Why de-commission defunct or failed bore wells?
Well decommissioning is a process that seeks to restore the hydro-geological characteristics of the area to its original state prior to well construction and to prevent the possibility of the abandoned well acting as a conduit for the movement of contaminants from the surface or the cross-contamination of aquifers.
- It ensures the safety of those in the vicinity of the bore well.
- It prevents surface water infiltration and vertical movement of polluted or contaminated water into an aquifer and contamination of the groundwater quality regime of the area.
- Proper decommissioning conserves the aquifer yield and hydraulic head as well as the groundwater quality parameters.
- It prevents fall of animals and small kids into the bore wells. An abandoned well, not property decommissioned, poses a potential risk and there have been several deadly incidents of children accidentally falling into bore wells in the last few years.
What are the goals for well decommissioning?
The goal of decommissioning a bore/tube well is to restore the hydro-geological properties of the site. There are three relevant objectives:
- The first is to prevent the vertical movement of water in the vadose zone. This prevents surface water from rapidly moving downwards and bypassing attenuation processes.
- The second is the restoration of aquitards. This is similar to the first goal in that it seeks to prevent the vertical flow of water.
- The third goal is to maintain the flow of water through the aquifer system.
This is frequently less of a concern as the well itself is a point of impact for the aquifer which usually extends some distance in both cardinal directions and as such the single point impact should be minimal to maintain the overall conditions of the aquifer. In some formations, such as highly fractured bedrock, fluid-based grouts may flow deep into the aquifer and have impacts on the aquifer performance.
What are appropriate sealant materials for bore well decommissioning?
In theory, decommissioning of wells suggests restoring it in a manner that reflects the original hydro-geological state and involves mimicking the layers of strata that existed previously. In most cases, the key focus is on preventing the unwanted movement of polluted or contaminated water in a vertical direction. As mentioned above, the localized impediment of aquifers is of less concern. This can be done by focusing on filling up the inside of the casing with an impermeable material.
The materials to be considered are natural clay materials and cuttings from new wells, bentonite high solid grouts, bentonite chips and pellets, neat cement concrete, and mixtures of sand and cement or bentonite grouts.
The various sealant material which can be used for scientific closure of bore wells are:
(A) A non‐toxic, commercially available material or a mixture of materials, including
- Bentonite clay,
- Bentonite clay and water mixture,
- Bentonite clay and sand and water mixture,
- Neat cement grout,
- Sand cement grout, and
- Concrete grout, or
(B) A non‐toxic material or a mixture of materials that have a lower permeability than the surrounding geologic formation to be sealed.
Products such as cement grouts and bentonite high solid grouts can be mixed into a slurry and pumped into a well using a tremie pipe which fills the well from the bottom-up (lifting the pipe during the process). This ensures that the entire space is filled and that any water in the well is pushed upwards and out of the well as the grout fills it. Both require specialized equipment for mixing and pumping. The concerns with using cement-based grout are that there is contraction when drying and also that it may not lead to bonding to casing walls.
Recent research done as part of some grout studies have indicated that cement-based grouts were more effective than bentonite clay-based products in sealing the annular space in wells, particularly when the cement was mixed with a small percentage of sand. Challenges owing to contraction and cracking were less if added with the sand mixture. Also, various tests conducted by adding bentonite grout to the cement mix (a common practice in the industry) resulted in the sealant being less effective.
Bentonite grout slurries, which had previously been lauded as more effective as they remained more malleable and bonded with the casing better, was proved to be far less effective than cement-based grouts as moisture was wicked away by the vadose zone materials. The bentonite grouts (of various solid contents) contracted more than the cement due to this wicking action and thus, created voids, and also failed to bond to the casing.
However, bentonite grout may fare significantly better in decommissioning operations inside of the casing because it will stay hydrated much longer as opposed to the materials that are in direct contact with the vadose zone. However, in many decommissioning approaches, there are efforts to ensure the annulus is filled by removing or puncturing the casing. This would open up the problem of moisture being wicked away by the vadose zone materials.
Various experimental studies have suggested that bentonite products with very high levels of solids, such as a mixture of sand to bentonite of a ratio of 4: 1, may be effective. Bentonite chips and pellets are inexpensive for grouting wells as no equipment is needed for their installation. They are a viable and low-cost option when the goal is to fill the space inside the casing or when filling a hole that the casing has been removed from.
What are the processes involved in decommissioning the bore well or tube well?
Step 1: Remove all pumping equipment and casing if any, from the defunct well. Thoroughly flush out the well using a high-pressure air compressor.
Step 2: Measure the total depth and the diameter of the abandoned well and the non-pumping static water level. If possible, compare these figures with the information sheet on the original drilling report. Confirm whether the well is open to its original depth.
Step 3: Use these figures to decide which plugging material is appropriate and how much you will need. Whether or not the casing can be successfully pulled out will also determine which material to be used and what method is appropriate for placing the sealant into the well. If the casing cannot be removed, choose a slurry that can be pumped under pressure into the well so that any space around the outside of the casing will also get filled in due to high compressive pressure during the sealing process.
Step 4: If possible, remove the well casing thoroughly, or
Step 5: Disinfect the bore well. The water in the well must be flushed and cleaned of foreign materials, then disinfect with 200mg of chlorine per litre of water in the well.
Step 6: Place the plugging material into the well. It must be introduced at the bottom of the well and placed progressively upwards to the ground surface. The only exception to this rule is when the plugging material being used is a bentonite pellet that has been designed and manufactured for pouring into the well from the ground surface. Using bentonite and cement grout we should be able to inject the grout from the base of the borehole to the surface using a tremie pipe in conjunction with a Putzmeister pressure grouting unit. This ensures all gaps are filled with fast setting grout, removing all contaminant’s; boreholes are then capped with concrete, tarmac or turf (as per existing surroundings) at the surface.
Step 7: If the casing was not able to be removed, dig around it and cut it off a minimum of 0.5 m (20 in.) below the ground surface (see the figure below), cutting off the casing and mounding the area with clay).
Step 8: Backfill and mound this portion of the hole with material appropriate for the intended use of the land (i.e., clay) (see figure below).
Step 9: Establish clear soil binding vegetation above the mound for increasing the stability of the land surface.
Figure: This depicts the cutting-off of the casing on top and the mounding of the clay material as grout, followed by soil and vegetation growth as total scientific closure of abandoned bore well.
About the author
Murali Kochukrishnan is Head-Projects (Geology, Hydrogeology & Environment) at IL&FS Environmental Infrastructure & Services Limited (Kerala Region); Website: www.ilfsindia.com; Email: Murali.Krishnan@ilfsindia.com