The natural fracture system of coal is the primary conduit for gas flow. Low permeability is in many cases attributed to low fracture porosity and/or connectivity. Mineral occluded coal cleats are known to considerably reduce coal permeability. Whilst cleat demineralisation has been found to increase coal permeability, its influence on compressibility is poorly understood.
In this study, we investigated the influence of mineral dissolution and secondary mineralisation on the compressibility of coal (Cf) following acidification with hydrochloric and hydrofluoric acid (HCl-HF). In-situ HCl core flooding and core immersion in 3% and 15% HF yielded a less compressible core (Cf = 0.006 from 0.020 bar-1) with sustained, enhanced permeability to brine (k = 0.40 from 0.10 mD). We attribute improved stress resilience and better fluid flow characteristics to the dissolution and subsequent secondary mineralisation of the core's circumferential periphery.
Predictive geochemical speciation using OLI Analyzer 9.1, for surveying mineral solubilities and precipitation tendencies, identified the formation of radio-dense neofluoride salts K2SiF6 and CaF2 Structural modifications and mineralogical changes detected from scanning electron microscopy-electron diffraction spectroscopy (SEM-EDS), confirmed the presence of these salts. Results suggest that mineral alteration and subsequent secondary mineralisation of the core periphery following HCl-HF acidisation yielded well-formed crystalline salts, apparently serving as in-situ generated proppants buttressing newly created void spaces for enhanced fluid flow and improved resistance to increasing confining stress.