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.
Please cite this article as: Turner, L.G., Steel, K.M., A study into the effect of cleat demineralisation by hydrochloric acid on the permeability Abstract 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.
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