Pore Scale View of Fluid Displacement Fronts in Porous Media

Abstract:

The macroscopically smooth and regular motion of fluid fronts in porous media is composed of abrupt pore-scale interfacial jumps involving intense interfacial energy release marked by pressure bursts and acoustic emissions. The characteristics of these pore scale events affect residual phase entrapment and the resulting unsaturated transport properties behind the front. Experimental studies using acoustic emissions technique (AE), rapid imaging, and pressure measurements help characterize pore scale processes during drainage and imbibition in model porous media. Imbibition and drainage produce different AE signatures (obeying a power law). For rapid drainage, AE signals persist long after cessation of front motion indicative of redistribution and interfacial relaxation. Rapid imaging revealed that interfacial jumps exceed mean front velocity and are highly inertial (Re>1000). Imaged pore invasion volumes and pore volumes deduced from waiting times between pressure fluctuations were in remarkable agreement with geometric pores. Differences between invaded volumes and geometrical pores increase with increasing capillary numbers due to shorter pore evacuation times and onset of simultaneous invasion events. A new mechanistic model for interfacial motions through a pore-throat network enabled systematic evaluation of inertia in interfacial dynamics. Results suggest that in contrast to great sensitivity of pore scale dynamics to variations in pore geometry and boundary conditions, inertia exerts only a minor effect on average phase entrapment. Pore scale invasion events paint a complex picture of rapid and inertial motions and provide new insights on mechanisms at displacement fronts essential for improving the macroscopic description of multiphase flow in porous media.