SUGGESTED METHODS FOR SEALING THE ANNULAR SPACE
Prior to sealing, the annular space should be flushed to remove any loose formation material or drilling mud that might obstruct the operation. The use of centralizers devices which are affixed to the casing at regular intervals to prevent it from touching the wall of the hole, thereby keeping the casing centered in the borehole are recommended. This assures that the seal is not less than the desired minimum thickness. It is particularly significant for large diameter wells where the casing exceeds 10 inches in diameter.
The use of the tremie or grout pipe for the introduction of the sealing material into the annular space is preferred. Where a tremie or grout pipe is used, the minimum annular space should be 2 inches and the minimum tremie size should be a nominal 1-1/2 inches in diameter.
Gravity installation without a grout pipe or tremie should not be attempted when the sealing interval contains water or cannot be visually inspected (with the aid of a mirror or light). Where sealing material is to be introduced under water or the interval cannot be observed from the surface, methods involving positive placement (by a tremie or grout pipe, pumping or other application of pressure) must be used.
The sealing material must always be introduced at the bottom of the interval to be sealed. This prevents bridging (jamming) or segregation (separation of large aggregate from the mixture in sand-cement or concrete grouts) of the sealing material and eliminates gaps.
Sealing should be accomplished in one continuous operation. Where the sealing interval will exceed 100 feet in length, consideration must be given to the collapse strength of the casing. Further, because of the weight of such extensive seals, consideration must also be given to the installation of stronger retaining devices and to staging the placement of the seal (as, for example, the installation of a short segment of rapid- setting sealant in advance of the main body of sealing material; the former becomes a foundation to support the extensive seal).
Sealing Methods
The following methods can be used to seal the upper portion of the annular space. Except for the first, these methods are illustrated on Figure 10. The first method is frequently used where short seals, under 20 feet deep, are placed in dry material.
