Evaluating free gas in coal measure sandstones: a hybrid geophysical logging approach

Abstract Gas management in underground coal mining remains a critical challenge, particularly with respect to free compressed gas in sandstones within coal measures—a factor less considered in current mining practices. The free gas can quickly escape and migrate into the working face from the roof/floor, complicating its measurement and leading to additional specific gas emissions (SGE). This study proposes a hybrid method that uses geophysical logs to estimate free gas within these sandstone formations under constrained conditions. The method involves parameters derived from empirical models and calibrated with data-driven techniques, along with the 4 log-based indicators for gas presence; its effectiveness was demonstrated through the application in 4 boreholes at a case study coal mine, known for the occurrence of free gas within the sandstone formations. The gas indicators via empirical and analytical methods collectively filtered out 20–100% non-gassy zones on top of gas estimates, meanwhile providing uncertainties to account for the best and worst scenarios. The results suggest the second borehole (BH2) to be notably gassy (0.3–1.0 $${m}^{3}/t$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> <mml:mo>/</mml:mo> <mml:mi>t</mml:mi> </mml:mrow> </mml:math> at maximum) within its sandstone formations consistent with the general field observations in the area. This hybrid approach offers a valuable enhancement to gas content estimation, enabling a more detailed evaluation of coal measure sandstones, improving gas risk management, and enhancing SGE prediction accuracy for the mining industry.