Permian shales of Barakar formation in India were investigated to study their pore structure to understand their potential for natural gas production and possible CO 2 sequestration. The studied shale samples with variable clay content were of early mature stage and contained low (<2%) total organic carbon. Initially, a combination of small-angle neutron scattering (SANS) and low-pressure gas adsorption (LPGA) was used to identify the pore sizes and fractal dimensions of Indian shales. It was found that the quenched surface density functional theory model in the LPGA method gave better pore size distribution (PSD) estimates over the nonlocal density functional theory model. The micropores and smaller mesopores contribute the most to the total pore volume and the surface area of the studied shale samples. The average pore size decreased with an increase in pore volume. The fractal studies using SANS reveal that all studied shales possess similar fractal dimension despite being different in mineralogy, maturity, and total pore volume. The PSD and its possible relation with the mineral composition and the accessibility of the pores in terms of gas storage have been elucidated. Pore morphology was analyzed using image analysis of field emission scanning electron microscopy and low-pressure adsorption, corroborated by SANS results. The effects of dissolution and deposition probability on the fractal dimension of the shale were interpreted using the Monte Carlo-based computer modeling. The fractal dimension was higher in the case of shales that underwent simultaneous dissolution and deposition processes.
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 CO 2 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 N 2 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 CO 2 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.
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