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• A novel approach for modelling subsidence induced by fluid extraction from coal seams. • Transient groundwater and subsidence models are linked. • Coal shrinkage induced by gas desorption can significantly impact subsidence. Several coal seams are contained within the formations of Great Artesian Basin (GAB), the largest natural underground water reservoir in Australia and the world. The extraction of coal seam gas (CSG) and its associated water from thousands of wells within the coal measures of the GAB has led to tens of millimetres of subsidence in CSG development areas. Since highly developed farming systems located in these areas rely on very low slope landforms, even this scale of subsidence has caused significant community concern about the potential for CSG extraction to impair farming operations and productivity through changes in land slope and drainage. Coal seam compaction associated with dewatering and gas extraction has two key components: poromechanical compaction and desorption-induced bulk shrinkage. The former results from pore pressure depletion (due to dewatering and gas extraction) and an increase of vertical effective stress, while the latter is induced by gas desorption from the coal matrix, leading to further deformation. While poromechanical compaction of fluid-bearing formations has been extensively addressed in the literature, relatively little research has been conducted on the role of coal shrinkage in CSG-induced subsidence. This paper introduces an innovative practical modelling approach for assessing CSG-induced subsidence at a subregional/regional scale. The approach utilises an analytical model for CSG-induced subsidence derived from constitutive stress–strain relations for poroelastic-sorptive media. This is a novel approach in the context of CSG-induced subsidence which considers both poromechanical compaction and desorption-induced shrinkage. A further distinction to previous work is the integration of the geomechanical-sorptive subsidence model with a numerical groundwater model. Based on this approach, this paper examines the effect of coal shrinkage on subsidence, and its proportions with respect to total compaction for one of the major coal measures in the Surat Basin. Input data are derived from three-dimensional geological, geomechanical, and groundwater models, as well as methane adsorption and desorption reports. Results show that (a) the impact of coal shrinkage on CSG-induced subsidence is likely to be significant in the study area and (b) the contribution of coal shrinkage to CSG-induced subsidence depends on gas content, Langmuir isotherm, shrinkage strain parameters, and the saturation state of coal. This study provides important insights into CSG-induced subsidence and lays the foundation for the development of robust, efficient, and localised predictive models to support environmental impact assessment and management. [ABSTRACT FROM AUTHOR]

• Demonstrated capability of three-phase foam flow in dewatering unconventional wells. • Examined flow regimes of foam flows in vertical tubing and annulus. • Examined the effect of pressure on dynamic and static foam flow characteristics. • Tested the effect of salt and particles on foam stability. This paper aims to investigate and demonstrate, for the first time, the feasibility of using an artificial lift system, referred to as foam assisted gas lift (FAGL) for dewatering unconventional gas wells to depressurise coal and allow gas to be produced. More specifically, this paper investigates the capability of a three-phase foam flow to lift gas, liquid and solid particulates to the surface to optimise gas production by minimising the flowing bottom-hole pressure (FBHP) and minimising downtime, operational costs and production losses caused by downhole pump failures. A laboratory set-up of a 9 m vertical tubing with 80 mm (3.1 in) ID and a 7 m annulus with a casing inner diameter of 170 mm (6.69 in) and tubing outer diameter of 70 mm (2.76 in) are used to investigate the foam flow behaviour in both geometries for a wide range of gas and water flowrates, 40–6500 L/min and 10–80 L/min, respectively. The 80 mm tubing is pressurised up to 6 bar, allowing examining the foam flow characteristics and foam stability at different depths of a gas well, by changing the operating pressure. Foam flow regimes in the tubing and the annulus, which are key in determining the FBHP are characterised by analysing differential pressure signals, collected at 100 Hz frequency. The addition of foamer (sodium dodecylbenzene sulfonate) significantly advanced the transition from churn to the annular flow regime, resulting in a lower required gas flow rate to obtain the least attainable FBHP. A model is developed to evaluate the FBHP of the FAGL, compared against the measured FBHP in 36 gas wells in Australia, equipped with down-hole pumps for dewatering. Our results indicate that FAGL is capable of maintaining a low FBHP for a wide range of gas and water flow rates. The effective particle lifting capacity of foam flow is also demonstrated. Further, the positive impact of water salinity (0.1 M NaCl) in the presence of particles (15–25 g/l) on foam integrity is presented. [ABSTRACT FROM AUTHOR]

This paper evaluates the potential methane storage capacity of six clay-rich interburden rock samples from coal seam gas (CSG) wells in the Surat Basin, Australia. Clay minerals identified in all samples included kaolinite, illite, smectite, and illite-smectite mixed-layers. The total organic carbon concentrations in these interburden rocks ranged from 0.66–1.19 wt%, and thus these rocks can be classified as fair to good hydrocarbon source rocks. The effective porosity of the rocks determined from mercury intrusion porosimetry and helium pycnometry varied from 6.8% to 12.5%, and included volumes of micropores and mesopores. The adsorption isotherm results indicated that the average adsorption capacity of six interburden was 3.64 cm3/g, a value corresponding to approximately 20% that of Surat Basin coal. Based on the clay compositions and porosity of the samples, the permeability of these Surat interburden rocks is estimated to be <5 nano Darcy using Yang and Aplin's empirical correlation, which was too low for reliable measurement in our laboratory core flooding apparatus even with a differential pressure of 10 bar applied over a shortened 20 mm length core. However, after stimulation by electrohydraulic discharge (EHD) shockwaves the permeability of one of the interburden samples (S2) increased to 0.6 ± 0.11 mD due to development of fractures and new pores by the EHD stimulation. We characterised the development of the fractures after EHD shockwaves using x-ray computer tomography. The findings of this study suggest that dynamic shockwaves such as those generated by EHD have potential to increase permeability of soft and clay-rich interburden layers in CSG reservoirs and other layered reservoirs. This potentially opens these ultra-tight gas resources to exploitation and recovery. • A suite of coal seam interburden samples has been characterised and investigated for gas potential appraisal. • After EHD stimulation, the confining-constrained interburden discs were successfully cracked with fractures/voidages. • The shock pressure at both radial and axial orientations was recorded to reveal the philosophy of interburden breakage. • As a pioneering trial to stimulate interburden under stress-state, EHD was employed on stimulating clay-rich interburden. [ABSTRACT FROM AUTHOR]

