Well Blowout

Gases entrapped in pore spaces resulting from phase separation of gases from liquids can reduce the permeability of a formation. This process was the major cause of clogging at groundwater recharge wells in the Grand Prairie Region in Arkansas.81 Normally, pressures in deep-well-injection zones are high enough to keep gases in solution, so phase separation is not a problem. However, it is possible for permeability to be reduced by air entrainment at the same time gases are generated by reactions between the injected waste and reservoir formation. The resulting pressure then forces waste and reservoir fluid up the injection well to the surface, causing a well blowout.

The hazard of well blowout is greatest if hydrochloric acid wastes exceeding certain temperature and concentration limits are injected into a carbonate formation. When carbonate dissolves in acid, carbon dioxide is formed. Normally, this gas remains dissolved in the formation waters at deep-well temperatures and pressures, but if the temperature exceeds 88°F or acid concentration exceeds 6% HCl, carbon dioxide will separate from the formation waters as a gas. The resulting gas accumulation can increase pressures to a point where, if injection stops or drops below the subsurface carbon dioxide pressure, a blowout can occur.

20.4.4 Influence of the Deep-Well Environment on Biodegradation

Biodegradation of hazardous organic compounds in groundwaters has been the subject of much research in recent years. Ghiorse and Wilson82 provide good general reviews of the topic. Unpolluted near-surface aquifers typically contain enough oxygen for aerobic processes to prevail. For example, Ghiorse and Wilson82 summarize biodegradation data on 38 trace organic contaminants in subsurface materials from pristine sites. At most sites aerobic degradation is observed. In contrast, the deep-well-injection environment is typically anaerobic.

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