Lawrence Berkeley National Laboratory

Numerical simulations of carbon dioxide (CO2) injection into a sandstone reservoir (∼2 km depth) were used to investigate the geochemical effects of trace amounts of mercury (Hg, 7 and 190 ppbV), with and without hydrogen sulfide (H2S, 200 ppm). Geochemical reaction-path modeling shows that cinnabar precipitates as soon as the Hg-bearing CO2 reacts with the formation. Mercury does not condense to liquid, and the net volume change from mineral dissolution and precipitation is found to be negligible. Two-dimensional radial reactive transport simulations of CO2 injection at a rate of 14.5 kg/s (∼0.5 Mt/y) into a 400-m-thick formation at 106°C and 215 bar, with varying amounts of Hg and H2S, show that porosity changes only by about ±0.05% absolute (i.e., new porosity% = initial porosity% ±0.05), and that Hg readily precipitates as cinnabar in a zone mostly coinciding with the single-phase CO2 plume. This essentially negligible porosity change is not expected to affect permeability and CO2 injectivity. The precipitation of minerals other than cinnabar dominates the evolution of porosity. Although the predicted porosity change is small, the dissolution and precipitation predicted for individual minerals is not negligible. The main reactions include the replacement of primarily Fe-chlorite by siderite, of calcite by dolomite, and of K-feldspar by muscovite. Chalcedony is also predicted to precipitate from the dissolution of feldspars. Except for some replacement of pyrite by ankerite when H2S is deficient, the cases with and without H2S show similar results. Experimental measurements are needed to decrease uncertainty in simulation results.