Quantitative analysis of colloid retention and pore fluid flow in monolithic, open-porous media by X-ray computed microtomography and magnetic resonance velocimetry

Abstract

X-ray computed microtomography (μCT) is a state-of-the-art technique for the experimental investigation of colloidal transport and retention in porous media. However, quantitative analysis of the amount of retained particles in laboratory CT scanners is problematic due to the polychromatic spectrum of the X-ray sources used, so such investigations are predominantly carried out in synchrotron based scanners. Synchrotron sources are expensive to operate and the measurement time at these facilities is severely limited, so that extensive parameter studies cannot usually take place here.In this work, we present an experimental investigation of deep bed filtration processes in a laboratory scale, conducted in porous, monolithic ceramics, including the quantification of retained particles. Experimental insights were given by an innovative combination of μCT and magnetic resonance velocimetry (MRV). Image analysis was carried using a novel approach for detection and quantification of even very low concentrated colloidal depositions, hereby correcting for beamhardening artifacts. Axial deposition profiles are presented, revealing a hyper-exponential behavior, thus indicating unfavorable attachment conditions and a transport-length dependent distribution of the filter efficiency kf. Due to the monolithic, porous media, further evaluation of colloid deposition is based on individual pore objects. It was found that not all pores contributed equally to colloid deposition, numerous pores even remained empty. It is very likely, that those pores are not well incorporated in the porous network and thus cannot be reached by fluid flow. The structural data of the porous media and colloidal depositions was correlated with 3D velocity information. It could be shown that the filter efficiency for particle retention is a function of time and axial position of the individual pore, and changes over the duration of the process. At a certain point in the filtration process, the rate of change of the volumetric particle fraction per pore becomes negative, predominantly at the media's inlet, while it remains positive throughout deeper pore layers. Thus, the rate of change is governed by a complex relocation process along the porous medium.