COMPLIANCE INFO - Laserfiche WebLink

Home Browse Search

well or borehole is drilled using casing advance, the drive casing is withdrawn back to the surface by hydraulic jacking (pulling) as the well is constructed. Sometimes during withdrawal, the casing may break leaving the lower end in the open borehole. Common practice to decommission blank well casing (cable tool drilled wells or lost drive casing) is to simply place grout in the open hole to near-surface and cover with fill. In many instances, pea-gravel or earth collected on-site have been used to fill the open void for which the consequences can be tragic. Conversely, as with all wells, the removal of fill-types of material (i.e. rocks, brick, metal, wood, etc. ) from a cable tool well is particularly critical and should be approached with caution because cable tool wells have no cement annulus for support; in addition, they are generally old and probably have weaken casing because of age and corrosion. The use of reverse circulation techniques will remove all water from inside the casing and may cause casing collapse. This situation is even more critical below the water table where hydrostatic forces are generated. Mud rotary drilling is the simplest and probably best technique for removing foreign debris from a cable tool well. If a "perforate and grout technique" is employed to decommission cable tool wells or lost drive casing in the environmental field, it usually involves perforating the entire length of casing and pressure grouting in-place. Lost drive casing completely within a permeable geologic unit can generally be grouted in-place to achieve an adequate seal. However, if lost casing interconnects one or more aquitards, a more complex decommissioning may be involved. To adequately decommission lost drive casing requires an in-depth interpretation of its borehole position respective to geologic units (permeable and impermeable) and knowledge of the location of the water table or water-bearing zones. The significance of each aquitard that is interconnected needs to be evaluated. All aquitards that do not appear to be sticky clay should probably be sealed by perforating the drive casing at these intervals and grouting using a packer at moderate to low pressures. If several significant aquitards are interconnected by a lost casing, perforating and grouting activities may need to be completed in stages for each significant aquitard. It is noted, decommissioning techniques directed tdward sealing aquitards, as described for wells with extensive filter packs, should be the focus of effort. As with filter packed wells, perforating cable tool wells or lost drive casing adjacent to a permeable zone may severely limit efforts to achieve an adequate seal adjacent to aquitards because permeable geologic materials will more readily accept grout during the sealing operation. If lost drive casing is perforated adjacent to a permeable zone below the water table and water has been evacuated from inside the casing, formation materials will flow through the perforations into the casing. The situation thus created will be nearly impossible 5