LASER ABLATION LA-ICP-MS TO DERIVE DIFFUSION COEFFICIENTS IN SANDSTONES

Diffusion is an important transport process at small scales in porous media. Diffusion coefficients are necessary for predicting and modeling diffusion. They are functions of both the rock framework and the chemical species diffusing into it. In the past, generic values found in literature or from using empirical equations have been used to estimate diffusion coefficients. To test these assumptions, we use laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to trace cation diffusion in sandstone, similar to experiments performed by Wood (1993) and Garner (2007) on granites. Diffusivity of SrCl2 in CaCl2-saturated Hinckley and Breathitt Sandstone slabs, with and without fracture skins (altered rock fracture surfaces that may change diffusion rates into the rock), was tested experimentally as a function of time and ionic strength. Treated samples, bisected using a dry saw and dried, were then analyzed by LA-ICP-MS, the latter involving a series of diffusion-normal transects (15 mm line scans) at 2 mm spacings. Preliminary results for varying the time of diffusion indicate that concentration gradients decrease exponentially with increasing sample depth, in accordance with predicted distributions. Observations of some preferential flowpaths suggests that LA-ICP-MS is also useful in determining the effects of rock porosity and permeability on diffusion pathways. This method allows us to use actual representative rock samples and consider the effects of thin fracture skins, which is not possible with other methods of measuring diffusion.