Comprehensive characterization of pore and throat system for tight sandstone reservoirs and associated permeability determination method using SEM, rate-controlled mercury and high pressure mercury

Abstract

Unconventional resource formations like tight sandstone reservoirs have complex pore and throat structure that are hard to characterize in micro scale. In this research, the pore and throat system of tight sandstones was characterized comprehensively, analysed qualitatively using scanning electron microscopy (SEM) techniques and quantitatively through combined high pressure mercury (HPM) and rate-controlled mercury (RCM) methods. First, the throat, distinguished from the pore, was divided in several types for tight sandstones and visualized through SEM imaging. Then, an integrated approach was proposed to combine the results from both HPM and RCM measurements, resulting in overall size distributions of the pore and the throat with scales from nanometer to micrometer. Both overall size distribution using the new combining method represented the pore and the throat more accurately in wider ranges. The results also showed that for tight sandstone core samples with different permeability, the difference of pore size distribution was less obvious than that of the throat. Therefore, the quantitative characterization of the throat size distribution was ultimately employed in developing a new permeability model together with physical considerations. The results demonstrated that the permeability of tight sandstones was in general depend on the size distribution of relatively larger throat, denoted as the macrothroat. However, for extreme low permeability (<0.1mD) formation, the small throats like microthroats and mesothroats mainly contributed to the permeability since the contents of macrothroats are really low.