Characterization of composite coal seam III–IV of Ramagundam coalfield, Godavari Basin (India): an attempt to evaluate CBM prospects

Abstract

In the present investigation, composite coal seam III–IV of the Ramagundam coalfield of the Godavari Basin has been studied through scanning electron microscopy (SEM), optical microscopy, Fourier transform infrared spectroscopy (FTIR), proximate and ultimate analyses, porosimetry and permeametry. Vitrinite reflectance ($\overline{R }$ _r from 0.38 to 0.49%) and proximate parameters put sub-bituminous rank to this coal. Vitrinite (48.3% mmf basis) and inertinite (37.2% mmf basis) are abundant while liptinite (14.5% mmf basis) occurs in subordinated amount. Cleats and the fractures are more in the vitrain band but they are partially filled with mineral matter. Dead-end types of pores are common. Open and unmineralized cell pores and cavities in the fusain band offer adsorption sites and micro-permeability, but at places, they are broken, homogenized and mineral filled, which eventually reduce the permeability. Porosity (1.88–2.75%) and permeability (0.017–0.029 millidarcy, mD) substantiate the availability of adsorption sites but poor methane migration in coal seam. The compressive strength value (24.28–29.55 megapascal, MPa) gradually increases from the top towards the bottom of the coal seam and leads to compaction, and lesser permeability and porosity in the coal seam. The elastic modulus value suggests less cleat development (face and butt cleat). The studied coals are thermally less mature and yet to approach the oil window. Most of the samples show a volatile matter content more than 37.8%, a value below which thermogenic methane generation begins. Nevertheless, FTIR supports a moderate hydrocarbon-generating potential. Besides, the biogenic gases are likely to occur in variable quantity.