Analysis of micropore size distribution using Dubinin's theory of volume filling - Effect of particle size on pore characterization of organic-rich Indian shales

Abstract

Micropores play a significant role in adsorption and are abundant in a shale pore structure. The influence of particle size on pore structure parameters, especially micropores, is a subject of primary interest for shale gas characterisation. In this work, a total of 6 samples with varying mineralogical and organic content are selected from 2 Indian sedimentary basins, Cambay and Krishna-Godavari. Selected samples were crushed to different particle sizes 500-425 μm (M1), 425-250 μm (M2), 250-150 μm (M3), and 150-75 μm (M4) and low pressure N₂ and CO₂ adsorption experiments at 77 K and 273 K were performed on them. Through these experiments, it was observed that, with crushing of samples to smaller particle sizes low-pressure N₂ adsorption tend to show higher values for total pore volume and average pore diameter. The mean difference over all samples for average pore diameter between M4 and M1 particle size is closer to 1 nm. Similarly, the mean difference over all samples for total pore volume between M4 and M1 particle sizes of all samples is 0.005 cc/g, which is almost closer to their micropore volumes. Such inconsistencies in measurements can lead to serious errors in shale gas exploration. Further, the low pressure N₂ experiments indicated that M4 particle size samples show higher values of pore structure parameters, and low pressure CO₂ experiments show that M3 particle size shows higher micropore volumes. For further understanding of nano-sized pore contributions, micropore size distributions of the samples have been constructed using Stoeckli pore size distribution function. From these distributions and other analysis, it was concluded that the smallest particle size overestimates the pore structure parameters and M3 particle size is optimum for both low pressure N₂ and CO₂ adsorption.