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<br />water and saline aquifers, some at ~everal thousand feet below the <br />surface. If these are under artesian pressure, water mixing can occur <br />between aquifers, causing contamination of freshwater. Artesian pres- <br />~ sures can also force saline water to the surface, discharging over the <br />CJl land, with a potential for contaminating surface streams. Water from <br />t~ deep formations can be extremely saline, with concentration of salts of <br />c:> 10,000 or more milligrams per liter (mg/l). Surface contamination from <br />saline aquifers can be eliminated by plugging above and below the water- <br />producing geologic formation. If the well cannot be successfully plugged, <br />che saline water can be collected on the surface and disposed of in an <br />evaporation pond. <br /> <br />Before attempting to plug an abandoned oil well flowing under artesian <br />pressure, it should be recognized that a number of potential problems <br />can be encountered. A successful operation depends upon a knowledge of <br />the stratigraphic column (position and order of sequence of geologic <br />strata) from the bottom of the well to the surface. This would include <br />information on lithology (structure, mineral composition, color and <br />texture of rock formations), porosity, and permeability. The depths and <br />intervals of aquifers must be known, as well as the identification of <br />those under a hydraulic head (artesian). The date the well was drilled <br />and cased, the type of casing used (including diameter and material), <br />total depth of casing, and depth and interval of perforations or screens <br />must be known. Such information can be obtained from a detailed driller's <br />log (9). An analysis of chemical constituents in the water (corrosive <br />properties) will give a clue to the present condition of the casing (6). <br /> <br />The structure and mineral composition of rock units will affect whether <br />the walls of the hole are stable or may have caved in or will cave in <br />during plugging operations. Cement plugs may not effectively seal off <br />aquifers if the casing has rotted away or if the hole walls are not <br />stable. If the condition of the well is very poor, a new hole may need <br />to be drilled alongside it. This can greatly increase costs. Drilling <br />out an existing cased hole can be expensive if rotted casing, sloughing <br />of the hole walls, or other detrimental conditions cause equipment to be <br />lost in the hole during drilling. Problems can also occur if grouting <br />is not used or is improperly done, allowing saline water under artesian <br />pressure to flow around the casing or outside the well bore via fractures <br />and reach the surface or contaminate other aquifers. The Ground Water <br />Manual (6) gives a good explanation of how a cement-based grout between <br />the soil walls of the bore and the casing cannot only seal a well but <br />can also serve as a concrete casing if the steel casing is lost through <br />corrosion. <br /> <br />18 <br />