Evolution of coal strength with dewatering and coal seam gas depletion: an integrated approach
Jimmy Xuekai Li A , Thomas Flottmann B , Max Millen B , Shuai Chen A , Yixiao Huang A and Zhongwei Chen A *A
B
Dr Jimmy X. Li is a geoscientist and has 10 years of experience in the upstream oil and gas industry and 8 years of experience in academic research. His professional background spans a range of technical domains, including formation evaluation, petrophysics, and borehole geophysics, with a focus on both field operations and technical consulting. Between 2007 and 2017, he held various technical and management roles at Halliburton Energy Services Inc. Currently, Dr Li is a Postdoctoral Research Fellow at the School of Mechanical and Mining Engineering at the University of Queensland (UQ), where his research interests centre on geophysics, geomechanics, and geotechnical engineering. |
Thomas Flottmann is a structural geologist and holds a degree of MSc and PhD (WITS, Darmstadt, Frankfurt). Thomas has worked for over 40 years in academia and industry, with the last 15 years for Origin Energy in various technical/managerial roles. His technical focus is adaptation of geomechanical concepts for reservoir optimisation, fracture stimulation, and horizontal drilling operations. Thomas is Origin’s Chief Geoscientist. |
Max Millen is an operations geologist, currently working with Origin Energy on APLNG’s coal seam gas operations. He holds a BSc (Earth Science) and a MPhil (Science) from Queensland University of Technology. Previous work includes a multidisciplinary seal capacity assessment for CO2 sequestration targets. Max’s recent work has focused on reducing the geological and geomechanical uncertainty in support of drilling, completions, workover, and well abandonment operations. |
Shuai Chen is currently pursuing a PhD in Mining Engineering at the China University of Mining and Technology (CUMT). He earned his MSc in Mining Engineering from CUMT in 2020. From May 2023 to May 2024, he participated in a 1-year joint PhD program at UQ. His research focuses on micro-seismic monitoring, rockburst early warning through deep learning methods, and rockburst prevention using liquid nitrogen cyclic treatment. |
Yixiao Huang is a PhD candidate in the School of Mechanical and Mining Engineering at UQ. He graduated with a Bachelor of Engineering from UQ in 2021. His research focuses on investigating the impacts of rock weathering on the stability of underground excavations. |
Dr Zhongwei Chen is an Associate Professor at the School of Mechanical and Mining Engineering in UQ. Associate Professor Chen’s research interests are in the areas of resources geomechanics, fluid transport in fractured porous media, and applications of machine learning to underground mining, unconventional gas extraction, CO2 sequestration, and hydrogen underground storage. |
Abstract
Understanding the response of coal mechanical properties to dewatering and gas depletion is critical for estimating borehole stability and designing infill coal seam gas (CSG) wells. Despite its importance, the full impact of these processes on coal strength remains little explored. This study aims to quantify these effects through a combination of results from micro-computed tomography (micro-CT) imaging, sonic testing, and mechanical testing on coal samples. Micro-CT imaging provides insights into coal’s internal structure by focusing on parameters such as fracture porosity and fracture intensity (P32 factor). Sonic testing measures dynamic properties, including P-wave and S-wave velocities (Vp and Vs) and dynamic Young’s modulus (Ed), under both dry and wet conditions. Mechanical testing with acoustic emission monitoring evaluates static properties like Young’s modulus (Es) and uniaxial compressive strength (UCS). The key findings follow: (i) micro-CT imaging shows a strong correlation between coal fracture porosity and P32, offering detailed insights into the coal microstructure; (ii) mechanical testing reveals that dry samples exhibit a 10% higher Es and 31% greater UCS than wet samples, suggesting that dewatering increases coal strength but potentially also promotes embrittlement; and (iii) wet samples show higher Vp and Ed in sonic tests, indicating water saturation significantly influences sonic measurements. These findings improve the understanding of dewatering and gas depletion effects, laying the groundwork for more advanced geomechanical models in CSG operations.
Keywords: coal seam gas, coal strength, dewatering, micro-CT imaging, P-wave velocity, UCS, Young’s modulus.
Dr Jimmy X. Li is a geoscientist and has 10 years of experience in the upstream oil and gas industry and 8 years of experience in academic research. His professional background spans a range of technical domains, including formation evaluation, petrophysics, and borehole geophysics, with a focus on both field operations and technical consulting. Between 2007 and 2017, he held various technical and management roles at Halliburton Energy Services Inc. Currently, Dr Li is a Postdoctoral Research Fellow at the School of Mechanical and Mining Engineering at the University of Queensland (UQ), where his research interests centre on geophysics, geomechanics, and geotechnical engineering. |
Thomas Flottmann is a structural geologist and holds a degree of MSc and PhD (WITS, Darmstadt, Frankfurt). Thomas has worked for over 40 years in academia and industry, with the last 15 years for Origin Energy in various technical/managerial roles. His technical focus is adaptation of geomechanical concepts for reservoir optimisation, fracture stimulation, and horizontal drilling operations. Thomas is Origin’s Chief Geoscientist. |
Max Millen is an operations geologist, currently working with Origin Energy on APLNG’s coal seam gas operations. He holds a BSc (Earth Science) and a MPhil (Science) from Queensland University of Technology. Previous work includes a multidisciplinary seal capacity assessment for CO2 sequestration targets. Max’s recent work has focused on reducing the geological and geomechanical uncertainty in support of drilling, completions, workover, and well abandonment operations. |
Shuai Chen is currently pursuing a PhD in Mining Engineering at the China University of Mining and Technology (CUMT). He earned his MSc in Mining Engineering from CUMT in 2020. From May 2023 to May 2024, he participated in a 1-year joint PhD program at UQ. His research focuses on micro-seismic monitoring, rockburst early warning through deep learning methods, and rockburst prevention using liquid nitrogen cyclic treatment. |
Yixiao Huang is a PhD candidate in the School of Mechanical and Mining Engineering at UQ. He graduated with a Bachelor of Engineering from UQ in 2021. His research focuses on investigating the impacts of rock weathering on the stability of underground excavations. |
Dr Zhongwei Chen is an Associate Professor at the School of Mechanical and Mining Engineering in UQ. Associate Professor Chen’s research interests are in the areas of resources geomechanics, fluid transport in fractured porous media, and applications of machine learning to underground mining, unconventional gas extraction, CO2 sequestration, and hydrogen underground storage. |
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