Bowen Basin Coals Research Articles

Automated detection of regions with persistently enhanced methane concentrations using Sentinel-5 Precursor satellite data

Abstract. Methane (CH4) is an important anthropogenic greenhouse gas, and its rising concentration in the atmosphere contributes significantly to global warming. A comparatively small number of highly emitting persistent methane sources are responsible for a large share of global methane emissions. The identification and quantification of these sources, which often show large uncertainties regarding their emissions or locations, are important to support mitigating climate change. Daily global column-averaged dry air mole fractions of atmospheric methane (XCH4) are retrieved from radiance measurements of the TROPOspheric Monitoring Instrument (TROPOMI) on board on the Sentinel-5 Precursor (S5P) satellite with a moderately high spatial resolution, enabling the detection and quantification of localized methane sources. We developed a fully automated algorithm to detect regions with persistent methane enhancement and to quantify their emissions using a monthly TROPOMI XCH4 dataset from the years 2018–2021. We detect 217 potential persistent source regions (PPSRs), which account for approximately 20 % of the total bottom-up emissions. By comparing the PPSRs in a spatial analysis with anthropogenic and natural emission databases, we conclude that 7.8 % of the detected source regions are dominated by coal, 7.8 % by oil and gas, 30.4 % by other anthropogenic sources like landfills or agriculture, 7.3 % by wetlands, and 46.5 % by unknown sources. Many of the identified PPSRs are in well-known source regions, like the Permian Basin in the USA, which is a large production area for oil and gas; the Bowen Basin coal mining area in Australia; or the Pantanal Wetlands in Brazil. We perform a detailed analysis of the PPSRs with the 10 highest emission estimates, including the Sudd Wetland in South Sudan, an oil- and gas-dominated area on the west coast in Turkmenistan, and one of the largest coal production areas in the world, the Kuznetsk Basin in Russia. The calculated emission estimates of these source regions are in agreement within the uncertainties in results from other studies but are in most of the cases higher than the emissions reported by emission databases. We demonstrate that our algorithm is able to automatically detect and quantify persistent localized methane sources of different source type and shape, including larger-scale enhancements such as wetlands or extensive oil- and gas-production basins.

Oxidative effects of gaseous oxygen and ozone on Australian subbituminous and bituminous coals and the implications for coal seam permeability enhancement

The use of gaseous ozone as a means for enhancing coal seam permeability has been investigated, including the effect on subbituminous and bituminous coal from the Surat and Bowen Basins, Australia, respectively. To elucidate the oxidative mechanism, the chemical properties of the Surat coal before and after oxidation was examined using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) and X-ray Photoelectron Spectroscopy (XPS). Additionally, gas adsorption techniques and optical microscopy were used to assess physical changes arising from the ozone treatment. Although both the oxygen and ozone treatments (two days duration) increased the oxygen content in the molecular structure of the coal, the ozone introduced significantly more, specifically with the generation of aliphatic ester CO groups and aromatic carbonyl/carboxyl CO groups. In terms of the physical structure of the coal, the micropore volume was slightly reduced by the oxygen treatment but almost eliminated by the ozone treatment. Furthermore, oxygen treatment showed little impact on the coal mass loss while ozone oxidation caused a 0.7% loss in coal weight. Additional tests on a Bowen Basin coal showed an increase in the aperture (4.37%) of a cleat after ozone treatment. These results demonstrate the potential of ozone for coal seam permeability enhancement by merging micropores and increasing cleat apertures whilst maintaining the structural integrity of the coal.

A three-dimensional feature extraction-based method for coal cleat characterization using X-ray μCT and its application to a Bowen Basin coal specimen

