical annular spaces of 3 to 5 inches, the peak temperature will be reached in less than 10 hours.
The inside of the casing can be cooled in some completions. This can be done by running cool water through the casing to dissipate some of the heat in the casing. In some cases, involving deeper well installations, grouting might have to be done in stages to prevent the collapse of the casing due to the differential pressure between the grout in the annulus and water or air inside the casing.
This is exacerbated by the potential for loss of strength of the PVC as the grout cures. What is done inside the casing can have a significant effect on the grout temperature.
It is also important to understand the integrity of the borehole that is being grouted. In some formations, there can be openings that exist or washouts (possum bellies) that occur in the borehole (Figure 4). These can create hot spots where the heat of hydration can be significant.
Often bentonite chips or fine sand are placed on top of the filter pack prior to grouting to prevent cement from migrating into the filter pack. Often 2 feet of fine-grained sand is placed on filter pack and between a cement seal or bentonite to prevent the breakthrough of the seal during placement of the concrete grouts.
Grouting with Bentonite Grout
Grouting with bentonite is like that of Portland cement, but it sets up to more of a ductile material as compared to cement grouts, especially below the water table. Bentonite grout exerts a pressure like cement grouts when placed, but it starts to set up and become a solid within an hour.
Benefits of bentonite grout
- Faster setting time: Bentonite grout sets faster than cement grout, allowing for quicker well construction.
- Heat of hydration: Bentonite grout does not experience the heat of hydration that cement grout does, reducing the risk of damage to the casing and surrounding formations.
- Hydrostatic pressure: Bentonite grout has a lower specific gravity (9.2) compared to cement, resulting in lower hydrostatic pressure on the well structure.
- Adhesion and swelling: Bentonite adheres to both walls of the annulus and swells over time, ensuring a tight seal that prevents contamination. Unlike cement, which can shrink and crack, bentonite remains stable and chemically inert.
- Cost: Bentonite grout is generally less expensive than cement grout, making it a cost-effective option for well construction.
We will further discuss the use of bentonite grouts in an upcoming column where we discuss experiments to determine what downhole pressures are realized at depth.
Other recommended references for grouting are Groundwater & Wells, Third Edition, and the Nebraska Grout Task Force study completed by the State of Nebraska (Lackey et al. 2009). This study showed the integrity of various grouts, including both bentonite and cement for grouting wells.
References
Lackey, S.O., Myers, W.F., Christopherson, T.C. and Gottula, J.J. 2009. “Nebraska Grout Task Force—In-Situ Study of Grout Materials 2001–2006 and 2007 Dye Tests.” Conservation and Survey Division, University of Nebraska- Lincoln: Lincoln, Nebraska.
Minnesota Administrative Rules for Water Wells, 4725- 3050.
Molz, F.J. and Kurt, C.E., 1979. “Grout-Induced Temperature Rise Surrounding Wells.” Ground Water 17, no. 3, pp. 264–269.
Sterrett, R.J., 2007. Groundwater & Wells, Third Edition. Johnson Screens: New Brighton, Minnesota.
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Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at thom.hanna@johnsonscreens.com.
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