Oxidant stimulation of coal seams to increase coal seam permeability

Abstract

Low permeability renders a significant fraction of coal seam gas (CSG) resources sub-economic. An effective permeability enhancement strategy is thereby crucial in monetising low permeability CSG resources. Though employed in a number of coal basins, conventional low permeability stimulation treatments have not been ubiquitously successful. The natural cleat system of coal is the primary conduit for gas flow and low permeability is in many cases attributed to low cleat porosity and connectivity. Cleat demineralisation by acid has been addressed previously with mixed results. This thesis explores the possibility that coal cleats could be etched and the cleat aperture could be widened by use of oxidants. It could have high potential to increase the cleat porosity and connectivity, thus enhancing the coal permeability in the near well bore region where high permeability is critical.Two coal samples, from the Bowen (coal B) and Surat (coal S) basins in Queensland, Australia were selected. A screening method was designed based on time-lapse photography to assess the extent of coal particle size change immersed in potential oxidants, including sodium hypochlorite (NaClO), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), and potassium persulfate (K2S2O8). Results show coal solubilisation and the propensity to swell in all the oxidants as well as coal breakage in specific oxidants (NaClO and KMnO4). Both coals react vigorously with NaClO and KMnO4, where massive coal solubilisation occurs, but react only slightly with K2S2O8 and H2O2. Given the solid products of the KMnO4 reaction (MnO2), NaClO is selected as the most promising oxidant stimulant.After NaClO treatment, the total accessible pore volume of both coals increases. Pore size distributions indicate that oxidation can enlarge the pores, particularly for coal S, which was confirmed by scanning electron microscopy (SEM). A microfluidic cleat flow cell (CFC) was used to inject NaClO into artificial channels scribed on polished coal samples, and measured an increase in the widths of the channels after NaClO treatment. The channel aperture increase indicates that coal solubilisation/etching is a more dominant mechanism than coal swelling when coal is confined in coal cleats. The channel aperture of coal S increases more than coal B.To chemically interpret the various NaClO oxidation effects on different coals, molecular structures of the two studied coals and their oxidised residues (coal So and coal Bo) were examined using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and solid state 13C nuclear magnetic resonance spectroscopy (13C NMR). The results qualitatively and quantitatively illustrate that NaClO preferentially reacts with aliphatic chains and oxygen functional groups. After aliphatic chains, NaClO tends to attack small aromatic rings, evidenced by the loss of smaller clusters in coal S, leaving the larger aromatic clusters in the oxidised residue. The oxidised coal B turned out to have a similar molecular structure with coal S. Indeed, the subsequent oxidation of coal Bo gave a similar mass loss (42.5%) to raw coal S (45.2%). Futuremore, NaClO oxidation products were rich in carbonyl (C=O) and carboxylic (O-C=O) functional groups.Finally, based on coal B, the coal permeability variations after oxidation were examined with core flooding tests under confining pressure and the corresponding coal structure changes were compared using X-ray microcomputed tomography (mCT). NaClO oxidation was found to etch the cleat surface, widen cleat aperture and generate new horizontal fractures and/or void space. Nevertheless, these structural changes did not lead to a vertical permeability increase, which is proposed to be due to fines generation and blockage of cleats in the unreacted area. However, the horizontal permeability was found to increase under confining pressure. Minerals may play a role in acting as pillars to hold the cleat aperture when the cleat is etched and widened. Furthermore, NaClO oxidation is lithotype independent and attacks the coals that are more porous or have initial fractures.This thesis has confirmed the potential of oxidant stimulation to benefit coal permeability. Some uncertainties remain. For example, given the high heterogeneity of coal, flooding tests with different coal types should be conducted to establish relationships between permeability change and specific coal properties. In the future, the oxidant stimulation targeting at lower rank coals is expected to show substantial permeability increase as NaClO can cause substantial etching of cleats in lower rank coals.