Cleats are the dominant micro-fracture network controlling the macro-mechanical behavior of coal. Improved understanding of the spatial characteristics of cleat networks is therefore important to the coal mining industry. Discrete fracture networks (DFNs) are increasingly used in engineering analyses to spatially model fractures at various scales. The reliability of coal DFNs largely depends on the confidence in the input cleat statistics. Estimates of these parameters can be made from image-based three-dimensional (3D) characterization of coal cleats using X-ray micro-computed tomography (μCT). One key step in this process, after cleat extraction, is the separation of individual cleats, without which the cleats are a connected network and statistics for different cleat sets cannot be measured. In this paper, a feature extraction-based image processing method is introduced to identify and separate distinct cleat groups from 3D X-ray μCT images. Kernels (filters) representing explicit cleat features of coal are built and cleat separation is successfully achieved by convolutional operations on 3D coal images. The new method is applied to a coal specimen with 80 mm in diameter and 100 mm in length acquired from an Anglo American Steelmaking Coal mine in the Bowen Basin, Queensland, Australia. It is demonstrated that the new method produces reliable cleat separation capable of defining individual cleats and preserving 3D topology after separation. Bedding-parallel fractures are also identified and separated, which has historically been challenging to delineate and rarely reported. A variety of cleat/fracture statistics is measured which not only can quantitatively characterize the cleat/fracture system but also can be used for DFN modeling. Finally, variability and heterogeneity with respect to the core axis are investigated. Significant heterogeneity is observed and suggests that the representative elementary volume (REV) of the cleat groups for engineering purposes may be a complex problem requiring careful consideration.

A 3d Feature Extraction-Based Method for Coal Cleat Characterization Using X-Ray Μct and its Application to a Bowen Basin Coal Specimen

A 3d Feature Extraction-Based Method for Coal Cleat Characterization Using X-Ray Μct and its Application to a Bowen Basin Coal Specimen

CH4, CO2, N2 diffusion in Bowen Basin (Australia) coal: relationship between sorption kinetics of coal core and crushed coal particles

To improve the general understanding of diffusive processes in coal and investigate if representative reservoir properties can be estimated from measurements on crushed coal particles, adsorption and desorption experiments on crushed samples and large, intact core samples are carried out. Diffusion properties are analysed for methane, carbon dioxide and nitrogen up to a pressure of 10 MPa at a temperature of 35 °C. The samples are a low volatile bituminous coal and a medium volatile bituminous coal from Australia's Bowen Basin.A bidisperse model (a simplified version of the Ruckenstein model for micro-pore dominated flow), characterised by a ‘fast’ and a ‘slow’ fraction, is required to describe the observed diffusion behaviour. Results from the modelling show that effective diffusivities of the core samples are typically an order of magnitude lower than for the crushed coals, and the ‘slow’ effective diffusivities are typically at least one order of magnitude lower than the corresponding ‘fast’ effective diffusivities. A pressure dependency of the effective diffusivities is noted but an explicit relationship with gas content could not be established. This may be due to the experimental set-up, which does not eliminate the effect of differential pressure.Calculation of the diffusion path length for the coal core reveals that the diffusion path length in coal may not be constant but decreases with increasing matrix gas content as a result of coal swelling. Furthermore, there is a distinct difference between the diffusion paths derived from the ‘fast’ and the ‘slow’ fractions; the ‘slow’ diffusion path of the coal core is 100s of microns shorter than the ‘fast’ diffusion path. The faster diffusion associated with the longer diffusion path is interpreted to relate to micro-porosity, while slower diffusion via the shorter diffusion path is interpreted to relate to macro-porosity.

Discrete-NTG Truncated Gaussian Simulation: An Alternative Modeling Approach for Coal-Seam-Gas Unconventional Reservoirs, Bowen Basin, Eastern Australia

Summary Building a representative static model for predicting and monitoring the performance of coal-seam gas (CSG) fields presents several complex and unique challenges. The individual reservoirs possess different coal architectures, often with highly complex seam splitting, amalgamating, and structural deformation. Our objective was to develop an alternative approach that honored log and core data capturing both the lateral heterogeneity and the vertical signature of the Bowen Basin coals in Central Queensland. In some areas of the Bowen Basin, coals can be thick and laterally continuous; picking the top and base of each seam works well in small models with homogeneous coals. As seam geometries begin to increase in complexity and coals become more heterogeneous in nature with thinner seams in multiple packages, then a net-to-gross (NTG) approach is often more appropriate. Each method has its merits. The former approach describes the reservoir architecture but implies a certain degree of confidence in coal correlation. In a vast field with complex seam splitting and merging with abundant drilling data, it might not be a practical technique. The NTG method disregards coal-seam architecture and reservoir connectivity. The proposed workflow is a hybrid approach using discrete modeling algorithms and data-analysis tools on continuous NTG properties. The process operates on a relatively coarse chronostratigraphic framework in which coal is captured as contiguous discrete-NTG “facies.” The use of the truncated Gaussian model ensures the ordering of NTG facies and mimics transitions between coals and the surrounding interburdens. With the adoption of facies vertical proportion trends, we are able to replicate a similar coal-seam signature laterally away from the wellbore. The definition of a categorical coal model allows the proper scaling of seams with different coal-quality characteristics. With the successful geocellular-model construction and history match of two historical CSG fields in the Bowen Basin, the discrete-NTG truncated Gaussian simulation (TGS) approach has proved to be a valid alternative CSG-modeling technique.

