A rapid method for determining apparent diffusion coefficients in Chalk and other consolidated porous media

Abstract

The development of a method for the determination of the apparent diffusion coefficient, DA, for chloride in saturated Chalk cores is described. The method is rapid compared with other approaches, taking typically less than 24 h for a single determination. Cylindrical Chalk cores approximately 25 mm high by 25 mm in diameter, which are routinely used in porosity and permeability measurements and which had been pre-equilibrated with a 200 mg/L chloride solution, were sealed at both ends and attached to a slowly rotating spindle suspended in a reservoir. A chloride ion-selective electrode (ISE) connected to a data logger was used to record chloride diffusion out of the core. DA was estimated by analysing the change in chloride concentration in the reservoir with time. Diffusion coefficients were estimated for six Chalk samples from a range of Chalk lithologies. Sample porosities for these Chalks ranged from 32% to 48% and gas permeabilities from 0.3 to 8.2 × 10?9 m2. The DA was found to vary from 3.1 to 8.7 × 10?10 m2/s, a similar range to that observed by others. A bromide ISE was also used on one sample and found to give a similar DA to that obtained for chloride. This approach, which combines a rigorous mathematical model of diffusion with a relatively simple practical method, could easily be adapted for other ions and for other consolidated porous media.