Multi-scale Optical Analyses of Dynamic Gas Saturation During Air Sparging into Glass Beads

Abstract

A multi-scale optical imaging technique was developed allowing for the 2D observation of two phase flow in porous media at two different scales simultaneously: Using two coupled cameras, a 2D flow cell (0.5×0.5 m 2 ) is recorded entirely at the bench scale and at the pore scale with a spatial resolution of 0.5 and 0.01 mm, respectively. The technique is applied to study channelized gas flow in saturated glass beads. We analyze the phase distribution at the pore scale and derive a pixel-based method for the measurement of saturation at the larger scale. This method assumes linearity between the mean reflected light intensity and the local gas saturation if averaging is performed over representative areas (REV). The REV depends on the irregularity of the local pore structure and has a lower limit at the correlation length of the porous medium (somewhat above the size of the glass beads) and an upper limit which correlates with the width of gas channels. These limits could be quantified through optical analysis. The optical approach to estimate phase saturations was validated by gravimetric analysis where a characteristic ratio between the optically observed flow cell wall and the saturation within the bulk material was identified, which corresponds to the expectation based on geometrical considerations of the glass bead packing. Considering a transient flow experiment the optical method is demonstrated to be able to quantify the temporal evolution of the residual and the convective gas phase. We conclude that the new technique provides a valuable tool to improve our quantitative understanding of multiphase phenomena across different scales.

Pore‐scale and continuum modeling of gas flow pattern obtained by high‐resolution optical bench‐scale experiments

“…In order to compare the experimental gas saturation with the theoretical one, one has to calibrate the gray scale distribution. Assuming linearity that had been proven for the experimental range of flow rates and gas saturations, respectively [ Lazik et al , 2008], the calibration constant is obtained by integrating over the gray scale distribution and by constraining it to the gravimetrically measured steady state gas volumes. Having in mind the excellent agreement of experimental and theoretical results for the V g ( Q ) dependence, the insufficient agreement of the flow pattern is disappointing as shown in Figure 14.…”

“…The most striking result is that the experimental gas saturation profiles are more likely described by a relatively sharp transition from a region with constant gas saturation to a water‐saturated region, than by a smooth Gaussian transition. Obviously, this transition is caused by the channelized flow structure, where the channels have a nearly rate‐independent diameter and a constant gas saturation [see Lazik et al , 2008]. It seems to be thermodynamically more favorable to realize a nearly constant channel density, than a widespread Gaussian one; that is, before spreading in lateral direction the system “searches” for equiprobable channels with the same length.…”

“…Therefore it was possible to calibrate the optical gas saturation by the gravimetrical value. For more details of the experimental setup and multiscale analysis see Gei06, Lazik et al [2008], and Krauss [2006].…”

“…For the comparison between theory and experiment image processing was conducted to the gray scale images obtained by high‐resolution optical bench‐scale experiments. We note that the visualization experiments for the 0.5 mm GBS are published in previous papers; that is, Lazik et al [2008] presented a k space image analysis for injection rates Q = 10, 59, 233, and 844 mL/min investigating the spectrum and identifying different maxima that characterize mobile and trapped gas phases. Here, we conduct a real‐space image analysis for injection rates Q = 10, 59, 146, 233, and 844 mL/min analyzing the upscaled saturation profiles and comparing these with the theoretical ones.…”

On the role of caprock and fracture zones in dispersing gas plumes in the subsurface

“…The macroscopic average value of this residual saturation depends on the detailed dynamics and instabilities of the initial gas displacement front and of the subsequent water imbibition front, since both can lead to trapped pockets of gas [cf. Juanes et al, 2006;Lazik et al, 2008].…”