Full-scale nanopore system and fractal characteristics of clay-rich lacustrine shale combining FE-SEM, nano-CT, gas adsorption and mercury intrusion porosimetry

Abstract

Shale pore structures and irregularities are significant for gas adsorption and interstifial flow. High complexity and nonuniformity, wide pore scales, and multiple morphology hinder the thorough characterization of multi-scale pore structure and heterogeneity in shales. In this work, the geochemistry, pore structure and fractal characteristics of the Upper Cretaceous Nenjiang shale in the Songliao Basin of NE China were investigated, combining geochemistry experiments, physical property analysis, FE-SEM and nano-CT image observation, CO2/N2 gas adsorption, mercury intrusion porosimetry, and methane methane sorption analysis. The results show that the Nenjiang shales are low-mature (Ro of 0.55%–0.93%) and rich in organic matters (OMs) and clays. The size diameters of shale pores are generally distributed in sizes of 0.3–0.8, 1.4–4.5, 80–600 nm and 10–80 μm. Shale pores have strong heterogeneity and complexity, good connectivity and openness, mainly with inkbottle- and slit-shapes. Total pore volume (PV) is positive correlate with specific surface area (SSA), porosity and permeability, but negative correlate with the average pore size. The development of micro-, meso-, and macro-pores were dominated by the contents of OM, clays, and quartz, respectively. Positive correlations were found among fractal dimension, contents of OMs and clays, total PV, SSA, and methane adsorption capacity, which would provides better understanding on reservoir assessment and shale gas storage capacity.