Pore structure characteristics and fractal dimension analysis of low rank coal in the Lower Indus Basin, SE Pakistan

The significance of pore structure and fractal characteristics is important for understanding untapped Coalbed Methane (CBM) exploration. In this study, these features of low rank coal in the lower Indus Basin, SE Pakistan, were evaluated using drill core samples from several locations. The pore size distribution and fractal dimension on adsorption (AdDs) and desorption (DeDs) values were systematically analyzed by scanning electron microscopy (SEM), low pressure adsorption N-2, and CO2 methods for better evaluation of CBM and its pores geometrical mechanism. Coal petrographic properties, were investigated for macerals composition revealed a huminite domination. SEM analysis of various pore structures, showed first-developed pores present in the larger pores and thermogenic gas pores. CO2 density functional theory revealed micropore particle size peaks in three adsorption phases on incremental pore volume (0.47-0.64 nm,0.64-0.83 nm, 0.83-11.11 nm), where gas molecules distort bonding with increasing pore size and define the micropore surface complexities. The mesopore size distribution analysis of differential and incremental pore areas indicated a transition zone at 22.8 nm, suggesting major micro-mesopores within smaller size particles. Nonhomogeneous particle sizes caused a dynamic range of peak trends due to different kinds of porespheres are interconnected with macrosphere, which can trap accessible and inaccessible gas molecules in the different pores structures. Fractal dimension analysis demonstrated that Ds values linear correlation showed a good fit for the AdDs value was 0.98-0.99, and 0.96 to 0.99 for the DeDs value. This finding showed that average obtained surface complexity of AdDs was 2.35, and the DeDs was 2.44 with notably more complicated pore roughness and high semifusinite content, possibly resulting in the higher DeDs value because larger pore spheres consist of microspheres.