Creation of microchannels in Bowen Basin coals using UV laser and reactive ion etching

Abstract

We report a set of techniques to create microchannels (20–40μm) in Bowen Basin coals for use as experimental models that replicate natural cleats at sub-surface reservoir conditions. The model cleats were developed to allow small scale systematic laboratory investigations of water and gas relative permeability behaviour in coal seam gas reservoirs. The four cleat fabrication techniques evaluated were ultra violet (UV) laser, reactive ion etching in oxygen plasma (RIE), mechanical scratching with a tungsten carbide cutting tool and chemical etching with KMnO4. The mechanical and chemical methods were eliminated from the study as chemical etching produced large variations in cleat aperture and the mechanical method sheared off sections of the channel. The UV laser and RIE methods both produced channels that met the width, depth and shape criteria. Micro-Raman spectroscopy indicated that UV laser and RIE etching did not induce significant chemical changes on the coal surface. Soft-lithography moulds and SEM observations showed that the morphology of the walls in the RIE channel more closely resembled a natural cleat than the walls of the laser etched channel, which were much smoother. The visualisation of two-phase water and gas flow through the UV laser and RIE channels was demonstrated in a microfluidic device.