Measurement of CO_2 diffusivity in synthetic and saline aquifer solutions at reservoir conditions: the role of ion interactions

摘要

Storage and disposal of CO_2 as the main component of greenhouse gases in saline aquifers require careful measurement of diffusivity for predicting rate of transfer and cumulative amount of trapped gas. Little information is available on diffusion of CO_2 in highly concentrated saline aquifers at reservoir conditions. In this study, diffusivity of CO_2 was measured into different solutions, including saline aquifer taken from oil field, distilled water and synthetic solutions prepared from four most common ions, Mg~(2+), Ca~(2+), K~+, Na~+. The roles of salvation effect and hydration phenomenon were studied on diffusivity of dissolved CO_2. Synthetic solutions were prepared at concentration ranges of 83-200 g/l. Experimental measurements were reported at temperature and pressure ranges of 30-40 ℃ and 5,880-6,265 kPa, respectively. Results show that both type and concentration of ion affect CO_2 diffusivity. Diffusion coefficient was found dependent on effective radius of hydrated ions. Also, CO_2 diffusivity increase by increasing strength of bonds between ion and neighbor water molecules. Also, presence of ions in water solution creates hydration competition between solution metal ions and aqua ions from diffusive gas. The Mg~(2+) cation, which has strongest hydration competition among other ions, has an increasing effect on gas diffusivity into saline aquifer. However, increasing ion concentration in solution decreases diffusivity of CO_2 due to growth in fraction of contact ion pairs. Results of this study provide unique measures of CO_2 diffusion coefficient in saline aquifer at high pressure and temperature conditions and conceptual information about effect of each common saline formation ion on gas diffusivity.