Investigation of Biofilm Growth within a Monodisperse Porous Medium under Fluctuating Water Level Assessed by Means of MRI

Microorganisms settle in diverse partially saturated porous media in the form of biofilms. The alteration of hydraulic properties and diffusive transport processes occurs simultaneously with biofilm growth in porous media. Imaging methods offer the ability to directly visualize and quantify alterations on the pore scale. However, imaging methods have mainly observed biofilm growth in completely saturated porous media. The current study used magnetic resonance imaging (MRI) to dynamically visualize biofilm growth within a porous medium under alternating drainage and flushing events. Prior to the MRI experiments, the sample was cultivated for 6 days within a porous medium consisting of 2 mm glass spheres. Starting from day 6, growth was monitored using MRI over a period of 7 days. The approach allowed for a visualization of all fractions (biofilm, water, air, and porous material) after drainage as well as flushing events. Biofilm was found to preferentially grow within permanently wetted areas situated next to pore throats. Furthermore, an increase in the water retention and connectivity of the liquid phase was found. The largest liquid cluster covered 11% (day 6) and 91% (day 12) of the total retained water, suggesting that biofilm growth might improve diffusive transport processes within partially saturated porous media.

Advances and benefits of fractal models to predict streaming potentials in partially saturated porous media

<p>Understanding streaming potential generation in porous media is of high interest for hydrological and reservoir studies as it allows to relate water fluxes to measurable electrical potential distributions in subsurface geological settings. The evolution of streaming potential <span>stems</span> from electrokinetic coupling between water and electrical fluxes due to the presence of an electrical double layer at the interface between the mineral and the pore water. Two different approaches can be used to model and interpret the generation of the streaming potential in porous media: the classical coupling coefficient approach based on the Helmholtz-Smoluchowski equation, and the effective excess charge density. Recent studies based on both approaches use a mathematical up-scaling procedure that employs the so-called fractal theory. In these studies, the porous medium is represented by a bundle of tortuous capillaries characterized by a fractal capillary-size distribution law. The electrokinetic coupling between the fluid flow and electric current is obtained by averaging the processes that take place in a single capillary. In most cases, closed-form expressions for the electrokinetic parameters are obtained in terms of macroscopic hydraulic variables like permeability, saturation and porosity. In this presentation we propose a review of the existing fractal distribution models that predict the streaming potential in porous media and discuss their benefits compared against other published models.</p>