Characterization of unsaturated diffusivity of tight sandstones using neutron radiography

Water flow in unsaturated tight sandstones plays a significant role in the area of the secondary and enhanced hydrocarbon recovery as well as the geological storage of carbon dioxide and nuclear wastes. Although a few non-destructive techniques, such as nuclear magnetic resonance, magnetic resonance imaging and X-ray imaging, can be used to capture fluid transport in sub-micron pores, great challenges exist due to the presence of iron or the use of the contrast agents (e.g. cesium chloride and salts), resulting in inaccurate results or alteration of the wetting behavior of the porous media. In addition, models for describing diffusivity and water transport in unsaturated tight sandstones are also limited. In this work, the neutron radiography facility at China Advanced Research Reactor was used to determine water content profiles during the water imbibition in two types of tight sandstones: silty sandstone and coarse grained sandstone. The diffusivity was determined separately by three methods, including Matano's method, Meyer-Warwick method and a fractal method, which was introduced as probably the first attempt to relate the microstructure observed by the high resolution X-ray computed tomography (CT) with the unsaturated diffusivity function for the tested tight sandstones. The air-entry value and the fractal dimension used in the fractal model were calculated based on the results of mercury intrusion porosimetry and CT data, respectively. The results from neutron images illustrate that the fractal model can give a reasonable description of the diffusivity function for the tested sandstones. Meyer-Warwick model produces a little bit higher diffusivity value at low water content range. The fractal model works better for the silty sandstone. Results also show that the value of water diffusivity increases with the increase in volumetric water content for both tested tight sandstones. This work shows that neutron radiography offers a feasible and more reliable way for characterizing fluid flow in other tight geo-materials and the fractal model also provides an easier way to give a quantitative description of the diffusivity than the core-flooding or centrifuge drainage experiment.