Effective abandonment depends on knowledge of the well construction, geology and the hydrogeology. The importance of a full characterization increases as the complexity of the well construction, site geology and the risk of aquifer contamination increases. Construction information for wells drilled since 1966 may be available from the Bureau of Topographic and Geologic Survey's (BTGS) Water Well Inventory System database. Additional well construction data and information describing the hydrologic characteristics of geologic formations may be available from reports published by BTGS and the United States Geological Survey. Site or program records also may exist. The well should be positively identified before initiating the abandonment, and field information should be compared with any existing information.
Water levels and well depths can be measured with a well sounder or weighted tape measure. In critical situations, well construction details and hydrogeology can be determined with borehole geophysics or a downhole camera. For example, a caliper log, which is used to determine the borehole diameter, can be very helpful in locating cavernous areas in open hole wells.
If possible, the borehole must be cleared of obstructions prior to abandonment. Obstructions such as pumps, pipes, wiring and air lines must be pulled. Well preparation also may involve fishing obstacles out of the borehole. An attempt should be made to pull the casing when it will not jeopardize the integrity of the borehole. Before the casing is pulled, the well should be grouted to near the bottom of the casing. This will at least provide some seal if the well collapses after the casing is pulled.
The presence of nested or telescoped casing strings complicates well abandonment. Inner strings should be removed when possible, but only when removal will not jeopardize the abandonment of the well. If inner strings cannot be removed and sealing of the annular space is required, then the inner string should be vertically split (plastic cased wells) or cut (metal-cased wells) at intervals necessary to ensure complete filling of the annular space.
Damaged, poorly constructed or dilapidated wells may need to be re-drilled in order to apply proper abandonment techniques. Also, in situations where intermixing of aquifers is likely, the borehole may need to be re-drilled.
Materials and Methods
Aggregate -Materials that eliminate the physical hazard and open space of the borehole, but do not prevent the flow of water through the well bore, are categorized as aggregate. Aggregates consist of sand, crushed stone or similar material that is used to fill the well. Aggregates should be uncontaminated and of consistent size to minimize bridging during placement.Aggregate usually is not placed in wells smaller than two inches in diameter. Nominal size of the aggregate should be no more than one-fourth of the minimum well diameter through which it must pass during placement. Because aggregate usually is poured from the top of the well, care must be taken to prevent bridging by slowly pouring the aggregate and monitoring the progress with frequent depth measurements.
Aggregates may be used in the following circumstances:
- There is no need to penetrate or seal fractures, joints or other openings in the interval to be filled.
- A watertight seal is not required in the interval to be filled.
- The hole is caving.
- The interval does not penetrate a perched or confined aquifer.
- The interval does not penetrate more than one aquifer.
If aggregate is used, a casing seal should be installed. The use of aggregate and a casing seal must be consistent with the future land use.
Sealants - Sealants are used in well abandonment to provide a watertight barrier to the migration of water in the well bore, in the annular spaces or in fractures and openings adjacent to the well bore. Sealants usually consist of portland cement-based grouts, bentonite clay or combinations of these substances. Additives frequently are used to enhance or delay specific properties such as viscosity, setting time, shrinkage or strength.
Sealing mixtures should be formulated to minimize shrinkage and ensure compatibility with the chemistry of the ground water in the well.
A grout pump and tremie pipe are preferred for delivering grout to the bottom of the well. This method ensures the positive displacement of the water in the well, and will minimize dilution or separation of the grout.
If aggregate is to be placed above sealant, a sufficient amount of curing time should pass before placing the aggregate above the seal. Curing time for grout using Type 1 cement typically is 24 hours to 48 hours, and 12 hours for Type III cement.
Bridge Seals - A bridge seal can be used to isolate cavernous sections of a well, to isolate two producing zones in the well, or to provide the structural integrity necessary to support overlying materials (and thus protect underlying aggregate or sealants from excessive compressive forces). Bridge seals usually are constructed by installing an expandable plug made of wood, neoprene or a pneumatic or other mechanical packer. Additional aggregate can be placed above the bridge.
Some Recommendations
The complexity of the abandonment procedure depends primarily on the hydrogeology, geology, well construction and the ground water quality. Four principal complicating factors have been identified; they include 1) artesian conditions, 2) multiple aquifers, 3) cavernous rocks and 4) the threat or presence of contamination. The recommended procedures for abandoning wells will be more rigorous with the presence of one or more complicating factors. The procedures may vary from a simple casing seal above aggregate to entirely grouting a well using a tremie pipe after existing casing has been ripped or perforated.Casing Seal - The transition from well casing to open borehole is the most suspect zone for migration of water. In order to minimize the movement of water (contaminated or otherwise) from the overlying less consolidated materials to the lower water-bearing units, this zone must be sealed. Generally, this can be accomplished by filling at least the upper 10 feet of open borehole and the lower 5 feet of casing with sealant. The length of open borehole sealed should be increased if extenuating circumstances exist. Such circumstances would include a history of bacterial contamination, saprolitic bedrock or possibly deep fracture zones. Water-bearing zones reported in the upper 20 feet or so of open borehole are indications of fractures and would warrant additional sealant. Casing that is deteriorated should be sealed along its entire length.
