Coal permeability stimulation by NaClO oxidation

Abstract

Many prospective coal seams have limited permeability and are thus marginal for economic coal seam gas (CSG) extraction. To enhance seam permeability, various CSG stimulation techniques, including hydraulic fracturing, cavity well completions and horizontal wells, have been used commercially, but these techniques are not successful everywhere. Alternately, oxidant stimulation to enhance coal seam permeability may have potential benefit. In this paper, we report the oxidation of a cubic coal sample from the gas-bearing Bandanna Formation with 1% NaClO. Permeability variation were measured over time with core stimulation/flooding tests. Coal structural changes, captured via X-ray microcomputed tomography (µCT), were used to analyse and model the impact of oxidant stimulation on coal permeability. We found that NaClO stimulation was able to widen fracture aperture and generate additional cracks and/or void space. Analyses of the coal after reaction indicated improved pore connectivity. This resulted in a significantly higher permeability, as shown by both core flood tests and flow modelling/simulations using GeoDict’s FlowDict module. Although this paper confirms oxidant stimulation has the potential to improve coal seam permeability, the optimum volume and reaction time, pressure and temperature conditions and suitability for use in various coal types requires further research.