INDUCING FRACTURES AND INCREASING CLEAT APERTURES IN A BITUMINOUS COAL (UNDER HYDROSTATIC STRESS CONDITIONS) VIA APPLICATION OF MICROWAVE ENERGY

摘要

For the degassing of coal seams, either prior to mining or in unminable seams to obtain coalbed methane, it is the cleat frequency, aperture, connectivity, and mineral occlusions that influence coals permeability to gases. Unfortunately many potential coalbeds have limited permeability, thus they are often marginal for economic methane extraction. They also present a challenge in the case of enhanced coalbed methane production, with concurrent CO2 sequestration, due to limited CO2 injectivity. Microwave energy can, in the absence of confining stress, induce fractures in coal. Here, creation of new fractures and increasing existing cleat apertures via short burst, high-energy microwave energy was evaluated for both hydrostatically stressed and unstressed North American bituminous coal cores. A microwave-transparent argon gas pressurized (1000 psi) polycarbonate vessel, simulating hydrostatic stress of 1,875 foot depth, was utilized. Cleat frequency and distribution was examined for two cores via micro-focused X-ray computed tomography. Evaluations occurred before and after microwave exposure with and without the application of hydrostatic stress during exposure. Optical microscopy was performed for tomography cleat aperture calibration and also to examine any lithoptypes influences on fracture: initiation, propagation, frequency, and orientation. It was confirmed that new fractures are induced via high-energy microwave exposure in an unconfined bituminous core and that the aperture increased in existing cleats. Cleat/fracture volume, following microwave exposure increased from 1.8% to 16.1% of the unconfined core volume. For the first time, similar observations of fracture generation and aperture enhancement in coal were also determined for exposure under hydrostatic stress conditions. An existing cleat aperture increased from 0.17 mm to 0.32 mm, after short microwave-bursts occurring under a simulated hydrostatic stress. The cleat/fracture volume increased from 0.5% to 5.5%. Optical microscopy indicated that fracture initiated likely occurred in at least some cases at fusain microlithotypes. Presumably this was due to the open pore volumes and potential for bulk water presence or steam pressure buildup in these locations. For the major induced fractures, they were mostly horizontal (parallel to the bedding plane) and often contained within lithotype bands. Thus it appears likely that microwaves have the potential to enhance the communication between horizontal wellbore and existing cleat network, in coal seams at depth, for improved gas recovery or CO2 injection.