If the casing is to be pulled, the sealant used should remain fluid for a period of time adequate for removal of the casing. If the casing is to remain, then whenever feasible, it should be cut off below land surface. After the casing seal discussed above achieves adequate strength, the open casing should, at a minimum, be filled with aggregate. It is strongly suggested that a sealant be used in the upper 2 feet to 5 feet of casing.
Unconfined or Semi-Confined Conditions - When applicable, unconfined wells in non-contaminated areas may be satisfactorily abandoned using aggregate materials up to 10 feet to 15 feet below the ground surface. This would apply mainly to domestic wells, as well as test borings or wells not covered by existing regulations. Monitoring wells that are not covered by specific regulatory programs and are located at sites with no known contamination might be abandoned in this manner. Above the aggregate, the casing seal should be installed. A sealant may be used over the entire depth.
Contaminated Sites - An abandoned, contaminated well often mixes contaminated ground water with uncontaminated ground water. Complete and uniform sealing of the well from the bottom to the surface is required. Therefore, proper well preparation must be done before the well is sealed with a proper sealant.
Cavernous Rocks - Problems can arise when filling wells that penetrate cavernous rock. Although such wells usually are located in carbonate terrain, voids can occur in areas that have been deep mined. Care must be taken to ensure that aggregates and sealants are of a size and consistency to prevent their removal by water flowing in the void. Large voids or high flow velocities warrant placement of a bridge in competent rock over the void. Aggregate and sealants then can be placed above the bridge.
Multiple Aquifers - The main goal in sealing wells that extend into more than one aquifer is to prevent the flow of ground water from one aquifer to another. If no appreciable movement of water is encountered, and there is no threat of ground water contamination, sealing with concrete, neat cement, grout or alternating layers of these materials and aggregate will prove satisfactory. When ground water velocities are high, the procedures for wells with artesian flow are recommended. If alternating plugs (or bridges) and aggregate layers are used, the plugs should be placed in known nonproductive horizons or, if locations of the nonproductive horizons are not known, at frequent intervals.
Flowing Wells - The sealing of artesian wells requires special attention. The flow of ground water may be sufficient to make sealing by gravity placement of concrete, cement grout, neat cement, clay or sand impractical. In such wells, large stone aggregate (not more than 1⁄4 of the diameter of the hole), well packers (pneumatic or other) or wooden plugs will be needed to restrict the flow and thereby permit the gravity placement of sealing material above the zone where water is produced. If plugs are used, they should be several times longer than the diameter of the well in order to prevent tilting. Seals should be designed to withstand the maximum anticipated hydraulic head of the artesian aquifer.
Because it is very important in wells of this type to prevent circulation between water-yielding zones, or loss of water to the surface or to the annular spacing outside of the casing, it is recommended that pressure grouting with cement be done using the minimum volume of water during mixing that will permit handling.
In wells in which the hydrostatic head producing flow to the surface is low, the movement of water may be stopped by extending the well casing to an elevation above the artesian pressure surface.
Multiple Complications - Wells with one or more of the above complicating factors that are to be abandoned in areas with contaminated ground water or in areas where the ground water is at a high risk for future contamination, require the most rigorous abandonment procedures. In general, the entire length of these wells should be sealed.
When the threat of contamination has been established, the elimination of a potential flow path is critical. For example, a contaminated well in a karst terrain must be sealed carefully to avoid worsening the situation. In general, the entire lengths of these wells should be sealed. In some situations, a bridge seal may have to be installed, and casing may have to be perforated. In each case, a prudent method should be selected that will eliminate all potential vertical flow paths.
Monitoring Wells - Monitoring wells should be abandoned in accordance with the rules and regulations of the program under which they were installed and operated.
Monitoring wells that were installed and continue to function as designed usually can be abandoned in place. Exceptions would include wells whose design precludes complete and effective placement of sealant and wells in locations subject to future disturbance that could compromise the abandonment. In such instances, all tubing, screens, casings, aggregate, backfilling and sealant should be cleaned from the boring, and the hole should be completely filled with an appropriate sealant.
Monitoring wells that are abandoned in place should be completely filled with sealant. Screened intervals can be backfilled with inert aggregate if sealant will alter the ground water chemistry and thereby jeopardize ongoing monitoring at the facility. Intervals between screens, and between the last screen and the surface, must be filled with sealant. Generally, sealant must be emplaced from the bottom of the interval being sealed. Protective casings, riser pipes, tubing and other appurtenances at the surface that could not be removed should be cut off below grade after the sealant has properly set. When the abandonment will be completed below the finished grade, the area of the boring should be covered with a layer of bentonite, grout, concrete or other sealant before backfilling to grade.
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