Impact of Gas Composition on Outburst Propensity of Coal

Pre-drainage of coal seam gas is practised in Australian underground coal mines to reduce gas content below the outburst threshold limit values (TLV) to mitigate coal and gas outburst risk. The current TLVs assume that coal containing CO2 seam gas has a significantly higher risk of outburst compared to coal containing CH4 seam gas. Some field observations have cast doubt on the current outburst TLVs in CO2 rich coal seams however, there has been limited research in this area to challenge the claims made by the past researchers that coal containing high concentrations of CO2 are more prone to outburst than coal containing CH4.

The objective of this project was to demonstrate, through laboratory testing and review of historical outburst event data, that carbon dioxide (CO2) rich coal does not represent a significantly greater outburst risk than methane (CH4) rich coal, in equivalent coal seam/sample conditions. A brief literature review is presented on field observations of mining operations with and without outburst incidents; a series of representative bulk coal samples, obtained from underground coal mine sites in NSW and Qld, have been subjected to various characterisation studies to better understand the impact of seam gas composition on coal outburst propensity. These characterisations allow comparison to be made with both prior literature and laboratory analysis results, and coal samples collected in this project.

Through extensive laboratory testing and review of historical outburst event and field data, it was found that seam gas composition alone does not have a significant impact on outburst propensity of coal, specifically, in equivalent geological conditions at the same measured gas content, coal rich in CO2 seam gas does not represent a significantly greater outburst risk than coal rich in CH4 seam gas. Other factors, such as the presence of geological structure, higher indicated proportions of lost or desorbed gas (i.e. higher than average Q1 and Q2 component percentages of QT), low coal toughness and the presence of abnormally higher proportions of fine particles in core samples, are indicators of areas of increased outburst risk.

Careful consideration of site and seam specific conditions, many of which are already measured or collected as part of standard outburst risk management practice, should be incorporated into the assessment of any proposed increase of threshold limit values (TLVs) based on total gas content and gas composition alone. Specifically, the initial gas desorption values such as Q1 and Q2 (and/or IDR30), and the integrity and appearance of core sample including presence of above average amounts of fine particles.

Significant factors identified as having greater impact on outburst propensity, which industry should consider in comprehensive assessment of outburst risk, include the presence and nature of geological structures and drilling anomalies, coal toughness and seam gas pressure, and significant changes in stress orientation and magnitude.

Laboratory isotherm experiments conducted confirm that the capacity of any coal to adsorb carbon dioxide is much greater than its capacity to adsorb methane. This is mainly due to the relative size, structure, and energy levels of CO2 gas molecules relative to CH4 gas molecules. Small coal particle sizes typically used for isotherm tests may, however, significantly overestimate sorption capacity and therefore is unlikely to be truly representative of the in-situ seam gas conditions and desorption characteristics.

Langmuir volumes and pressures of 30 m3/t and 2 MPa, and 20 m3/t and 2 MPa, were recorded for CO2 and CH4 respectively, with the ratio of VLCO2/VLCH4 ranging between 1.58 and 2.14 for coal tested. With reference to pure CO2 and CH4 isotherms determined experimentally for a fixed gas content under the same sample condition, the equilibrium gas pressure of a CH4 saturated coal sample is observed to be significantly greater than the equivalent CO2 saturated sample. In other words, for a given gas content, the equilibrium gas pressure of a CH4 rich coal sample is significantly greater than the equivalent CO2 rich coal sample. Therefore, for a given gas pressure providing the energy to induce an outburst, it would be reasonable to suggest that CH4 rich coal actually contains greater outburst initiating energy.

However, it can be observed from the isotherm test results that once in-situ equilibrium gas pressures fall below 1 MPa, for each incremental gas pressure reduction of 100 kPa, the potential volume of CO2 released would typically be significantly greater than volume of CH4 released. This is an important consideration in situations where rapid gas pressure reduction occurs due to rapid loss of confinement, slip or displacement on bedding planes or plies, and horizontal dilation of coal at the working face. The accessibility to, and availability of, adsorption sites or coal internal surface area is found to be a key limiting parameter in determining the maximum gas sorption capacity. Gas storage capacity, and hence the magnitude of potential available stored gas volume, is observed to be a complex combination of gas composition, coal properties and coal structure.

The relative timeframe in which desorption, diffusion, and Darcian based gas transport processes occur, materially affects the magnitude of energy available to contribute to outburst. Specifically, the amount of gas available to transition quickly from adsorbed to free state is significantly increased with decreasing particle size. Sorption hysteresis testing was conducted for both crushed and intact samples. It was observed that coal particle size has the greatest impact on sorption hysteresis. The larger the coal particle size, the stronger the sorption hysteresis. Maximum sorption pressure was also observed to strongly influence sorption hysteresis. The greater the sorption pressure, the larger the sorption hysteresis. CO2 sorption was observed to have greater sorption hysteresis than CH4 sorption.

Coal toughness testing using a drop hammer rig similar to the Protodyakonov test apparatus was undertaken in this study. The coal toughness coefficient (f) index is used internationally as an indicator of greater risk of coal and gas outburst, and coal having a lower toughness coefficient f, of <0.5, is usually regarded as having a higher potential of outburst. Testing of three different coal seam samples collected from Australian mines in this study did not reveal any result for f index of <0.5, however, test results did align with suggestion that outburst risk increases as f  value decreases, due to the increased number of smaller particles generated under the test conditions. Coal toughness is only one parameter to evaluate coal outburst proneness, but this research suggests alignment of toughness index f, with other parameters is worthy of further investigation for Australian coal mines.