Pore-scale displacement processes and relaxation dynamics in multiphase flow imaged at sub-second temporal resolution

Abstract:

With recent advances at X-ray micro-computed tomography (μCT) synchrotron beam lines, it is now possible to study pore-scale flow in porous rock under dynamic flow conditions. The collection of 4 dimensional (4D) data allows for the direct 3D visualization of fluid-fluid displacement in porous rock as a function of time. However, even state-of-the-art fast-μCT scans require between one and a few seconds to complete and the (much faster) fluid movement occurring during that time interval is manifested as imaging artifacts in the reconstructed 3D volume. We present an approach to analyze the 2D radiograph data collected during fast-μCT to study the pore-scale displacement dynamics on the time scale of 40 milliseconds which is near the intrinsic time scale of individual Haines jumps. We present a methodology to identify the time intervals at which pore scale displacement events in the observed field of view occur and hence, how reconstruction intervals can be chosen to avoid fluid-movement induced reconstruction artifacts. We further quantify the size, order, frequency, and location of fluid-fluid displacement at the millisecond time scale. We observe that after a displacement event, the pore scale fluid distribution relaxes to (quasi-) equilibrium in cascades of pore-scale fluid re-arrangements with an average relaxation time for the whole cascade between 0.5 and 2.0 seconds. These findings help to identify the flow regimes and intrinsic time and length scales relevant to fractional flow.