History, Geology, In Situ Stress Pattern, Gas Content and Permeability of Coal Seam Gas Basins in Australia: A Review

Abstract

Coal seam gas (CSG), also known as coalbed methane (CBM), is an important source of gas supply to the liquefied natural gas (LNG) exporting facilities in eastern Australia and to the Australian domestic market. In late 2018, Australia became the largest exporter of LNG in the world. 29% of the country’s LNG nameplate capacity is in three east coast facilities that are supplied primarily by coal seam gas. Six geological basins including Bowen, Sydney, Gunnedah, Surat, Cooper and Gloucester host the majority of CSG resources in Australia. The Bowen and Surat basins contain an estimated 40Tcf of CSG whereas other basins contain relatively minor accumulations. In the Cooper Basin of South Australia, thick and laterally extensive Permian deep coal seams (>2 km) are currently underdeveloped resources. Since 2013, gas production exclusively from deep coal seams has been tested as a single add-on fracture stimulation in vertical well completions across the Cooper Basin. The rates and reserves achieved since 2013 demonstrate a robust statistical distribution (>130 hydraulic fracture stages), the mean of which, is economically viable. The geological characteristics including coal rank, thickness and hydrogeology as well as the present-day stress pattern create favourable conditions for CSG production. Detailed analyses of high-resolution borehole image log data reveal that there are major perturbations in maximum horizontal stress (SHmax) orientation, both spatially and with depth in Australian CSG basins, which is critical in hydraulic fracture stimulation and geomechanical modelling. Within a basin, significant variability in gas content and permeability may be observed with depth. The major reasons for such variabilities are coal rank, sealing capacity of overlying formations, measurement methods, thermal effects of magmatic intrusions, geological structures and stress regime. Field studies in Australia show permeability may enhance throughout depletion in CSG fields and the functional form of permeability versus reservoir pressure is exponential, consistent with observations in North American CSG fields.

Characteristics of in situ stress field of coalbed methane reservoir and its influence on permeability in western Guizhou coalfield, China

“…As a momentous parameter for evaluating the stability of underground engineering, in situ stress measurement is very important for evaluating the permeability, recoverability and fracturing ability of coal reservoirs (Meng et al, 2010;Wang et al, 2019;Sang et al, 2022). The influence of in situ stress is also reflected in areas such as gas content (Bell, 2006;Salmachi et al, 2021), coal structure (Hu et al, 2017;Lv et al, 2021), wellbore track and borehole stability (Zoback et al, 2003;Liu et al, 2004). Stress has three main impacts on CBM exploration and development: stress on coal reservoir pressure and pore and fracture development characteristics; the influence of stress on the series transfer process of desorption, diffusion and seepage of coalbed methane; the influence of stress on the present underground occurrence state and availability of natural fractures, as well as the morphology and extension direction of artificial fractures formed by hydraulic fracturing, etc.…”

Drainage Type Classification and Key Controlling Factors of Productivity for CBM Wells in the Zheng Zhuang Area, Southern Qinshui Basin, North China

“…Twenty-nine percent of Australia’s LNG nameplate capacity is in three east coast facilities that are supplied primarily by coal seam gas. 14 Since 2002, the Lower Cretaceous Mannville Group coals in the central Alberta Plains have been the focus for CBM production in Canada. 15 Over the past 25 years, China has developed its substantial CBM resources and realized commercial exploitation in several basins and regions, such as Qinshui, Ordos, Junggar, Tuha, and western Guizhou.…”

“…In the United States, commercial production began in the Black Warrior Basin, San Juan Basin, and Powder River Basin. ,, Six geological basins including Bowen, Sydney, Gunnedah, Surat, Cooper, and Gloucester host the majority of coal seam gas resources in Australia. Twenty-nine percent of Australia’s LNG nameplate capacity is in three east coast facilities that are supplied primarily by coal seam gas . Since 2002, the Lower Cretaceous Mannville Group coals in the central Alberta Plains have been the focus for CBM production in Canada .…”

Gas Migration Patterns with Different Borehole Sizes in Underground Coal Seams: Numerical Simulations and Field Observations

“…The structures of coal seams have been studied by many scholars and, currently, it is widely accepted that the dual-porosity system is suitable for describing the coal porosity structure [7,8]. Coal blocks are cut by fractures or cleats, and coal seams consist of coal matrices and coal fractures.…”