Impact of capillary trapping on CSG recovery: an overlooked phenomenon

The impact of water on gas production has been commonly reflected using relative permeability curves, which are obtained by measuring the flow behaviour of each phase through a core sample. This approach reflects the overall response of a core to flow but is unable to capture the capillary trapping phenomenon at the microscale, which is expected to vary significantly for coals with different microstructures. The overlook of trapping effect could potentially overestimate gas production, a topic that does not appear to be well explored. In this work, the impact of capillary trapping on gas recovery was investigated numerically through a fully coupled water-gas two-phase flow model. The characterisation of a coal microstructure, surface chemistry (e.g. coal wettability) and an isotherm curve was summarised for Bowen Basin coal. Various sensitivity studies were then conducted at coal matrix scale to quantify the amount of gas trapped by capillary forces under different reservoir conditions and production controls. Our results show that for the studied coal parameters from the Bowen Basin, the capillary trapping effect hinders gas breakthrough noticeably, causing unwanted high abandonment pressure and reduction in gas recovery rate. Among all investigated parameters, pore size has the most important effect on trapped gas percentage. If taking 3 MPa as initial reservoir pressure, 300 kPa as the abandonment pressure baseline, 63.58 kPa as the gas breakthrough pressure, then the trapped gas accounts up to 4.02% of the total predicted gas; the trapped gas percentage will increase considerably if the saturation of gas is very low, although this variability is largely dependent on reservoir condition.

Compositional variations and carbon isotope reversal in coal and shale gas reservoirs of the Bowen and Beetaloo basins, Australia

Abstract Stable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. Many isotope classification systems used for gas to source rock correlation and thermal maturity determination are primarily based on empirical observations made in conventional reservoirs and the kinetic isotope effects observed during pyrolysis experiments performed on source rocks. However, such relationships may not be readily applicable to onshore unconventional reservoirs due to the strong molecular and isotope fractionation that occur during extensive gas expulsion associated with basin uplift and depressurization. Degassing studies of freshly recovered core samples can provide useful insight into the behaviour of gas molecules in unconventional reservoirs during basin uplift. The analyses of Australian coal and marine shale samples demonstrate that during desorption both molecular and isotopic compositions of gas change at variable rates. Gas initially desorbed from the samples is mostly CH 4 , whereas later desorbed gas becomes increasingly enriched in C 2 H 6 and higher hydrocarbons. Hydrocarbon molecules also fractionate according to their isotopic composition, where the early released gas is enriched in 12 C causing the remaining gas in the reservoir to be enriched in the heavier 13 C isotope. During the release of gas from the Bowen Basin coals the C isotope ratio of CH 4 ( δ 13 C 1 ) changes by up to 21‰ (VPDB), whereas that for C 2 H 6 ( δ 13 C 2 ) and C 3 H 8 ( δ 13 C 3 ) changes by <6‰. Similar changes in the isotope composition can be seen during the release of gas from marine source rocks of the Beetaloo Sub-basin. In a fully gas-mature middle Velkerri shale sample, δ 13 C 1 changes by up to 28‰ and δ 13 C 2 by up to 3‰ with no appreciable change occurring in δ 13 C 3 . The extent of molecular fractionation during gas flow through carbonaceous rocks is primarily related to the adsorption–desorption properties of organic matter and diffusivity through the overall rock matrix. Using the current dataset, the magnitude of the contributions exerted by the desorption and diffusion processes cannot be readily distinguished. However, both Bowen Basin coals and Beetaloo Sub-basin shale show similar fractionation effects during gas flow, where the heavier alkane molecules, including those containing more 13 C, desorb and move slowly compared with the lighter components, in particular CH 4 . Different rates of isotope fractionation between hydrocarbon molecules during gas flow cause the shape of compound-specific-isotope (CSI) curve to change with time. Early released gas is characterized by a normal CSI trend where the short-chain hydrocarbons are isotopically lighter compared with the longer-chain hydrocarbons. Because CH 4 and C 2 H 6 molecules enriched in 12 C desorb and diffuse more readily than the heavier hydrocarbons (including those enriched 13 C), the gas remaining in the coal and shale samples after extensive desorption shows a reversed CSI trend where CH 4 and C 2 H 6 are isotopically heavier compared with the longer chain hydrocarbons. Reversed isotope trends may also develop over geological time, particularly where the source rock is fully gas-mature and has expelled hydrocarbons due to prolonged degassing. As seen in the Beetaloo Sub-basin, the CSI trend in the dry-gas-mature Velkerri shale is reversed, possibly due to the loss of a large proportion of originally generated CH 4 during post-Cambrian basin uplift.

