Methods For Grouting Casing
In general, it is good practice to place a grout seal around the upper portion of well casing to help prevent contaminated surface water from entering the well. In other situations, grout may be placed around the entire length of casing or within the casing itself. These situations are outlined below and various techniques for placing grout are .described
The most common use of grout is to seal the annular space between the top of the filter pack and the ground surface. For shallow wells the water table does not extend far above the filter pack, it is often possible to mix cement and water (no sand or gravel) into a thin paste and pour it into the annular space. However, in deep wells in which the gravel filter pack is far below the level of water in the annulus, this procedure would lead to separation of the sand and cement leading to formation of a poor seal. To :prevent this, the following procedure can be followed
- Ream-out the borehole to 18 cm (7 in) diameter.
- Insert a 7.6 cm (3 in) diameter well screen and casing and “float-in” your gravel pack.
- Insert a 1 inch diameter “tremie” pipe down the annular space to the top of the gravel filter pack (the outer diameter of the couplings for the 7.6 cm (3 in) PVC schedule 40 pipe is 10.8 cm (4.25 in). In a 18 cm (7 in) borehole, this will leave a 3 cm (1.25 in) gap to insert your tremie pipe)
- Using a funnel, slowly pour cement grout into the tremie line. Gradually lift the line ensuring that the bottom of the line stays below the level of cement accumulating in the annular space.
- When the annular space is filled, remove and wash the tremie line.
Modified Halliburton Wiper Plug Method
When heavily contaminated soil or a severely polluted surficial aquifer are encountered, well casing should be securely grouted into an underlying rock or clay layer. This should also be done when moving from mud rotary to air hammer drilling to ensure that the hole does not cave-in. In order to securely grout a casing, the following procedure :should be followed
- Unless you have special small bits, ensure that you use 10 cm (4 in) diameter casing … you will not be able to drill-out the cement inside 7.6 cm (3 in) diameter casing!!
- Make sure that the seals on the drill rig and mud pump hoses are in good condition and that they are tight.
- Make an adaptor so that the drill pipe fitting on the LS-100 can be attached with a water-tight seal onto the top of the casing. If this is not possible, have an extra 3m (10 ft) length of casing with a glued-on coupler and a 3 m (10 ft) ladder ready.
- Calculate the volume of the inside of the casing. The volume of a 10 cm (4 in) diameter casing is 8 litres/meter (0.09 ft3/ft or 0.64 USG/ft). Subtract 8 litres from this estimated volume and then measure-out this amount of water into a barrel.
- Calculate the volume of the annular space. The annular space for a 15.24 cm (6 in) diameter borehole with a 10.16 cm (4 in) diameter casing is 8 litres/meter (0.09 ft3/ft or 0.64 USG/ft). Multiply this number by 1.3 (30%) to allow for wash-out down hole etc. If you suspect that there has been much caving in the hole, increase the volume by up to 100%.
- Place the well casing into the hole, ensuring that it is suspended about 0.3 meters (1 ft) about the bottom of the borehole.
- Mix-up a volume of cement equal to the volume estimated above. Neat cement grout yields about 37 litres (9.7 USG or 8.1 IMG) of slurry per 94 lb bag (1 ft3) which is mixed with 23 litres (6 USG or 5 IMG) water. Do NOTadd any sand or gravel. Mix it smooth (no lumps) until it is like thin pudding or thin yogurt.
- Pour the cement grout down the inside of the casing.
- After the cement is placed in the casing, lower the drill rig swivel/quill until the drill pipe fitting makes a water tight seal with the adaptor on top of the well casing. Then pump the measured volume of water down into the well casing behind the cement. This can be done by filling all the hoses with water and then placing both the suction hose and the recirculation hose into the barrel containing the measured volume of water. Start the mud pump and then use the 3-way valve on the drill rig to control the rate at which water is pumped into the casing. SLOW pumping is GOOD!
- If it is not possible to create a water tight seal with the drill pip fitting, wait until the cement is inside the casing and then tightly push or glue on the extra 3.05 m (10 ft) length of well casing. Ensure that you accounted for this extra length of casing when you calculated the volume of water to add. Standing on top of the ladder, slowly add water into the casing until the entire measured volume of water has been added. Since cement weighs 14 lbs/gallon and water weighs 8.3 lbs/gallon, the extra 3.05 m (10 ft) of water will push the cement down an extra meter (3 feet).
- Once all the water has been added, gently push the casing downward to ensure that it is seated in the rock or confining clay layer. This helps ensure that there is a complete seal at the bottom of the casing.
- Then use several shovel full of dirt to plug-off the ditch leading from the borehole to the mud pits.
- If water is leaking out of the plumbing joints and/or drilling mud is draining back down into the annulus, then cement is leaking back up the casing. Tighten joints to reduce leakage. While some leakage is not desirable, it is not a crisis either… it will merely require the drilling out of a little extra cement.
- Wait 24 hours before you start to drill inside the casing. If in doubt, wait longer rather than shorter. If you start drilling and find that the cement is not set-up, wait another 12 hours.
- Drill-out the cement using a 10.16 cm (4 in) roller bit.
Wiper Plug Method
If for some reason the Modified Halliburton Wiper Plug Method described above will not work, consider the following method which uses a plug to push the grout down into the :casing and up the annular space
- Take 1 or 2 empty cement sacks and wad them into a tight ball.
- Once the measured volume of grout is placed in the casing, insert the cement sack ball (the “wiper plug”) in the casing.
- Use drill pipe to slowly force the wiper plug down the casing until it is just above the bottom of the casing.
- Use several shovels full of dirt to plug-off the ditch leading from the borehole to the mud pits.
- Leave the drill pipe in-place for 1-2 hours to let the cement set enough so that it wouldn’t come back up the casing when the drill pipe is removed.
- Do not leave the drill pipe in the casing for an extended period of time. If you do and some cement leaks back through or around the wiper plug …. it will set around the drill pipe and make it a permanent installation!
- After 1-2 hours, slowly start to pull-up of the drill pipe. If drilling mud ponded around the top of the borehole starts to drain back down into the annulus, then cement is leaking back up the casing. Leave the drill pipe in the hole for another 15-30 minutes and try removing it again. Remember… some leakage is better then cementing your drill pipe into the casing!
Cement “Plastic Bag Plug” Method
Sometimes a borehole is drilled into poor quality water and it is determined that the best course of action is to backfill the lower portion of the hole and set the well screen at a higher elevation where better quality water is expected. When a wooden drive shoe is used at the bottom of a casing string, it is desirable to place a 30 – 61 cm (1 – 2 ft) cement plug in the bottom of the well to ensure that sediment cannot enter the well :when the plug rots out. Both these procedures nvolve all or some of the following steps
- Backfill the hole with cuttings until you are 61 cm (2 ft) below the desired completion depth of the borehole where you want the bottom of the screen to be.
- Mix-up 0.4 ft3 (11 litres) of real thick cement & water mixture (no sand or gravel).
- Pour the cement into small plastic bags the easily fit down inside the borehole.
- Run the drill pipe with a blade bit down the borehole and move the pipe up and down to chop-up the bags and mix-up the cement.
- Remove the drill pipe and wait 1 hour for the cement to set.
Insert the well screen and casing (with casing bottom plug) down into the borehole. Suspend the .casing 6 inches off the bottom of the borehole and then put in the filter pack