CO2 storage and gas diffusivity properties of coals from Sydney Basin, Australia
Introduction
About 300 Mt of coal is produced in Australia annually with almost half of the production from Sydney Basin coalfields. The Sydney Basin, which is located in New South Wales (NSW), contains four major coalfields: Hunter, Newcastle, Western and Southern Coalfields. All coals are Permian and their rank is generally medium to high volatile bituminous, except for the Southern Coalfield coals which are generally low to high volatile bituminous. Coal in the Southern Coalfields of Sydney Basin is > 300 m deep and is mined underground whereas most coal in northern coalfields is extracted from open-cut mines. The open-cut mining produces half of the total coal production of NSW.
The current study was undertaken on coals obtained from underground faces, in-seam boreholes and surface exploration boreholes in the Sydney Basin and involves investigations on factors affecting CO2 storage and injectability in coal, in particular the adsorption and diffusivity properties of coal. CO2 adsorption and diffusivity properties were determined for coals ranging in depth from about 27 to 723 m. The results from the deep coal seams are applicable to CO2 sequestration projects, whereas the data from the shallow seams are useful in assessing the extent of fugitive CO2 emissions into the atmosphere from surface coal mining.
The maximum quantity of gas that can be stored in a given coal is mainly a function of its adsorption capacity, though at high pressures significant amounts of gas can also be stored in the pore system if the pores are not saturated with water. At a given set of P–T conditions, a coal can adsorb higher amounts of CO2 than CH4, depending on coal rank; high to medium volatile bituminous rank coals store more gas than sub-bituminous coal and anthracite. The desorption rates of CO2 and CH4 for coal are also considerably different (Williams et al., 1995).
During the early stages of coalification, coal generates large amounts of carbon dioxide, but most of this is lost due to dissolution in mobile water and migration out of the coal seam. The origin of CO2 in some coalfields of the world (such as Australia, Poland and France) has been subject to comprehensive investigations by researchers worldwide, initially because of the safety hazard of this gas in underground mining operations, as evidenced by the tens of thousands of gas outbursts over the last century (Lama and Saghafi, 2002). Clayton (1998) reviewed the geochemistry of seam gas and based on the work of other researchers (Kotarba and Rice, 1995a, Kotarba and Rice, 1995b, James and Burns, 1984, Smith et al., 1985, Whiticar et al., 1986, Smith and Pallasser, 1996) listed four sources for CO2 gas in coal seams: a) decarboxylation reactions of kerogen and soluble organic matter during burial heating of the coal, b) mineral reactions such as thermal decomposition or dissolution of carbonates or other metamorphic reactions, c) bacterial oxidation of organic matter and d) magmatic intrusion.
One particularity of Australian coalfields is the occurrence of large amounts of CO2 in many coal seams; in some cases close to 100% of the seam gas. It is believed that CO2 in Sydney Basin coals is mainly derived from magmatic sources as the high CO2 content areas generally have isotopic compositions of δ13C of about − 7‰ (Smith and Gould, 1980, Smith et al., 1985). Also high concentrations of CO2 are localised laterally along some major faults. Mining experience in Australia shows that CO2 content can vary significantly within short distances in the same seam and within the same coal mine (Lama and Saghafi, 2002). Differences in the behaviour of CH4 and CO2 have been noted for more than a century from coal mining operations worldwide. It has been found that gas outbursts can occur at lower gas contents for CO2 than for CH4, and the presence of high CO2 content in coal seams has been the cause of numerous gas outbursts during underground coal mining. During the last 50 years, many outbursts occurred in Australian mines. In some instances where the dominant gas is CO2, outbursts happened more frequently. For instance, at Tahmoor, Metropolitan and Westcliff collieries in Illawarra coalfield in the southern part of the Sydney Basin, gas outbursts were caused mainly by CO2. However there were also some occurrences of CH4 outbursts in Australian coals. In some instances, the large number of CH4 outbursts has led to complete closure of the mine, for example the Leichhardt colliery, Bowen Basin, Queensland (Moore and Hanes, 1980). Due to the common occurrence of CO2 in Australian coal seams and its implications for coal mining, the mechanism of CO2 storage and flow in coal has been investigated during the last two decades (e.g. Lama and Bodzinoy, 1996, Saghafi and Williams, 1998). However most of these studies have concentrated on the behaviour of methane in coal, particularly for mine safety and coal bed methane (CBM) production. In the last decade, the impetus for enhanced coalbed methane recovery (ECBM) and increasing interest in the CO2 sequestration to abate greenhouse gas emissions has led to further research into physical chemistry of the interaction of gas with coal.
