Monitoring Well Abandonment
A monitoring well must be investigated before it is destroyed or abandoned to determine both its condition and the details of its construction. The well will be sounded (its depth measured with a weighted line or appropriate method) immediately before it is destroyed to make sure that it contains no obstructions that could interfere with filling and sealing.
If pollutants or contaminants are known or suspected to be present in a well to be destroyed, all materials removed from a well scheduled for abandonment need to be disposed of properly.
Where possible and practical, the monitoring well can be destroyed by removing all material within the original borehole - including the well casing, filter pack and annular seal. Material in the borehole may be removed by means of over-reaming (with a hollow-stem auger). The hole then can be completely filled with grouting material or cuttings. The sealing material should be pumped under pressure to ensure that the monitoring well is properly filled and sealed.
If the casing, filter pack and annular seal materials cannot or should not be removed (where good surface seal has been placed), they may be left in place. The casing left in place may require perforation or puncturing to allow proper placement of sealing materials. Where the casing is left in the hole, the casing may be cut at the surface.
The monitoring well should be filled to the surface with cement grout, or within 20 feet of the surface with bentonite grout. After the placement of the bentonite grout (if used), the remaining portion of the well then should be sealed with a Portland Type I, II or Type I/II cement with 2 percent to 5 percent bentonite.
General requirements for abandoning RFI boreholes - a borehole for the immediate or short-term determination of contamination conditions at a site, or a borehole used in a corrective action or remediation - are similar to those of a monitoring well. The borehole will be inspected immediately before filling and sealing operations. All obstructions that could interfere with filling and sealing operations are to be removed before filling and sealing. All waste generated from borehole and well abandonment will be disposed of properly. RFI boreholes can be completely filled with grouting material and/or cuttings.
The monitoring well or RFI boring can be filled with appropriate sealing and fill material using a tremie pipe and proceeding to fill upward from the bottom of the well or boring. Sealing material may be placed by free fall only where the interval to be sealed is dry and no more than 30 feet in depth. Place the sealing material in one continuous operation (or “pour”) from the bottom to the top of the well or boring, unless conditions in the well or boring dictate that sealing operations be conducted in a staged manner.
If subsurface pressure produces a flow of ground water into a well or boring that is significant, use special care to restrict the flow while placing sealing and fill material. Verify that the volume of sealing and fill material placed during abandonment operations equals or exceeds the volume to be filled and sealed. This is to help determine whether the well or boring has been properly destroyed and that no jamming or bridging of the fill or sealing material has occurred.
Heavy TrafficThe following options may be exercised for destroying monitoring wells and RFI boreholes in urban areas and near active technical areas:
- The upper surface of the sealing material may end at a depth of 5 feet below ground surface.
- If the well casing was not extracted during abandonment and sealing operations, a hole can be excavated around the well casing to a depth of 5 feet below ground surface after sealing operations have been completed and the sealing material has adequately set and cured.
- The exposed well casing then may be removed by cutting the casing at the bottom of the excavation.
- The excavation will be back-filled and compacted with clean, native soil or other suitable material.
Whenever work is interrupted by such events as overnight shutdown, poor weather or other delays, the monitoring well or borehole opening and any associated excavations should be covered at the surface. This is to ensure public safety and to prevent the entry of foreign material, water, pollutants and contaminants. The cover will be held in place or weighted down in such a manner that it cannot be easily removed, except by equipment or tools.
Sealing material used for abandoning monitoring wells or boreholes of a depth greater than 200 feet should consist of neat cement, with a 2 percent to 5 percent bentonite mixture or high-solids bentonite grout. In shallow and dry RFI boreholes (non-monitoring wells or boreholes where the casing has been removed) less than 200 feet in depth, cuttings can be replaced - if they can be placed (with certainty) without bridging. Dry bentonite may be added to the cuttings if desired.
