Caustic Waste Injected into Dolomite

Velsicol Chemical Corporation maintained two injection wells at its plant near Marshall, Illinois, to dispose of caustic wastes from pesticide production, as well as contaminated surface runoff. In September 1965, the company began to inject the wastes into Devonian dolomites of the Grand Tower Formation at a depth of about 2600 feet. The wells accepted about 6 million gallons of waste monthly. 

The waste contained about 3.5% dissolved solids, 1.7% chlorides, 0.4% sodium hydroxide, and tens to hundreds of ppm of chlorinated hydrocarbons and chlordane its pH was generally greater than 13 (Brower et al., 1989). At the time of drilling, analysis of formation samples indicated that the injection zone was composed of nearly pure dolomite [CaMg(CO3)2]. The carbonate formation was thought to be safe for accepting an alkaline waste water because carbonates are considered stable at high pH. 

With time, however, the company encountered problems, including caving of the formation into the wellbore and the loss of permeability in zones that had accepted fluid. In June 1987, a number of sidewall cores were taken from the formation (Mehnert et al., 1990). Mineralogic analysis by x-ray diffraction 

The plant eventually closed for environmental reasons, including surface contamination unrelated to the injection wells, causing a loss of jobs in an economically depressed area. Could geochemical modeling techniques have predicted the wells deterioration Using REACT, we trace the irreversible reaction of dolomite into the NaOH-NaCl waste. To calculate the waste s initial state and then titrate dolomite into it, we enter the commands 

The fluid contains arbitrarily small amounts of Ca++, Mg++, and HCO3, as is necessary in order for the program to be able to recognize dolomite. The initial magnesium content is set small to assure that the hydroxide mineral brucite is not supersaturated in the alkaline fluid. 

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