The application of natural landform analogy and geology‐based spoil classification to improve surface stability of elevated spoil landforms in the Bowen Basin, Australia—A review

AbstractLarge‐scale open cut mining has occurred within the Bowen Basin for over 4 decades, transitioning from shallow mining depths and limited spoil elevation to increased mining depths, prestripping and increasingly elevated mesa‐like landforms.As a result of this evolution, the stabilisation of modern constructed landforms is no longer assured through the establishment of vegetation alone. The selection of resilient fragmental spoil types for the construction of final landform surfaces, and as cladding for stabilising steep erosive batters, is a practical methodology that has the potential to significantly improve rehabilitation outcomes, by increasing surface rock cover, roughness, and infiltration and reducing erodibility. An understanding of the properties and behaviour of individual spoil materials disturbed during mining is required.Relevant information from published literature on the geological origins, lithology, and weathering characteristics of individual strata within the Bowen Basin Coal Measures (and younger overlying weathered strata) has been reviewed, related to natural landforms and applied to the surface stability of major strata types when disturbed by mining.A spoil classification derived from geological characteristics and weathering behaviour of identifiable lithologic components has been reviewed and refined, demonstrating the application of use of geological information.This classification system is a tool for the allocation of spoil types and use of categories that have application in premine feasibility investigations, landform design, and material selection and placement. The logic of classifying materials based on their stability in the natural landscape has wider relevance to other mining areas where elevated landforms of sedimentary material are constructed.

Image processing based characterisation of coal cleat networks

Characterisation of the cleat network serves as the basis for estimating the hydraulic and mechanical seam properties which in turn are fundamental for flow and geomechanical modelling in the context of underground coal mining. Cleat and cleat network geometry can be described as a function of frequency, aperture, size, orientation relative to in situ stresses, connectivity and porosity, with mineralised and un-mineralised cleats occurring. To describe these properties, CT-scans of core samples of a Bowen Basin coal in central Queensland, Australia, are analysed.A unique image processing workflow method is introduced to extract the key statistical parameters of perpendicular butt and face cleats present in a two-dimensional image. As face and butt cleats have different characteristics, the presented method distinguishes face cleats and butt cleats by direction and present detailed data for both cleat types. The results comprise cleat length, apertures, sizes, intensities, densities, shape parameter, spacing, orientation and connectivity and are therefore more comprehensive than previous cleat descriptions. Three generally different cleat geometries are considered within this study, one sample shows perpendicular face and butt cleats, the second two sets of face cleats intersected by butt cleats and the third parallel face cleats only.

Biogenic methane production from Bowen Basin coal waste materials

A microbial consortium derived from sewage sludge from the treatment of wastewater (Luggage Point Wastewater Treatment Plant, Brisbane, Australia) has been applied to Jameson Cell (J-cell) rejects (Ro,max=0.96±0.008) of a Bowen Basin coal preparation plant to assess the potential for biogenic methane production. A maximum methane yield of 26.20μmol/g J-cell rejects (0.64m3CH4/ton) was observed, suggesting biogenic methane production from coal waste materials is a feasible process if yields can be improved. Molecular analysis performed on the microbial consortium showed similar microbial community compositions to those observed in natural coal bed environments. The study demonstrates that Australian coal waste materials can be used as a viable feedstock for biogenic methane production using microorganisms that are not native within the coal beds.