The demand for natural gas in Queensland, Australia has historically been supplied from conventional reservoirs. However, depletion in conventional sources has led producers to turn to extensive supplies in Queensland's coal resources. These coal seam gas (CSG) developments not only represent new supplies for the domestic market in eastern Australia, they are also the first time that CSG (aka coal bed methane or CBM) has been liquefied to serve the expanding world LNG market. In order to make this development occur, considerable infrastructure had to be installed, with field developments still on-going. This AUD$60 billion investment precipitated a major overhaul of state regulations to provide not only a safe and clean operating environment, but also to allay the concerns of certain stakeholders. The gas is primarily produced from thin high permeability coals in the Jurassic-age Walloon Coal Measures in the Surat Basin and from several relatively thick Permian-age coal seams in the Bowen Basin, of which the Baralaba Coal Measures and the Bandanna formation are the most important. There are numerous technical challenges with this production, such as fines production from the inter-burden clays, which can form a thick paste that is difficult to pump. Salt extraction by reverse osmosis, from associated water produced to depressurise the coal seams and enable the flow of gas, allows for the beneficial use of the water. Technical challenges also include mathematical modelling of the counter-current two-phase flow (gas and water) in the well annuli because conventional models in simulators only handle co-current two-phase flow in the well-bores. Also, the subject of on-going investigations is decommissioning of the large number of shallow wells over the next few decades in a safe and cost effective manner, with compressed bentonite being a promising option for well plugging. As with any major commercial development, in addition to the technical challenges there have been social challenges as well. These include interaction and coexistence of extensive surface operations with an established agricultural sector, interactions between gas production and ground water aquifers in water-stressed areas, and the cumulative social and economic impacts of 3 large projects on a rural area. Ultimately, the State of Queensland expects to produce more than 1800 BCF/annum, of which about 1400 BCF/annum will be exported as LNG. Depending on the demand and well productivity, up to 1000 CSG wells may be drilled per year for the next thirty years. A review of CSG resources, development, and challenges is presented in this paper to provide context for a stream of research findings that are emerging on the Queensland CSG experience. [ABSTRACT FROM AUTHOR]

The sustainable in situ regeneration of microbial (biogenic) methane or microbially enhanced coal bed methane (‘MECoM’) is an emerging concept being investigated globally. Promising results and recommendations of a preliminary culture study of Walloon Subgroup co-produced coal seam gas (CSG) waters from the Surat Basin, Queensland established the presence of viable methanogens and suggested that in situ methanogenesis could be stimulated using physical and chemical reservoir treatments. This paper represents the culmination of a stepwise basin analysis project ultimately aimed at siting potential in situ bioreactor locations in the eastern Surat Basin. The integration of stratigraphically located data on the molecular and isotopic composition of desorbed gases, host coal properties and spatially associated waters through the core production zone has allowed the spatial variability and relative influence of hydro-geological factors on methanogenesis to be evaluated in detail for the first time. Higher gas contents and systematically enriched CH 4 and CO 2 carbon isotopic compositions in the stratigraphically central coal seams (oldest-youngest: in the upper Taroom Coal Measures, Tangalooma Sandstone lower Juandah Coal Measures) are best explained by increased rates of microbial CO 2 reduction and substrate depletion. There is a building case to trial MECoM in the central coal seams in a depleted/underperforming well in an area of high permeability. Integrated microbiological, chemical engineering, hydro-chemical and geological studies are ongoing to further enhance understanding of Walloon Subgroup CSG and the bioreactor potential of the Surat Basin. [ABSTRACT FROM AUTHOR]

In-situ stress pattern and subsurface fractures control the production of Coal Seam Gas (CSG) reservoirs and, hence, comprehensive analyses are required for better development of these reservoirs. The Surat Basin is a prolific CSG province with major economic impacts in eastern Australia. Recent studies have reported significant stress variations in this basin due to local geological structures. However, a systematic analysis of localised stress perturbations within the basin, and their impacts on reservoir permeability have not been presented thus far in the published literature. In this paper, we combine different sets of data including conventional log data, wellbore image logs, 2D seismic lines and interpreted permeability (from well test data) in order to characterise the role of geological structures in the subsurface fluid flow of the Walloon Coal Measures in eastern Surat Basin. Analysis of 7.3 km of borehole image log across 33 vertical wells revealed a regional ENE-WSW orientation of the maximum horizontal stress (S Hmax) in the study area. However, there are massive variability in the S Hmax orientation, spatially and with depth. Analysis of 2D seismic lines highlight the role of geological structures in stress perturbation, and the interpretation of natural fractures using image logs show that most of the open fractures are oriented sub-parallel to the acute angle of local S Hmax orientation. Interpreted well test data in the study wells demonstrates that most of the high permeability data points are located where in-situ stress rotation is observed. This indicates the influence of local S Hmax orientation responding to geological structures on the subsurface fluid flow in the eastern part of the Surat Basin. • Variability of S Hmax orientation laterally and with depth. • Small scale geological structures control the local in-situ stress pattern. • Open fractures are oriented sub-parallel to the acute angle of local S Hmax orientation. • Most of the high permeability data points are located where in-situ stress rotation is observed. [ABSTRACT FROM AUTHOR]