Acid-induced mineral alteration and its influence on the permeability and compressibility of coal

• Mineral alteration by HF acid yielded relatively large crystalline minerals.
• 15% HF-acidising has enhanced coal permeability and reduced compressibility.
• Enhanced permeability is attributed to kaolinite (Al2Si2O5(OH)4) dissolution.
• Improved stress resilience is ascribed to hieratite (K2SiF6) precipitation.

The natural fracture system in coal serves as the primary conduit for water and gas flow in coal seam gas fields. For low permeability coal with highly mineralised fracture systems, the dissolution and/or modification of mineral occlusions could potentially enhance permeability and improve stress resilience. This study investigated the effect of mineral alteration by hydrochloric and hydrofluoric acid (HCl–HF) on fracture compressibility and coal permeability. Coal core immersion in 15% HF- 4% KCl solution has enhanced coal permeability to brine from 0.10 to 0.45 mD and reduced fracture compressibility from 0.020 to 0.006 bar-1. Enhanced permeability and improved stress resilience were attributed to kaolinite (Al2Si2O5(OH)4) dissolution and hieratite (K2SiF6) precipitation, respectively. Geochemical speciation, simulating HF interactions with coal fracture minerals, predicted the occurrence and prevalence of both dissolution and precipitation reactions. Scanning electron microscopy-energy dispersive spectroscopy confirmed the mineral alteration phenomena. Identification of resultant structural changes and the differentiation of chemical from physical effects were elucidated using X-ray computed tomography. The overall findings show that mineral alteration by HF yielded relatively large, crystalline minerals that appeared to provide structural support to fractures, resulting in enhanced fluid flow and improved resistance to compression.

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