A spatial assessment framework for evaluating flood risk under extreme climates

Australian coal mines have been facing a major challenge of increasing risk of flooding caused by intensive rainfall events in recent years. In light of growing climate change concerns and the predicted escalation of flooding, estimating flood inundation risk becomes essential for understanding sustainable mine water management in the Australian mining sector. This research develops a spatial multi-criteria decision making prototype for the evaluation of flooding risk at a regional scale using the Bowen Basin and its surroundings in Queensland as a case study. Spatial gridded data, including climate, hydrology, topography, vegetation and soils, were collected and processed in ArcGIS. Several indices were derived based on time series of observations and spatial modeling taking account of extreme rainfall, evapotranspiration, stream flow, potential soil water retention, elevation and slope generated from a digital elevation model (DEM), as well as drainage density and proximity extracted from a river network. These spatial indices were weighted using the analytical hierarchy process (AHP) and integrated in an AHP-based suitability assessment (AHP-SA) model under the spatial risk evaluation framework. A regional flooding risk map was delineated to represent likely impacts of criterion indices at different risk levels, which was verified using the maximum inundation extent detectable by a time series of remote sensing imagery. The result provides baseline information to help Bowen Basin coal mines identify and assess flooding risk when making adaptation strategies and implementing mitigation measures in future. The framework and methodology developed in this research offers the Australian mining industry, and social and environmental studies around the world, an effective way to produce reliable assessment on flood risk for managing uncertainty in water availability under climate change.

Thermodynamic models for swelling of unconfined coal due to adsorption of mixed gases

Permeability evolution in coal seams during CO2-Enhanced Coalbed Methane (ECBM) production is strongly influenced by swelling/shrinkage effects related to sorption and desorption of both CO2 and CH4. Other gases, such as N2, have also been proposed for injection in ECBM operations. In addition, water vapour will almost always be naturally present. Much work has been done on the sorption/swelling response of coal exposed to pure gases. However, to address in-situ coal-seam conditions realistically, an understanding of the swelling behaviour of coal matrix material due to multiple-species adsorption is needed. Here, we construct new thermodynamic models for swelling of unconfined coal due to adsorption of multiple gas/fluid species, addressing the equilibrium state and focusing on a binary gas mixture. Four models are derived, covering three possible end-member interactions, plus a generalized case. The end-member models considered correspond to adsorption of gases α and β at fully separate sites, at fully shared sites and at partially shared sites (Models 1–3, respectively). We compare our model predictions with literature data on the swelling behaviour of Bowen Basin coal exposed to CH4/CO2 mixtures at total pressures up to 15 MPa and at 55 °C. The results show that swelling of the Bowen Basin coal exposed to CO2/CH4 mixtures is best explained by Model 3, which in turn implies that adsorption and swelling is in this case determined by both partial pressure and selective adsorption (affinity) effects. Model 3 is easy to parameterize and its applicability to specific coals and gas mixtures is easily evaluated. Though more difficult to parameterize, our generalized swelling model (Model 4) can be applied to any coal rank, and to any binary gas mixture. It therefore offers an important tool for modeling swelling and permeability evolution during ECBM operations.

Creation of microchannels in Bowen Basin coals using UV laser and reactive ion etching

We report a set of techniques to create microchannels (20–40μm) in Bowen Basin coals for use as experimental models that replicate natural cleats at sub-surface reservoir conditions. The model cleats were developed to allow small scale systematic laboratory investigations of water and gas relative permeability behaviour in coal seam gas reservoirs. The four cleat fabrication techniques evaluated were ultra violet (UV) laser, reactive ion etching in oxygen plasma (RIE), mechanical scratching with a tungsten carbide cutting tool and chemical etching with KMnO4. The mechanical and chemical methods were eliminated from the study as chemical etching produced large variations in cleat aperture and the mechanical method sheared off sections of the channel. The UV laser and RIE methods both produced channels that met the width, depth and shape criteria. Micro-Raman spectroscopy indicated that UV laser and RIE etching did not induce significant chemical changes on the coal surface. Soft-lithography moulds and SEM observations showed that the morphology of the walls in the RIE channel more closely resembled a natural cleat than the walls of the laser etched channel, which were much smoother. The visualisation of two-phase water and gas flow through the UV laser and RIE channels was demonstrated in a microfluidic device.

