ANALYSIS OF PERMEABILITY CONTROLS ON TRANSPORT THROUGH LABORATORY-SCALE CROSS-BEDDED SANDSTONE

Heterogeneities occur over a range of spatial scales, each with potential to influence patterns of subsurface flow and transport. Understanding such controls is fundamental to developing accurate models for design, forecasting or prediction. Here we integrate highly resolved permeability and porosity measurements, visual rock attributes and numerical modeling to explore the effects of small-scale heterogeneity on solute transport. A 30 cm by 30 cm by 2 cm slab of Massillon Sandstone was selected for the research due to the distinct cross bedding and nested scales of heterogeneity.

Conservative solute transport experiments were conducted with potassium iodide to study the effects of heterogeneous permeability and porosity on solute transport. Experiments are run both perpendicular and parallel to cross-bedded features of the sandstone. Porosity and tracer distribution at sub-millimeter resolutions are obtained through x-ray adsorption imaging. Permeability heterogeneity was characterized by 8649 measurements collected with a gas permeameter using a 1.5 mm inner tip seal radius. The measured heterogeneity is used to numerically simulate experimental tracer tests. Simulations are run with MODFLOW and numerical results are compared to experimentally derived tracer distributions.

Initial results yield poor comparisons between the measured and modeled tracer distributions. A thorough investigation of potential experimental errors, including boundary conditions, permeability measurements, and tracer injection distributions, suggest that the problems may result from permeability heterogeneity at scales smaller than can be resolved with the 1.5 mm tip seal. Numerical models show that boundaries between cross-stratified beds have a large impact on fluid flow, and that lower permeability layers along these boundaries may account for the fluid transport seen in the experiments. Thin sections show concentrations of hematitic cement and clays in these regions, which are oriented parallel to the direction of gas permeameter injection, possibly biasing permeability measurements.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.