The capacity of coal to store CO2 and its ability to allow movement of gas through its pore and fissure systems are two major factors that influence the release of gas during coal mining and storage of gas in coal seams (CO2 sequestration). Storage and migration properties for coal are represented by adsorption and diffusivity measurements and these can be considered as primary coal reservoir properties that are required for any meaningful evaluation and predictive modeling of coal gas reservoirs. They depend largely on coal type, rank, pore structure and any pore filling, present as a consequence of paleo-fluid flow and mineralisation. External factors such as mining methods and stress regimes, as well as the flow of meteoric water, also affect these properties, but are of secondary importance.
The aim of the present investigation is to characterise Sydney Basin coals according to these two parameters, particularly for CO2. The measurement methodology and mathematical expressions were developed and applied to 27 coal samples. After a brief discussion on the nature of gas in coal, this paper presents the measuring systems, methodology and implications of the measurements.
Section snippets
The nature of gas in coal
Gas contained in deep, high rank coals is generally of thermogenic origin, i.e. generated during the process of organic metamorphism. For shallow coals, gas is mainly a byproduct of anaerobic biogenic activity. Worldwide, seam gas mainly consists of methane with lesser amount of carbon dioxide. Nitrogen is present in some areas, but only in small quantities (< 10% of the total gas volume). Other higher hydrocarbons (C2+) may also be present in some deeper (> 600 m) Australian coals, but they
Adsorption isotherm
The technique developed to measure adsorption isotherms is based on a gravimetric method (Saghafi, 2003). This method involves direct measurement of the increase in weight of coal as it is saturated with gas at increasing gas pressures. The equation of state is derived from the density of the free gas phase and is measured simultaneously in a reference empty container held at the same pressure and temperature as the coal container. Approximately 300 g of coal is used for the experiments and
Coal sampling and measuring conditions
In the present study gas adsorption isotherms for 27 samples from 17 Permian coal seams from the Sydney Basin were measured. These samples were obtained from depths between 27 m and 723 m. Table 1, Table 2 summarise sample details, including results of proximate analyses and coal seam depths. Fig. 4 shows the relationship of volatile matter of the coals with depth. The coals are mostly of high to medium volatile bituminous rank with mean maximum vitrinite reflectance in oil (Ro max) ranging
Comparison of properties of main seam gases
Australian coal seams mainly contain methane, carbon dioxide and lesser amounts of nitrogen. The physical properties of these gases are shown in Table 5. Measurements of adsorption isotherms using all the gases were undertaken on four of the coals including one of the contact metamorphosed coals (Table 6). Gas adsorption capacity of these coals, represented by their Langmuir volumes, show that, for the non-metamorphosed coals, the adsorption capacity is 1.6 to 1.8 times higher for CO2 than for
Summary and conclusions
CO2 storage and diffusivity properties of 27 coal samples from 17 coal seams in Sydney Basin, NSW, Australia were measured using a gravimetric method and a new diffusivity system based on Fick's diffusion law.
The measurements were undertaken at CO2 sub-critical pressure and temperature conditions, namely at gas pressures below 6 MPa and at a temperature of 27 °C for most of the coals. The coals analysed are of high volatile bituminous to low volatile bituminous rank (Ro max = 0.66 to 1.45%) and
Acknowledgment
The authors wish to thank the coal mining industry in Sydney Basin, Australia for providing the coal samples used in this study. Also our thanks are extended to Neil Sherwood and Nigel Russell of CSIRO Petroleum for their invaluable editorial comments.
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