Application of Airborne EM to Bowen Basin Coal Projects

In 2011, several Bowen Basin coal exploration projects in central Queensland were surveyed with the SkyTEM airborne electromagnetic system. The aim of these surveys was to trial the effectiveness of the method in for coal applications, with specific focus on mapping the key aspects of the weathering, overburden and hydrology that are most important for effective development of a coal project. The first survey, which was over an area near Moranbah with 150m of Tertiary overburden with extensive basalt flows, mapped crucial hydrological and geological features important to efficient further development of the project: These included a) a convoluted network of basalt-filled palaeochannels, some of which extend through and into the coal measures b) location, extent and thickness of major saline aquifers, c) depth to base of Tertiary. Subsequent surveys over different parts of the Bowen basin, showed that SkyTEM was a very effective tool for mapping the thickness of weathering, a parameter very important for drilling out coal resources. The results are surprising in that they not only provide a detailed map of depth of weathering (including volumes of lateritic sediments requiring excavation), but also map more subtle geological features such as synclines (previously only loosely inferred from surface mapping), major faults, previously unknown dykes, dip and strike of the main stratigraphic units, fresh water aquifers and even possibly also various coal seams (generally because they are saline aquifers). These surveys represent, at least to the authors? knowledge, the first example of modern helicopter-borne transient EM in the Australian coal fields. The results from this project and subsequent projects are surprising in that they have had unexpected applications above and beyond those which were originally forecast. Thus, with informed and thorough use, airborne EM promises to deliver valuable geological and geotechnical information to aid in the further development of Australia?s coal resources.

Automated Interpretation of 3D seismic data using genetic algorithms

SummaryOver the past twenty-five years Geoscientists have acquired more than 550,000 square kilometers of 3D seismic data [APPEA statistics] over continental and offshore Australia in the pursuit of mineral and petroleum deposits. Whether the target is hydrocarbons of any phase (solid, liquid or gas) or minerals, the information extracted from the 3D seismic data when integrated with other geological and geophysical data helps form models of the subsurface. These models are the foundation upon which decisions are made, directing the design and the development of many aspects of the mine site. The success of these activities often depends upon the accuracy of these models.Many advances in acquisition, processing and interpretation methods have been implemented since the first 3D seismic surveys were acquired in Australia during the 1980s. As a consequence of these advances, the geoscientist today is faced with dramatic increases in the volumes of high quality data available for analysis. However, the time available for the thorough examination, analysis, extraction and integration of the information from these large often multi-volume datasets is always limited and is becoming more problematic. Typically, the geoscientist will spend most of their available time extracting information from small portions of these datasets with a disproportionately amount of time spent thinking about the significance of the results.Fortunately, Geoscientists are not the only, or the first Scientists, to face challenges associated with the analysis of large amounts of data. Specifically, ideas developed during the course of the thirteen (13) year Human Genome Project (HGP) have been adapted to help interpret seismic data by automatically segmenting and identifying all surfaces within a 3D volume of data; which is then stored into a visual database. Using this technology enables the Geoscientist the ability to analyse large amounts of data in a unbiased manner and thereby incorporate much more data into their models. The application of this patented technology is discussed and demonstrated on several 3D seismic datasets collected over several Queensland Bowen Basin Coal Mine sites.

The role of private corporations in regional planning and development: Opportunities and challenges for the governance of housing and land use

There is a broad consensus that the corporate sector has an active role in facilitating community development through corporate-community investment. However, there remains uncertainty as to how much the sector should invest without taking on responsibilities and influencing decisions which are traditionally within the realm of the state. This paper explores the contemporary role of mining companies in regional governance through a case study of housing and residential land use planning and supply within the Bowen Basin coal mining and coal seam gas extraction region in Queensland, Australia. Mining companies were found to have a broad role in planning and development with far-reaching implications for land use and the wider regional community. These arrangements have emerged in response to corporate sensibilities, some recent policy requirements and most importantly, in pragmatic response to the institutional void apparent in many regional communities. In the absence of an effective regional framework with a clear and defined role for governments and corporations, powerful corporate interests risk effectively ‘capturing’ the regional development agenda within a framework that works on short-term paternalism rather than long-term regional partnerships. This also has implications for the efficiency of governance. Stronger institutional arrangements need to be developed to strengthen the capacity of the state to oversee these relationships. Regional governance and planning theory also needs to address this deficit.

