Gas Wettability of Coal and Implications for Gas Desorption and Drainage

A key parameter affecting the flow of gas in coal is the wetting potential of gas, in comparision to water, to spread over the wall of coal micropores and microfissures. Wettability is quantified in terms of the contact angle of the fluid interface with the solid surface. A fluid with a small angle of contact would spread over the pore walls and eventually displace the non-wetting fluid. Depending on the nature of the coal, gas type and environmental conditions in coal reservoirs, either water or the gas phase could wet coal more strongly. Furthermore, in mixed gas conditions, one gas may be more strongly attached to coal than the other gases. In water-saturated coal, gas desorption in small pores -where most adsorbed gas is stored - can be totally inhibited by water if it is a strong wetting phase. Reducing the hydraulic head (drawdown to achieve the gas desorption pressure) should allow desorption of gas in larger fractures, whereas in small pores, gas desorption could be inhibited by capillary pressure due to the effect of interfacial tension and gas-wetting properties of coal. In this study, we built a new system to quantify the wettability of coal by gas. The contact angle of the water-gas interface with the coal surface inside the gas phase was measured using a captive gas bubble technique. The contact angles of CH4 and CO2 bubbles in water with a coal from the Sydney Basin were measured at different gas-water pressures of up to 15 MPa for CH4 and 6.1 MPa for CO2. The results show that as gas bubbles dissolve in water, the contact angle of the bubble with the coal surface reduces. The contact angle values were smaller for CO2 gas than CH4, and in general, the contact angle value decreases as gas–water pressure increases

A study of potential occurrence of biogenic methane in coal seams

A significant proportion of the total gas emitted from coal mining, particularly for shallow seams (depth), is believed to have been generated from microbial activities within the coal seams and water filling the pores and fractures in coal. To investigate the potential and extent of gas generation in coal due to microbial activities, we developed a method to culture and monitor the production of biogenic methane in coal. We then applied the method to study the process of biogenic methane generation in coals from a mining region in New South Wales. Fresh coal core samples were collected from an exploration borehole drilled into a sequence of coal seams at a greenfield site where five coal seams were located between the depths of 50 to 250m. The formation water was collected from an adjacent borehole drilled into the same sequence of coals. The coal samples were crushed and mixed with formation water and other solutions in glass vials, and then placed in pre-designed incubator at in-situ temperature to allow the production of methane over the life of the project. The results of measurements show that biogenic activities take place and that methane is generated. Methane continued to be produced throughout the life of the project for the studied coals

Developing a New Method to Identify the Source of Gas Emissions into Longwall and Goaf from Surrounding Strata

During coal mining, strata is fractured and gas trapped in the roof and floor of coal seams travels into the workings. Depending on the extent and shape of fractured zones suitable gas drainage patterns are required to maximise the gas capture from strata but also to minimise the cost of operations. In this paper a new method to identify gas emitting zones/seams in the embedding strata and gas migration pathways is presented. The developed method was used in a coal mine in the Southern Coalfield of the Sydney Basin. Geochemical properties of gas trapped in coal seams above and below the mining horizon were analysed and compared with similar properties of gas collected from goaf areas. This study shows that using this method it is possible to identify the source of gas in goaf areas and thus determine the extent of fracturing in the strata around the mined seam

A new method to determine the depth of the de-stressed gas-emitting zone in the underburden of a longwall coal mine

Underground coal mining induces de-stressing and fracturing of strata above and below the targeted seams. This creates a gas-emission zone, which contains gas-bearing coal seams and strata in the roof and floor of the mined (working) seam. In longwall mining, most of the gas released from the emission zone escapes into the coal face and the goaf (caved-in area) behind the coal face, where it presents a safety issue. Depending on the extent and shape of the emission zone, various gas drainage strategies could be applied to maximise capture of the gas from the emitting seams. We developed and trialled a new method to identify the gas emission zone in the underburden of an underground mine in the Sydney Basin, Australia. In this operation, all coal seams are located below the major targeted seam for mining. By measuring the isotopic and molecular composition of gas desorbed from coal cores from exploration drilling and gas collected from the goaf, we identified the source of gas and quantified the limit of the emission zone in the underburden of the working coal seam. This has allowed drainage to be focused and limited to the required depth. Our study will assist others to plan the required depth of gas drainage drilling below the floor of mined seams

Geological controls on coal seam gas distribution in the Hunter coalfield, Sydney basin, NSW

