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. i�I vl.-L1PIF �dR•91U'LJJ'Ji JN 1'IQ�j 1��90 1f •1V f'. v✓ - <br /> screened (or perorated) sections of the well which were designed <br /> for intake. These intervals usually require a separate episode of <br /> grouting to establish a floor for pressure grouting aquitards. <br /> Perforating well casing and pressure grouting using packers always <br /> requires numerous phased activities which makes the process time <br /> consuming, expensive, and often frustrating. <br /> For some wells with extensive filter packs, explosives have been <br /> used to perforate casing at aquitard intervals so that they may be <br /> cemented. In this process, explosive chaxges are attached to a <br /> wire line that is hung in the well with charges adjacent. to <br /> aquitards and then detonated. when using explosives, the well bore <br /> should always be filled with water or cement during the actual <br /> blast so that geologic materials outside the well, especially below <br /> the water table, will not collapse back into the well after <br /> blasting. A collapsed well severely complicates and may prevent an <br /> effective decommissioning. After blasting, the top of the well <br /> casing should be sealed, preferably by welding, and cement pumped <br /> into the casing under pressure. The pressure causes cement to flow <br /> through breaches in the casing and invade and seal Off aquitards. <br /> For wells deeper than 200 feet, injection pressures at the bottom <br /> of the well are much greater than the top which may reduce the <br /> effectiveness of pressure sealing shallow aeuitards. <br /> Unfortunately, blasting does not provide for the confidence that is <br /> achieved by a phased decommissioning effort where the casing is <br /> mechanically perforated adjacent to aquitards and pressure grouted <br /> in lifts using a packer. Also, using explosives for <br /> decommissioning purposes is essentially a single-step aggressive <br /> activity. unforseen mechanical breakdowns may severely compromise <br /> the process and generate additional corrective activities and <br /> expense. For these reasons, the use of explosives is generally <br /> discouraged. <br /> =f a well filter pack annulus is at least four inches in diameter, <br /> dual-tube air rotary drilling (reverse circulation) can be used to <br /> only remove filter pack material. This technique creates an open <br /> annulus where the casing may be lifted and the open borehole <br /> sealed. For wells with `filter packs to ground surface, dual-tube <br /> air rotary drilling may be used to remove annulus materials so that <br /> a near-surface sanitary seal can be installed. This technique <br /> should be used with caution and only if the relative consolidation <br /> of geologic materials penetrated by the well are known to be <br /> competent. The removal of a well filter pack material for <br /> decommissioning or workover opera dg;_init <br /> tions should ely not be <br /> used where geologic materials are unconsolidated because the <br /> borehole may collapse. A collapsed borehole would severely <br /> complicate and possibly prevent an effective well decommissioning <br /> or annular seals. <br /> Cable Tool wells and Blank Casing <br /> @� , <br /> 14 ts'e and seine types of rotate drilling employ a casing that <br /> is driven into the 9T0=d to iaeap+ t�ha borahole open. Once a Cable <br /> tool well is drilled, the casing is left in the borehole. After a <br /> 4 <br />