Modeling solute diffusion in the presence of pore-scale heterogeneity: method development and an application to the Culebra dolomite Member of the Rustler Formation, New Mexico, USA

Previous studies have revealed the presence of pore-scale variability in di ffusivity in the Culebra (dolomite) member of the Rustler Formation, NM. In this study, right laboratory-scale diffusion experiments on five Culebra s amples were analyzed using a methodology for modeling solute diffusion thro ugh porous media in the presence of multiple matrix diffusivities. D-p. A l ognormal distribution of D-p is assumed within each of the lab samples. The estimated standard deviation (sigma (d)) of ln(D-p) within each sample ran ges from 0 to 1, with most values lying between 0.5 and 1. The variability over all samples leads to a combined sigma (d) in the range of 1.0-1.2. whi ch is consistent with the distribution of independently determined formatio n factor measurements for similar Culebra samples. A comparison of our esti mation results to other rock properties suggests that. at the lab-scale. th e geometric mean of D-p increases with bulk porosity and the quantity of ma croscopic features such as vugs and fractures. However, sigma (d) appears t o be determined by variability within such macroscopic features and/or by m icropore-scale heterogeneity. In addition, comparison of these experiments to those at larger spatial scales suggests that increasing sample volume re sults in an increase in sigma (d). (C) 2001 Elsevier Science B.V. All right s reserved.