Fractal and pore dispositions of coal seams with significance to coalbed methane plays of East Bokaro, Jharkhand, India

In India, much attention has been paid to the Gondwana coals of Permian age of Damodar valley, which comprises the major CBM producing blocks. East Bokaro Coalfield is part of Damodar valley and one of the promising potential CBM plays. In this study, the technological properties, TOC content, thermal maturity (Ro%), low pressure N2 sorption isotherm and SEM analyses were performed to find out the significance of fractal dimension, pore dispositions and associated geological influence on pore structure and fractal dimensions for better understanding of gas adsorption capacities of coal seams of East Bokaro. The micropore and mesopore specific surface area obtained through DR and multi-point BET ranges from 0.18 to 8.86 m2/g and 0.42–7.41 m2/g respectively, signifying dominance of micro and mesopores involvement in specific surface area. The BJH and DFT pore volume varying from 0.002 to 0.007 and 0.001–0.007 cc/g respectively. It is observed that the considerable pore volume contributed by mesopores of the range 3–50 nm and least fractions of macropores ranges from 50 to 278 nm. There is a tremendous increase in pore volume, when the pore size in between 4 and 16 nm, these pores are particularly more suitable for gas storage and associated with carbon rich vitrinite maceral. The pore size distribution obtained through BJH, DFT, DA, DR and average pore diameter values ranges from 2.99 to 10.20, 1.77–3.17, 1.42–2.48, 0.46–3.28 and 4.48–16.43 nm respectively. The opening of the hysteresis loop at lower pressure ∼0.2 (P/P0) and closing of hysteresis loop between 0.6 and 0.8 (P/P0) acutely connects to the adsorption branch is revealing of cavitation and irregularities in pore structures, consequently rapid decline in desorption curve due to the condensation and evaporation characteristics of micro and mesopores, often associated with combined slit and bottle neck pores and bottle-neck open ended shaped mesopores. The variations in correlation coefficients of two separate linear segments of fractal dimensions (D1 and D2), summarizes the suitability of D1, for pore structure analysis. Nevertheless, the low values of fractal dimensions (D1 = 1.1600–2.5450 and D2 = 1.5580–2.8730), signifies the pore surfaces are controlled by heterogeneity of coal composition. However, the correlations of D2 with ash, average pore diameter, specific surface area, Ro%, pore volume and depth accentuated that D2 may be considered for CBM reservoir characterization of East Bokaro Coalfield. The very good correlations between depth with D2, total pore volume and thermal maturity (Ro%) signifies the intensification of fractal dimensions as a function of reduction and variations in pore diameter with increasing degree of coalification. The effects of rank upon fractal dimensions are mainly due to structured variety of micropore and mesopore of coals. SEM studies supports the variations in surface morphology, pore structure and fractal dimension controlled by complex coal compositional parameters like carbon content, volatile matter, thermal maturity and pore size distribution