Geochemical Interactions of Two Deep-Well Injected Wastes with Geological Formations: Long-Term Laboratory Studies

Abstract

Two liquid hazardous wastes, an alkaline brinelike solution and a dilute acidic waste, were mixed with finely ground «250 jlm) rock samples of three injection-related lithologies, sandstone, dolomite, and siltstone. The batch experiments were conducted for 155 to 230 days at 325°K and 10.8 MPa pressure. The pH and inorganic chemical composition of the alkaline waste were not significantly altered after 230 days of mixing. The acidic waste was neutralized by carbonate dissolution and transformed into a nonhazardous waste. Mixing the alkaline waste with the solid phases yielded several reaction products: brucite (Mg(OH)2)' calcite (CaC03), and possibly a sodium metasilicate. Claylike minerals formed in the sandstone; trace levels of hydrotalcite (M96AI2C03(OHh6'4H20) may have formed in the siltstone. Mixing the alkaline waste with a synthetic brine yielded brucite, calcite, and whewellite (CaC20 4·H20). The thermodynamic model PHROPITZ predicted that brucite and calcite would precipitate from solution in the dolomite and siltstone mixtures and in the alkaline waste-brine system. After 155 days of solid-liquid contact, the dilute acidic waste had not significantly altered the mineralogical composition of any of the three rock types. The model PHREEOE indicated that calcite was thermodynamically stable in the dolomite and siltstone mixtures, and it was detected in small quantities in the aged-solid samples. The sandstone-waste system appeared to equilibrate with amorphous silica, whereas silica equilibria may not have been attained in the dolomite and siltstone systems. Computer models like PHROPITZ and PHREEOE may be useful tools for estimating mineral equilibria in deep-well scenarios, but there is a need to expand the database used in these kinds of calculations. The predicted equilibria must be interpreted with caution.