Recovery of trace metals in formation waters using acid gases from natural gas

The maximum contents of Pb (360 mg l^?1), Zn (360 mg l^?1) and Ag (7.9 mg l^?1) in formation waters from the Alberta basin were high enough to suggest that it would be of interest to test the concept of recovering these metals by passing natural gas through the water, thereby precipitating the metal sulphides as the result of contact with hydrogen sulphide. The idea was to see if these metals could be recovered from formation water co-produced with crude oil prior to disposal of the water in deep formations, with the possibility of the sale of the metals partially offsetting the cost of disposal. It was proposed to use natural gas with a relatively small amount of hydrogen sulphide (insufficient for sulphur recovery) that must be removed by flaring before the gas is utilized. Accordingly, a database of 694 formation waters with major, minor and trace components was searched for appropriate analyses for detailed study. Of the nine analyses selected the majority were from Devonian and Granite Wash aquifers in the Peace River Arch area of northern Alberta, Canada. Modelling with PATH.ARC showed that there is a consistent set and order of precipitation reactions, in spite of the differences among the formation waters. As would be expected intuitively, acid gas addition makes the formation water more acidic, and metallic sulphide minerals are precipitated. Depending on the initial composition, the end minerals are any of galena, sphalerite, acanthite, covellite and pyrite. These are the minerals that must be beneficiated to recover the metals. A preliminary evaluation of the dollar value of the recovered metals shows that although the absolute values are low, there may be an advantage to recovering the metals if the waters are already being handled at the surface.