Brittleness index of coal from the Upper Silesian Coal Basin

Abstract

A growing interest in the field of coal bed methane (CBM) extraction in Poland shows the demand for rock mechanics data, used to design hydraulic fracturing operations. The elastic response of the rock is typically determined by sonic logging calibrated with laboratory tests. This paper presents the laboratory ultrasonic measurements of the core samples, performed to determine the elastic moduli and brittleness index (BI) of the coal. Tests were performed on 20 core plugs from four coal mines located in the central and southern part of the Upper Silesian Coal Basin (USCB), Poland, characterized by varied maceral composition and mineral additives. The samples were cored out in three directions: perpendicular, parallel, and at a 45° angle to the bedding planes, and tested with the given effective pressure. The majority of the samples were saturated by water with a potassium chloride additive (swelling inhibitor). A P- and S-wave velocity upward trend was observed when the mineral content in the samples increased. Elevated velocities in samples of high mineral content resulted in exceeding the Ed to vd limits for coal as proposed in literature. With increased BI, upward trends in the liptinite and inertinite content as well as a downward trend in the vitrinite content were observed. The dynamic elastic moduli of the measured samples were compared to the available literature data.