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

Abstract

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.

Impure CO2 reaction of feldspar, clay, and organic matter rich cap-rocks: Decreases in the fraction of accessible mesopores measured by SANS

“…In contrast, carbonate-rich cap-rock and reservoir core from the Netherlands showed an increase in permeability after reaction with SO 2 -CO 2 and brine due to strong carbonate mineral dissolution (Bolourinejad and Herber, 2015). In a different application, a permeability increase was observed in coal and attributed to calcite dissolution in coal cleats after reaction with HCl; however, permeability subsequently decreased in coals where kaolinite was also present in the cleats via kaolinite fines migration and pore clogging (Turner and Steel, 2016). These various changes in accessible porosity and permeability indicate that several factors including mineral content, organic matter content,…”

Experimental Study of the Pore Structure and Permeability of Coal by Acidizing

“…Balucan et al presented an acidizing means consisting of a pre-flush (HCl), main flush (HCl-HF), and an overflush (HCl) to improve production in the oil and gas industry. Turner et al launched for the first time an acid-induced mineral alteration of the microstructure of coal using scanning electron microscopy and X-ray [24,25]. It is conceivable that cleats demineralization using hydrochloric acid and hydrofluoric acid can be an effective means to overcome low permeability provided cleat connectivity and permeability are characterized.…”

Oxidant stimulation of coal seams to increase coal seam permeability

“…Vasyuchkov (1985) reported aqueous HCl (2, 4 and 6%) solutions could enhance the coal permeability dramatically from less than 0.1 mD to over 15.1 mD [12]. Turner et al (2016) There was clear evidence for cleat opening via mineral dissolution as well as mineral mobilization-accumulation. Based on the X-ray microcomputed tomography (μCT) image analyses, they conducted flow simulation and reported that acidizing could enhance permeability not only in the vertical direction but also in the lateral or horizontal direction [13].…”

“…For example, few of their results is about the horizontal permeability which is parallel to the face cleats and more realistic than the reported vertical permeability. Additionally, in the core flooding tests conducted by Turner et al (2016), the HCl was injected at the same direction with brine [11], which, to mimic how stimulation would be conducted downhole, should be adopt forward flow to characterise initial permeability, reverse flow with acid to simulate the downhole treatment, and then forward flow again to characterise the final permeability.…”

“…and BV2 used in this study had been flooded with 1% HCl previously by Turner et al (2016) where they were labelled as sample "A1" and "A2" [11]. In the current research, only oxidant (1% NaClO) is injected into the two core samples.…”