Chemical stimulation for enhancing coal seam permeability: Laboratory study into permeability variation and coal structure examination

Abstract Many potential coalbeds have sub-economic permeability for coal seam gas (CSG) extraction even though all the other key characteristics (gas content, thickness etc.) can meet the requirement for successful production. To enhance coal seam permeability various CSG stimulation techniques, including hydraulic fracturing, cavity well completions and horizontal wells, have been used to enhance production, but none is ubiquitously successful. The potential of chemical stimulation including both acid (1% HCl) and oxidant (1% NaClO) stimulation are examined in this paper as an alternative method to enhance coal seam permeability. Acid stimulation targets the removal of certain minerals in coal cleats, while the oxidant dissolves the coal matrix, thereby etching pre-existing coal cleats and perhaps forming new ones. The permeability variation during stimulations are measured with core flooding tests and the corresponding coal structural changes are examined using X-ray microcomputed tomography (μCT) techniques. The chemical reaction mechanisms are confirmed by elemental analysis via inductively coupled plasma optical emission spectroscopy (ICP-OES) and dissolved organic carbon (DOC) in the effluent. Acid stimulation was performed horizontally with cube samples cut from coal cores (Bowen Basin, Australia) and it exhibited a positive effect on coal permeability, which is proposed to result from demineralisation as shown by before and after CT scanning observations together with high calcium (Ca) content in the effluent. NaClO oxidation was able to etch pre-existing coal cleat surfaces, widen cleat apertures and generate new horizontal fractures and/or void space. These changes were caused primarily by chemical attack, confirmed by the high DOC concentration in the effluent. Oxidant stimulation caused a decrease in vertical permeability and an increase in horizontal permeability for some cores. The decrease in permeability is proposed to be due to a combination of increased void space weakening the coal associated with movement of the coal into newly created space against confining pressure. Minerals in the cleats appear to play a role keeping void space open and preventing collapse under confining pressure. Furthermore, the results for these samples indicate that NaClO oxidation appears to be lithotype independent and preferentially attacks coal sections that have more initial pores or fractures.