Geochemical changes in experimental water or HCl reactions of the Cooper Basin REM shales

Abstract

Production or flow-back water from shale gas stimulation has been reported in the USA to contain high salinity, TDS and variable concentrations of potentially toxic metals such as uranium, barium, or lead. The shales of the Cooper Basin REM sequence have a high prospectively for unconventional gas. Water reactions of shale core samples were performed in mildly oxic conditions and at elevated temperature and pressure to investigate potential for metal mobilisation. A relatively higher concentration of lead, vanadium, chromium, zinc and copper, was measured in the Roseneath shale core, which on water reaction mobilized a higher concentration of dissolved uranium, cadmium, cobalt, chromium, zinc, copper and nickel. Mobilized metal concentrations were however generally <10% of the amount available in the core. Relatively higher concentrations of calcium, magnesium and sodium were mobilized from the Murteree shale. Mobilised uranium and barium however remained low. SEM-EDS indicated that the carbonates were most reactive, with dissolution creating porosity in the Roseneath shale. Fe-rich precipitates were also formed in both cases. We found the presence and type of carbonate and sulphide minerals have a strong control on water chemistry and generated acidity. When reacted with HCl, the Epsilon Formation sample showed strong corrosion of carbonates leaving pits, with no Fe-rich precipitates formed. Experimental fluid-rock interactions can give understanding and insights to the potential chemistry of flow back water which may be different from different shale formations.