Preferential flow paths in a complex coal seam system

Abstract

Prior to commencing coal mining operations, measurements of permeable geological structures are an important task in the mine site exploration to facilitate the understanding of potential mining impacts on the adjacent river and groundwater systems. These potential environmental impacts are likely to be controlled predominately by preferential flow zones provided by faults and fractures, which are known to enhance the transport of gas and water. However, detailed mapping of these geological structures is rarely carried out in mining areas. In order to develop a feasible and rapid survey method, two field campaigns, involving soil gas mapping and gamma ray survey, were designed and carried out a at coal mine site in the Hunter River Valley, NSW, Australia. All measured parameters (radon, gamma ray and CO 2 ) revealed similar anomaly distribution patterns though measured by different techniques over different time periods (1.5 years elapsed between two field campaigns). Geophysical modelling (based on borehole logging results) and soil gas mapping were conducted independently, and both results indicated the existence of geological structures crossing the sampling region. Results of this study confirm the applicability of high intensity soil gas mapping methods for locating active tectonics faults covered by alluvium in the decametre scale. Additionally, compared with labour-intensive soil gas mapping methods, the gamma ray survey is capable of providing relatively reliable results in a fast and convenient way to cover a relatively large field site.The basic principles and controlling parameters for gas seepage from deep formations to the surface are not fully understood. In order to better understand gas transport mechanisms in the subsurface environment so that the soil gas mapping data can be better interpreted, laboratory experiments were conducted to investigate the flow of discrete microbubbles through a saturated porous medium as part of a rapid gas transport pathway from deep ground formations to the ground surface. During the experiments, bubbles, released from a point source, moved upward through a quasi-2D flume filled with transparent water-based gelbeads and formed a distinct plume that could be well captured by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviour between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width and breakthrough time of plume front. The ii experiments also revealed circulations of ambient pore water induced by the bubble flow.Based on a simple momentum exchange model, we showed that the relationship between the mean pore-water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of soil gas mapping and gas emission (including hydrocarbon gases) from deep ground formations as well as fund...

Discrete Bubble Flow in Granular Porous Media via Multiphase Computational Fluid Dynamic Simulation

“…Numerical simulations of gas transport in porous media have been extensively studied previously. This section reviews two conventional gas transport models: the continuum model of two-phase flow in porous media (Yan et al, 2022c) and the continuum "colloidal" discrete bubbles transport model (Ma, 2012) in order to identify their advantages and disadvantages. This review contributed to the development of a new model that could overcome those shortcomings.…”

“…Although many studies have comprehensively studied the former three mechanisms Mogro-Campero, 1978, 1979;Saunders et al, 1999), the basic principles and controlling parameters for bubbly flow from deep ground formations to the ground surface, proposed by Etiope and Martinelli (2002), have not been comprehensively investigated and developed. This research objective is, so far, still at an initial stage with a few academic attempts (Ma, 2015;Mahabadi et al, 2018;Chang and Lindquist, 2020;Yi et al, 2022).…”

Measurement of In-Situ Flow Rate in Borehole by Heat Pulse Flowmeter: Field-Case Study and Reflection

“…After unloading the overburdened geological strata above the coal seam layer, groundwater drainage from the ambient geoenvironmental system (e.g., the adjacent aquifer and river) will further exacerbate the surface water and groundwater-dependent ecosystem [2]. With an aim to appropriately assess and manage such risks, the novel in-situ geological investigation (GI) methods have been encouraged to conduct before the open-cut mining operation [3][4][5][6].…”