Pore Characteristics of Distinct Thermally Mature Shales: Influence of Particle Size on Low-Pressure CO2 and N2 Adsorption

Abstract

The influence of crushed sample particle size on low-pressure gas adsorption and desorption behavior of shales and their measurement is an issue of significant current interest in this new era focused on shale gas and oil resources. Here we study two samples of distinct Indian shales, with different organic contents, ages, levels of thermal maturity, and pore-size distributions crushed to four different particle size ranges [S1 (1 mm to 500 μm), S2 (500–212 μm), S3 (212–75 μm), and S4 (75–53 μm)]. Low-pressure gas adsorption analysis with nitrogen and carbon dioxide gases reveals significant and complex impacts of particle-crush sizes on the measured pore structure characteristics for the two shales. The CO2 results suggest that at the smallest (S4) particle-crush size evaluated, low-pressure gas adsorption measurements record more, finer nanopores (i.e., less than about 8 Å), fewer larger nanopores (i.e., greater than about 8 Å), and a lower overall nanopore surface volume. The N2 results show an overall increase in macro-pore volume at the smallest particle-crush size. The results imply that while more, smaller pores are exposed to gas adsorption at the smaller crush sizes, a significant number of nanopores are in some way altered and are not recorded as part of the measured nanopore-size distribution >8 Å by low-pressure CO2 adsorption analysis. Fractal dimensions of one shale varied across a range of particle-crush sizes, whereas the fractal dimensions of the other shale studied did not. The analyses suggest that low-pressure gas adsorption results conducted with samples of very small particle-crush sizes should be viewed with caution.