INDUCING FRACTURES AND CLEAT APERTURE ENHANCEMENT IN BITUMINOUS COAL VIA THE APPLICATION OF MICROWAVE ENERGY APPLIED UNDER HYDROSTATIC STRESS CONDITIONS
Open Access
- Author:
- Kumar, Hemant
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 22, 2010
- Committee Members:
- Jonathan P Mathews, Thesis Advisor/Co-Advisor
Jonathan P Mathews, Thesis Advisor/Co-Advisor - Keywords:
- Fracture
Bituminous coal
Microwave
Coalbed Methane - Abstract:
- For the degassing of coal seams, either prior to mining or in unminable seams as a means 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 and thus are often marginal for economic methane extraction or limited in CO2 injectivity in the case of enhanced coalbed methane production or CO2 sequestration. Microwave energy has been shown to induce fractures in coal in the absence of confining stress. 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 (1,000 psi) polycarbonate vessel, simulating hydrostatic stress of 1,800 foot depth, was utilized. Cleat frequency and distribution were examined via X-ray computed tomography before and after short burst microwave exposure for two cores with and without the application of hydrostatic stress. 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 the unconfined core and that the aperture increased in existing cleats. Cleat/fracture volume, determined from micro-focused X-ray computed tomography, following microwave exposure increased from 1.8% to 16.1% of the unconfined core volume. Similar observations of fracture generation and aperture enhancement were determined for the stressed coal, for the first time. After the short microwave-bursts cleat/fracture volume was increased from 0.5% to 5.5% under the application of hydrostatic stress of 1000 psi. Optical microscopy indicated that fracture initiated likely occurred, in at least some cases, at inertinite. 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, unlike natural cleats. Cleat aperture enhancement were observed for both cores being on the order of 400% for unstressed core and lower values of around 100% for the single cleat in the cores while exposed under stress. Thus, it appears likely that microwaves have the potential to enhance the communication between bore and existing cleat networks in coal seams at depth for improved gas recovery or CO2 injection.