Diffusion of dissolved CO2 in caprock

Abstract

Diffusion of dissolved CO2 is a possible transport mechanism in caprocks which should be evaluated precisely for long term storage of CO2. It is also a mechanism controlling the caprock geochemical reactivity in the vicinity of the top of the storage where CO2 accumulates driven by gravity. As a first step, we studied the diffusion properties of water in two low porosity caprock samples, giving an upper limit of the diffusion of CO2 dissolved in water. Two methods were used and compared on the same sample: standard through diffusion experiments with tritiated water, and low field NMR experiments with deuterated water. In through diffusion experiments, the sample is placed between two reservoirs; tritiated water is injected in one reservoir and its concentration measured in the other. In the NMR experiments, the sample is immersed in deuterated water and the water concentration inside the sample measured as a function of time. The NMR experiments also provide a measurement of porosity and pore size distribution, gradually invaded by D2O; in the present experiments, we observed that a small porosity fraction could not be exchanged with D2O. Both techniques give a comparable value of pore diffusion, namely 2.4 10−10 m2/s and 1.5 10−10 m2/s respectively for NMR and through diffusion methods. The porosity of the sample was 6%. Finally, we give some arguments for modeling diffusivity, useful for numerical simulators. The effective diffusivity is primarily a function of porosity and we suggest a power law dependence. Due to the similarity between diffusion and electrical properties in porous media, the exponent of the power law can be inferred from existing literature.