Water used to prepare sealing mixtures generally should be of drinking-water quality and compatible with the type of sealing material used. It also should be free of petroleum and petroleum products and free of suspended matter. Water with a maximum of 2,000 mg/l of chloride and 1,500 mg/l of sulfate can be used for cement-based sealing mixtures.
Portland CementCement used in sealing mixtures should meet the requirements of American Society for Testing and Materials C150-92, Standard Specification for Portland cement, including the latest revisions thereof. Types of Portland cement available under ASTM C150-92 for general construction:
- Type I - General purpose. Similar to American Petroleum Institute (API) Class A.
- Type II - Moderate resistance to sulfate. Lower heat of hydration than Type I. This is similar to API Class B.
- Type I/II - General purpose, mixture of Types I and II. Cement grout used for borehole sealing and hole abandonment will have a mixture of 2 percent to 5 percent bentonite powder added and be thoroughly mixed before pumping.
- Neat Cement - For Types I and II Portland cement, neat cement will be mixed at a ratio of one 94-pound sack of Portland cement to 5 gallons to 8 gallons of clean water. Additional water may be required when bentonite is added.
Concrete is to be used for the surface pad only.
Cement-based sealing materials should be mixed thoroughly by machine to provide uniformity and ensure that no lumps exist before pumping. For abandonment where pumping is not required (30 ft. or less), the grout may be mixed by hand.
Ratios of the bentonite used with cement-based sealing materials can be varied from 2 percent to 5 percent. The bentonite should be dry-mixed before hydration and additional water may be required.
Curing accelerators may be used for temporary and short-term surface casing seals. Typical accelerators are calcium chloride, sodium chloride, aluminum powder or gypsum.
Bentonite ClayBentonite clay in gel or slurry form has some of the advantages of cement-based sealing material. A disadvantage is that the clay sometimes can separate from the clay-water mixture. Although many types of clay mixtures are available, none has sealing properties comparable to bentonite clay. Bentonite expands significantly in volume when hydrated. Only pure montmorillonite is an acceptable clay for bentonite annular seals.
Do not use bentonite clay seals where structural strength of the seal is required or where it will dry. Bentonite seals may have a tendency to dry, shrink and crack where subsurface moisture levels are low. Bentonite clay seals can be adversely affected by subsurface chemical conditions, as can cement-based materials.
Do not use bentonite clay as a sealing material if roots from trees and other deep-rooted plants might invade and disrupt the seal and/or damage the well casing. Roots can grow into a bentonite seal in some surrounding soil and vegetation conditions.
Do not use bentonite-based sealing material for sealing intervals of fractured rock or sealing intervals of highly unstable, unconsolidated material that could collapse and displace the sealing material. Bentonite clay should not be used as a sealing material where flowing water might erode it.
Specifically prepare bentonite clay products used for sealing material. Used drilling mud cannot be used for sealing purposes. Use only commercially prepared, powdered, granulated, pelletized or chipped/crushed bentonite comprised of sodium montmorillonite for annular seals. The largest dimension of pellets or chips will be less than 1⁄5 the radial thickness of the annular space into which they are placed.
Thoroughly mix bentonite slurry mixtures with clean water before placement. Add a sufficient amount of water to the bentonite to allow proper hydration. Depending on the bentonite sealing mixture used, add 1 gallon of water to about every 2 pounds of bentonite. Water added to bentonite for hydration will be of suitable quality - free of pollutants and contaminants.
Bentonite slurry preparations normally require between 30 minutes and one hour to adequately hydrate. Actual hydration time is a function of site conditions and the form of bentonite used.
Finely divided forms of bentonite generally require less time for hydration, if properly mixed.
Dry bentonite pellets or chips may be placed directly into the annular space below water, where a short section of annular space is to be sealed. Care must be taken to prevent bridging during the placement of bentonite-seal material.
Record-keepingAt a minimum, record the depth from surface to the bottom of the borehole, grout and bentonite/cuttings depth and location, ground surface construction details and actual composition of the grout and backfill.