Multiscale evolution observation of the pore structures of porous media under an electrokinetic treatment

Direct current (DC) stimulation is a promising electrokinetic method for enhancing oil recovery in porous media, especially in tight sandstones, due to the more remarkable effect of electroosmosis in materials with narrower pore throats. However, an interesting reversal of enhanced oil recovery (EOR) occurs in which oil recovery begins to decrease with DC voltages; this phenomenon hinders the application of this electrokinetic technology in petroleum industry. To date, the reason for this phenomenon remains unclear. Thus, in this study, we applied nuclear magnetic resonance (NMR) and in-situ scanning electron microscope (SEM) methods to investigate the underlying mechanisms. We classified and inspected the evolution of pore structures in tight sandstones at multiple scales ranging from the nanoscale to micron scale as the strength of DC voltage increases. The effects of both low DC and high DC voltages were analyzed in detail. Results show that the size and number of nanopores increased as the DC voltage increases. A low DC voltage can transform submicropores to micropores, and a high DC voltage can fasten this transformation. Furthermore, a low DC voltage increased the number and size of micropores, but a high voltage decreased the number and size of micropores. This study provides insights into changes in the pore structures of reservoir rocks under the treatment of the DC voltage, helping develop DC voltage-related technologies to recover hydrocarbons from porous media.