Evaluation of the origin, distribution, migration and accumulation of coal seam gas (CSG), contributes to a better understanding of the CO2 storage potential of coals and the production of CH4 from coal seams. This study aimed to analyse the origin and distribution of the dominant gas components in coal seams of the Hunter Coalfield of the Sydney Basin, and to discuss the implications of these findings. The geological, coal and CSG reservoir properties were analysed using statistical methods, and burial history models were used to further explain the origin and temporal evolution of the gas. The Hunter Coalfield has been divided into five CSG compartments on the basis of geology, and gas content and composition trends. Gas distribution in the coalfield is compartmentalised, with coal and CSG reservoir properties influencing gas origin and distribution to varying degrees within these compartments. Coals in the first compartment, located south of the Hunter River Cross Fault, are characterized on average by the highest gas contents (~9m3/t), adsorption capacities (~23m3/t), permeabilities (between ~100.0 and ~1.0mD) and vitrinite reflectances (0.56 to 1.15%) in the coalfield. Present day gas contents may partially reflect the ranks and adsorption capacities, with late stage biogenic gas generation replenishing CH4 volumes. Compartments 2, 3 and 4 are located in the central region of the coalfield, with the Hunter-Mooki Thrust Fault and Muswellbrook Anticlines in the east, and the Mount Ogilvie Fault in the west forming the main boundaries. Compartment 2 is characterised as the ‘CH4 rich’ compartment, and has burial history profiles consistent with a shallower depth of burial compared to other compartments. Gas contents in Compartment 3 are particularly low (average ~4m3/t) given that these coals have a similar burial history to those in Compartment 1. It appears that low permeabilities have restricted meteoric water recharge, inhibiting the generation of significant volumes of biogenic CH4. Compartment 4 is considered to be the ‘intermediate gas content’ compartment. The coals were exposed to a shallower maximum depth of burial and greater uplift compared to Compartment 1, but have reached sufficient maturity levels to generate significant volumes of thermogenic gas. It is likely that substantial volumes of heavy hydrocarbon gases have been lost, with the Mount Ogilvie Fault probably acting as the main migration pathway for gas escape. It also seems unlikely that significant biogenic gas generation has taken place in Compartment 4 due to limited cross-formational water flow to recharge deep aquifers, and low permeabilities. Compartment 5 possibly had a similar burial history to that of Compartment 3, and gas contents are comparable to those in Compartment 1, but the gases consist almost entirely of CO2. Despite the lack of data, the most reasonable explanation for the CSG distribution in this compartment is injection of CO2 from the numerous dykes and deep seated igneous intrusions which may have enhanced the adsorptive properties of the coals, and thus the injected CO2 was preferentially stored for millions of years. Generally, gas contents in the coalfield are depth related whereas no relationships have been observed between coal rank and adsorption capacity for the Hunter coals. Detailed studies on the coals of two local study areas occurring within two of the compartments have shown that the main coal and CSG reservoir properties controlling gas distribution in the coalfield are the coal maceral composition, rank, adsorption capacity and permeability. Results show that some relationship exists between gas content and liptinite content in the Glennies Creek area, and that the generation of biogenic CH4 might have taken place using hydrocarbons generated from the liptinite-rich coals. Thus the degree of biogenic gas generation is not only related to the capacity for meteoric water access. This trend was not observed for the Warkworth area, which shows gas distribution patterns similar to Compartment 1. The burial history of the Sydney Basin has been shown to have an overarching control on the temporal evolution of CSG in the Hunter Coalfield. Based on theoretical storage estimates and other factors affecting CO2 storage in coals, the southern and western compartments of the coalfield are considered most prospective, with the theoretical CO2 storage capacity for the coalfield estimated to be ~9512Mt. The southern region also shows the greatest potential for CH4 production given its high gas contents and enhanced permeability

Succession Patterns and Physical Niche Partitioning in Microbial Communities from Subsurface Coal Seams

Summary: The subsurface represents a largely unexplored frontier in microbiology. Here, coal seams present something of an oasis for microbial life, providing moisture, warmth, and abundant fossilized organic material. Microbes in coal seams are thought to syntrophically mobilize fossilized carbon from the geosphere to the biosphere. Despite the environmental and economic importance of this process, little is known about the microbial ecology of coal seams. In the current study, ecological succession and spatial niche partitioning are explored in three coal seam microbial communities. Scanning electron microscopic visualization and 16S rRNA sequencing track changes in microbial communities over time, revealing distinct attached and planktonic communities displaying patterns of ecological succession. Attachment to the coal surface is biofilm mediated on Surat coal, whereas microbes on Sydney and Gunnedah coal show different attachment processes. This study demonstrates that coal seam microbial communities undergo spatial niche partitioning during periods of succession as microbes colonize coal environments. : Coal Geochemistry; Biogeoscience; Microbiology; Microbiome Subject Areas: Coal Geochemistry, Biogeoscience, Microbiology, Microbiom

Revealing colonisation and biofilm formation of an adherent coal seam associated microbial community on a coal surface

The discovery that coal seam microbial communities contribute appreciably to coal seam methane (CSM) reserves worldwide has led to an increased interest in the coal seam microbiome. While studies to date have focussed on characterising the microbial communities in a mature state, very little has been reported on the physical niche partitioning and colonisation processes of these communities on coal surfaces. Coal represents a difficult substrate for microbial characterisation using classical techniques due to in its adsorptive nature and recalcitrance to reflectance and fluorescence-based microscopy. This study presents a new technique involving culturing on specially prepared polished coal disks which allows for examination of microbes adherent to the coal surface using both molecular and microscopic approaches. Using this technique we have investigated the colonisation process of the coal surface including evidence for the involvement of a biofilm and successional changes in abundance of several community members during colonisation.9 page(s