Dissolution of sandstone powders in deionised water over the range 50–350 °C

Sandstone dissolution is a common water–rock reaction in the Earth’s crust, but a thorough understanding of this phenomenon is constrained by poorly determined kinetic data. To this end, kinetic data were determined for the dissolution of arkosic sandstone powders in deionised water (pH was about 7.0–7.3 and electrical conductivity was between 0.95 and 1.00 μS/cm). Release rates of dissolved elements were determined over the range 50–350 °C at 20, 15, and 10 MPa using a column flow-through pressure vessel reactor. The conductivity of the outlet solution, measured at room temperature, is dependent on the charge of major cations such as Na+, K+, Ca2+ and Mg2+ at these conditions. The conductivity of the outlet solution was used to determine the steady state of the dissolution of sandstone powders. The pH values of the outlet solutions at the steady state, measured ex situ at room temperature, were about 7.7, 8.3, 8.4, 8.4 and 7.6 at 75, 100, 150, 200 and 250 °C, respectively, at 10 MPa. Silicon, Na, K, Ca, Al and Mg are the major ions found in the solution at low temperatures, but Si is the only major ion retained at higher temperatures (>150 °C). Compared with static experiments, the flowing dissolution experiments occurred at conditions far from equilibrium. The relationship between temperature and dissolution rates of arkosic sandstone powders was described as log R = 0.005469t − 10.50 where R is the dissolution rates of sandstone powders in kg/(m2 s), t is temperature in °C which ranged from 100 to 350 °C at 20 and 15 MPa, and the dissolution rates of sandstone powders were measured only for the major dissolved elements without oxygen in the outlet solutions. lease rates of Si and K increased sharply at temperatures over the 100–350 °C range. The release rates of Mn and Ni increased slowly at low temperatures (<200 °C) but increased sharply over the 200–350 °C range. The release rates of Na, Ca, Mg and U initially increased, then decreased. The release rates of Fe varied slightly from 100 to 250 °C, but increased sharply at 300 °C and 350 °C. A comparison of release rates of Si and Ca with temperature showed that the release rate of Si was lower than that of Ca at temperatures below 150 °C, but clearly higher at temperatures above 150 °C. This phenomenon can explain previous observations for deep layers of oil field drill cores that feldspar showed dissolution pits but no clear dissolution features were found on the surfaces of carbonate minerals. Apparent activation energies of Si release and K release in these sandstone powders dissolution experiments were derived to be 33 kJ/mol and 20 kJ/mol, respectively, over 75–350 °C and 10–20 MPa. The findings thus provide insights into the dissolution behaviours of sandstone powders in deionised water at conditions that are far from equilibrium.