Investigation of the mechanical behaviour of brine saturated brown coal under uni-axial compression
Coal seams are typically saturated with brine with varying NaCl concentrations and can be subjected to irreversible formation damage due to a number of physical and chemical interactions. Understanding the mechanical behaviour of a brine-saturated coal mass is thus important, when conducting operations on coal seams such as ones in-cooperated with hydraulic fracturing and CO2 sequestration projects. This paper investigates the mechanical behaviour of brown coal, saturated with varying NaCl concentration (i.e. 0%, 10% and 20% NaCl by weight) and subjected to monocyclic uni-axial compression. The coal volumetric strain, evaluated from ARAMIS photogrammetric analysis shows that the samples saturated at 0% and 10% brine concentrations exhibit a dilatancy behaviour because of the softer nature and induced high fracture density, which has caused a significant deformation in the coal mass by widening existing fractures and forming new fractures during compression. This can be happened due to significant growth of secondary cracks from pre-existing cracks and initial pores. The coal mass weakening due to softening at 0% (i.e. water) saturation and corrosive chemical interactions at 10% saturation under favourable chemical environment can cause these secondary crack propagations. The micro-CT image data of the corresponding failed samples shows a high fracture density, which confirms the formation of new cracks causing this dilatancy behaviour. In contrast, at 20% brine concentrations, compaction is the main mechanism of sample deformation, where the volumetric strain increases with increasing axial stress until the failure. However, the corresponding micro-CT images of the failed samples show a much denser fracture network at failure, which infers the initiation of micro-cracks during loading due to higher brittleness of the samples. The overall results conclude that the mechanical behaviour of coal mass can significantly be varied with the brine concentration of the saturation fluid and, thus be carefully evaluated when designing reservoir stimulation projects.