Permeability: mineralisation

There are some differences between low permeability and high permeability coal seams. These differences can be related to the presence of some specific pore and cleat fillings such as mylonite, the development of cleats and their mineralisation, and the mode of occurrence of minerals in coal macerals. Successful drainage and a suitable rate of gas flow through the coal can be influenced by coal microstructures, especially micro-cleat openings and mineral matter. In good drainage and high permeability coal seams, the micro-cleats are mostly empty, or only partly mineralised.

In coal seams with negligible quantities of mineral matter, the coal-bed gases flow and initially continue to flow when the pressure is lowered below the desorption pressure. The coal matrix shrinkage as a result of gas desorption may cause greater cleat openings than the effective stress. Titheridge (2004) stated, “In mineralised coal, the presence of calcite (or other minerals) in cleat or fractures adds an additional factor to the initial and subsequent drainage process. Mineralisation blocks cleat and fracture permeability routes that would otherwise transport gas”. Thus, an increased presence of mineral matter in coal would cause a reduction in coal permeability, and the degree of permeability will be proportional to the extent of mineralisation. Furthermore, mineral matter impedes the gases from leaving their place by affecting the desorption and shrinkage properties of the coal matrix.

Gamson, Beamish and Johnson (1993) study on Australian coals found that the amount of fracture infilling with minerals was one of the factors which influenced the effectiveness of methane flow through the coal matrix. They also noted that mineral matter such as clay, calcite and quartz block the methane flow path through cleats and interconnected pores by forming a compact amorphous or crystalline structure. The size of infillings influences gas diffusion as well as laminar flow in the coal matrix. Later Gurba (2002b) described her microscopic studies of some Australian coal samples and found that the Bulli seam had two different sets of cleats. One set of cleats is open and the other mineralised. Microscopic studies on coal samples from West Cliff Colliery showed micro-cleats totally mineralised by carbonates. Siderite nodules (Iron Carbonate) in the cleats were observed to cause difficulty in drilling and in drainage. Mylonite is also present in West Cliff coal samples, and mylonitic type coal could be prone to outbursts (Gurba, 2002a). Additionally, microscopic examination of the coal samples from difficult drainage areas has shown that the presence of mylonite in micro-cleats is also likely to cause difficulties in gas drainage. As revealed by electron microprobe analysis the mylonite in micro-cleats is cemented by calcite, dolomite or kaolinite. In the coal samples from Central Colliery in the Bowen Basin that were collected from the low permeability area and outburst prone zone, the cleats were totally filled with calcite. In Appin Colliery coals, carbonates were present in the cleats as well as mylonite, which was cemented by carbonates so that there was not much space for gas flow. Titheridge (2004), who did extensive work on Tahmoor Colliery Bulli coal and its calcite mineral matter, postulated that high fluid pressure was the major factor responsible for the fibrous veins in coal (sedimentary rock). He stated, “The origin of high fluid pressure was primarily due to the fluctuating NE-SW tensional – compressive stress field that was present during the burial phase of the Southern Sydney Basin”. Calcite in Tahmoor coal was formed from the combination of CO2 and water, for which one of the CO2 resources was magmatic.