Experimental study on the effects of an electric field on the pore characterization in anode, middle and cathode regions of tight sandstone samples

Abstract

Electric field stimulation is more favour for the enhancement of oil recovery from tight sandstones compared to conventional resources, since electroosmotic flow of fluids in smaller pores is stronger. However, the question concerning the character of electric field on the pore structure of tight sandstones (PSTS) remains open, especially the effects in three treated regions: the anode (A), middle (M), and cathode (C) regions. Therefore, in this study, for the first time, several techniques including nuclear magnetic resonance (NMR), high-pressure mercury intrusion (HPMI), and low-temperature nitrogen adsorption (LNA), were combined to explore this topic. First, the T2 relaxation time distribution was converted to the pore size distribution (PSD) through a power-law relation. After that, the changes in the PSTS before and after applying an electric field in the three treated regions were analyzed. The differences in the anode and cathode regions occur in the changes in pore types and pore volume. The reason for this difference is attributed to the effect of electrophoresis and the motion of polar water molecules. Apart from the effect of the dissolution of clay minerals due to the presence of hydrogen ions, the negatively charged particles move to the anode region to fill the pore space, consequently decreasing the number of micropores there. However, the escape of polar water molecules from the anode region increases the number of nanopores in the anode region. This study offers useful information concerning the application of electric fields, which could help electric field-related technologies greatly improve the oil recovery of unconventional resources.