Permeability increase in Bowen Basin coal as a result of matrix shrinkage during primary depletion

Matrix shrinkage in coal-seam gas reservoirs as a result of production, and the resultant impact on absolute permeability, is a current area of intense interest. A number of publications have documented the existence of this phenomenon. The exponential growth behaviour of coal seam permeability with reservoir pressure depletion has been observed previously at the Fairway wells in the San Juan basin. More recently, an exponential trend has also been confirmed for a group of 10 wells in a region northeast of Fairway. An increase in the absolute permeability of coal seam is a result of matrix shrinkage caused by gas desorption, which becomes a dominant factor on cleat permeability over the effective horizontal stress change during production. Bottom hole pressure recovery during shut-in periods can provide valuable data concerning the impact of coal seam gas production on reservoir permeability. Arrow Energy's historical production data, from the MGP field in the Bowen Basin, contains over 60 instances of data on pressure recovery during shut-in. These time lapse build-up periods have been analysed using the pseudo-pressure method (Kamal and Six, 1991) in an effort to gain an understanding of the extent and effect of this phenomenon within the company's existing assets. The method is based on a pseudo pressure function that uses the adsorption isotherm and the relative permeability relations of the reservoir. The infinite acting radial flow regime was identified and the gradient was recorded on a semi log plot of the pseudo pressure versus dimensionless time. Results from these tests show multi fold increase in absolute permeability as a function of the estimated reservoir pressure.

Occurrence of minerals within fractures and matrix of selected Bowen and Ruhr Basin coals

Abstract Late Permian age samples of black coal from the coeval Bandanna Formation and Baralaba Coal Measures of Queensland's Bowen Basin, as well as Carboniferous coal from Germany's Ruhr Basin, were investigated. This paper presents scanning electron microscope (SEM) analysis of the mode of occurrence of minerals within these coals, and characterisation of the natural fractures present within the samples. The coal samples located proximal to orogenic fronts in both basins are structurally complex, with multiple generations of mineralisation evident. Stable isotope analyses of carbonates from selected samples indicate that methanogenesis-related CO2 reduction has occurred at two of the Bowen Basin sampling locations. Ankerite precipitated from a magmatic fluid or basinal brine is associated with significant sulphide mineralisation within brecciated Ruhr coal proximal to major fault intersections with anticlines, and has been meteorically altered to ferroan calcite and sulphates in coal from immediately below the Carboniferous/Cretaceous unconformity. Similar alteration has occurred within a Bowen Basin coal sample close to the present day land surface. Variation of mineralisation within both the cleats and the coal matrix occurs on a fine scale, with crystals of multiple mineral species often being present within micrometres of each other.

GEOLOGICAL CONTROLS ON EXPLOITABLE COAL SEAM GAS DISTRIBUTION IN QUEENSLAND

Methane is present in all coals, but a number of geological factors influence the potential economic concentration of gas. The key factors are (1) depositional environment, (2) tectonic and structural setting, (3) rank and gas generation, (4) gas content, (5) permeability, and (6) hydrogeology. Commercial coal seam gas production in Queensland has been entirely from the Permian coals of the Bowen Basin, but the Jurassic coals of the Surat and Clarence-Moreton basins are poised to deliver commercial gas volumes.Depositional environments range from fluvial to delta plain to paralic and marginal marine—coals in the Bowen Basin are laterally more continuous than those in the Surat and Clarence-Moreton basins. The tectonic and structural settings are important as they control the coal characteristics both in terms of deposition and burial history. The important coal seam gas seams were deposited in a foreland setting in the Bowen Basin and an intracratonic setting in the Surat and Clarence-Moreton basins. Both of these settings resulted in widespread coal deposition. The complex burial history of the Bowen Basin has resulted in a wide range of coal ranks and properties. Rank in the Bowen Basin coal seam gas fields varies from vitrinite reflectance of 0.55% to >1.1% Rv and from Rv 0.35-0.6% in the Surat and Clarence-Moreton basins in Queensland. High vitrinite coals provide optimal gas generation and cleat formation. The commercial gas fields and the prospective ones contain coals with >60% vitrinite.Gas generation in the Queensland basins is complex with isotopic studies indicating that biogenic gas, thermogenic gas and mixed gases are present. Biogenic processes occur at depths of up to a kilometre. Gas content is important, but lower gas contents can be economic if deliverability is good. Free gas is also present. Drilling and production techniques play an important role in making lower gas content coals viable. Since the Bowen and Surat basins are in a compressive regime, permeability becomes a defining parameter. Areas where the compression is offset by tensional forces provide the best chances for commercial coal seam gas production. Tensional setting such as anticline or structural hinges are important plays. Hydrodynamics control the production rate though water quality varies between the fields.