X-ray μCT investigations of the effects of cleat demineralization by HCl acidizing on coal permeability

Demineralization of coal cleats via HCl acidizing is a potential stimulation strategy for improving the productivity of coal seam gas (CSG -also known as coalbed methane or CBM) wells. This preliminary study measured structural changes that HCl acidizing can produce in small-scale core samples, and investigated the mineral characteristics that control the extent of these changes. This is the first time sister cores have been explored in detail to unravel the mechanisms influencing the permeability changes. In this study, the acid-induced physicochemical changes in core samples from CSG producing coals were investigated to gain improved understanding of permeability enhancement in the vertical flow direction. In particular, this work was undertaken to explain the variability in permeability improvements when calcite from coal cleats was removed by 1% HCl. X-ray microcomputed tomography (mu CT) image analyses were used to reveal the mechanisms influencing permeability changes. There was clear evidence for cleat opening via mineral dissolution as well as mineral mobilization-accumulation. Mineral dissolution improved the overall porosity of coal by opening mineralized cleats resulting in improvements in vertical permeability. Cleaned-out master cleats afforded greater permeability enhancement compared to the demineralized orthogonal cleats within bright band layers of the coal samples. Image analysis revealed that HCl-insoluble minerals were mobilized and subsequently accumulated at pre-existing bottleneck regions. The mineral build-up in these bottleneck regions limited the extent of the permeability enhancement by acidizing. Flow simulations of the digitized coal materials indicated that acidizing could enhance permeability not only in the vertical direction but also in the lateral or horizontal direction. Overall, this study concluded that improving the flow path continuity was achievable by HCl acidizing, whereby calcite-filled master cleats were particularly attractive targets for maximizing vertical flow enhancements. Further research is required to assess application at larger scales, consider economic feasibility, and investigate environmental performance.