A study into the effect of cleat demineralisation by hydrochloric acid on the permeability of coal

Mineral occlusions in cleats are known to considerably reduce coal permeability. Sequential steady state core flooding experiments with aqueous hydrochloric acid (HCl) solutions were conducted on whole core samples from the Bowen Basin, Australia, to assess the effect of mineral dissolution on core permeability. Cleat minerals were characterised by scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS). Generally, the cleats contained kaolinite and carbonates which were present in various proportions as either single phases or mixed. An immediate increase in permeability was obtained after HCl was flooded through for the majority of tests. This increase coincided with heightened concentrations of Ca and Fe in the effluent and is attributed to the dissolution of acid soluble minerals, mainly calcite and siderite. In some cases the increase in permeability was very high (200 times increase) and sustained whilst in other cases there was a gradual decrease after the initial increase, resulting in either a small overall increase in permeability of about 20-30% or a decline in permeability relative to the original level of about 20-30%. A possible reason for permeability decline is that although dissolution of minerals allows more liquid into the core, it is not enhancing flow paths, i.e. restrictions to flow in the cleats still exist. Another reason could be destabilisation of insoluble minerals, causing them to become mobile fines in the liquid which migrate toward restrictions in the flow and then jam. High and sustained permeability increases are attributed to cleats containing solely calcite in addition to having high connectivity. This investigation has shown that cleat demineralisation using HCl can be an effective means to overcome low permeability provided cleat connectivity and mineralogy are characterised.