Chlorite dissolution rates under CO2 saturated conditions from 50 to 120 °C and 120 to 200 bar CO2

Abstract

Chlorite dissolution rates were measured in a series of batch reactor experiments testing the effect of pCO2, pH, chloride and bicarbonate concentrations and temperature. Chlorite is an important diagenetic mineral in sedimentary basins, often found cementing mineral grains and filling pore space in formations that may serve as reservoirs for storing carbon dioxide. Conflicting reports of whether chlorite acts as a barrier to reservoir rock reactivity or leads to enhanced porosity due to dissolution, after the injection of supercritical CO2 into a reservoir, makes studying the reactivity of chlorite in contact with CO2 saturated waters pertinent.Measured dissolution rates were initially rapid and decreased over time as the saturation state of solution relative to chlorite increased. Temperature had the strongest effect on dissolution rate, with an apparent activation energy of 16±0.5kJmol−1 and rate constant of logk0=−9.56±0.07molm−2s−1 assuming a rate law of the form: rate=k0exp(−EA/RT). The apparent activation energy is lower than previously accepted values, but is consistent with a study of chlorite dissolution using flow through techniques (Smith et al., 2013). Mineral dissolution rates are typically proton enhanced, but the lack of a significant pH effect or pCO2 effect on chlorite dissolution rate in this study suggests that the use of NaHCO3 to buffer the pH of CO2 saturated solutions led to an inhibition of mineral dissolution in competition with the expected pH effect. This is supported by the observed dissolution rate increasing dramatically (half a log unit) with the use of an organic acid buffer (KHpthalate) under CO2 free conditions. The effect of chloride (NaCl ∼5 to 50g/L) was found not to affect the dissolution rate of chlorite. Various empirical rate laws are proposed and fit to the data and lead to the development of a surface complex model describing proton promoted dissolution and bicarbonate inhibition of chlorite dissolution rates. The model can be applied to predict the rate of chlorite dissolution under elevated pCO2 conditions relevant to the storage of CO2 in reservoirs from 50 to 275°C in contact with fluids ranging in pH from 3.4 to 5.4.