Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining

Johnson Jr., Raymond L. (2014). Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2017.10

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Author Johnson Jr., Raymond L.
Thesis Title Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2017.10
Publication date 2014-01-31
Thesis type PhD Thesis
Supervisor Saiied Aminossadati
Zhongwei Chen
Joan Esterle
Total pages 281
Language eng
Subjects 091405 Mining Engineering
091406 Petroleum and Reservoir Engineering
040309 Petroleum and Coal Geology
Formatted abstract
Gas drainage has been performed in advance of coal mining for commercial production of gas and to reduce levels of gas to safe or regulatory levels required for coal mining. Inadequate pre-drainage can lead to higher levels of gas than may be adequately managed by ventilation systems, potential pressure scenarios, or even outbursts, any of which can affect mine safety and economics. Thus, it may be desirable to more rapidly depressurise the coal seam based on mine safety and production scheduling, currently requiring either stimulation of the coal or additional drilling of pre-drainage wells and potentially jeopardising project economics.

The aim of this research is to establish non-permeable barriers or ‘curtains’ to alter subterranean fluid flow patterns in coal as a means to enhance pre-drainage in advance of coal mining and reduce gas influx into areas of mining. The novel method of barrier placement proposed in this thesis utilises hydraulic fracturing with non-permeable materials from horizontal wells to create regions of permeability damage, reducing fluids influx into the drainage area. A modelling demonstration of a barrier placement is made with a generalised dataset derived from the Bowen Basin, Australia, a historical area of mining pre-drainage. The commercially available, planar, three-dimensional (3D) fracturing simulator applied in the thesis for barrier implementation was verified in a highly instrumented, hydraulic fracturing experiment in the Walloon Coals, Surat Basin, Queensland performed and referenced by the author.

Once the barrier is placed in the reservoir, the effectiveness and benefits of a barrier placement are modelled for the theoretical gas drainage scenario and demonstrate the benefits of the barrier placement. A commercially available reservoir modelling software is used to investigate a range of potential reservoir parameters (e.g., porosity, permeability, permeability anisotropy, reservoir dip, etc.) to assess the impact and sensitivity of parameters. To validate the effectiveness of the barrier placement, the thesis describes strategies and simulates potential pressure responses based on two potential analytical models in a commercially available, analytical well testing software using observation wells.

To quantify the economic benefit of this research to the mining industry, an economic modelling framework is developed based on a on a pre-drainage production case where gas, predominantly methane, is sold into the Eastern Australian gas market and water costs differ between pre- and post-drainage to capture surface drainage cost differentials and efficiencies versus underground drainage costs. The model highlights key uncertainties and variables affecting economic outcomes based on timing of the implementation relative to the start of mining and any emissions trading scheme (ETS) or carbon costing that may be in effect. Environmental benefits and cost savings are based on potentially reducing fuel or fugitive gas emissions under two scenarios: either nil to current European Union (EU) market price (as of July 2013) or EU market to Australian Treasury estimated long-term carbon costs (also as of July 2013).

Implementing the design, execution, evaluation and verification strategies for barrier placement outlined in this thesis reduces uncertainties around barrier placement, reservoir modelling, and overall gas pre-drainage effectiveness in coal mining pre-drainage applications. This research demonstrates that in moderate to high permeability cases, installation of barriers can increase the rate of pressure drainage within the perimeter curtained and relative to not installing the barriers.

Economic analysis using the full range of outputs from the reservoir modelling indicates that installing barriers around areas of moderate to high permeability coals can be cash positive with internal rate of return (IRR) and net present value (NPV) highly dependent on the amount of alternative mitigation employed and inputted monetary discount rate. Installing barriers around drainage areas in moderate to high permeability coals has a positive environmental impact by reducing ongoing gas influx (i.e., for fuel or as fugitive mine emissions) from the target seam into operating then abandoned mine works from the surrounding unmined and undrained coal reservoir.

Finally, the author makes recommendations for further research into flow modification materials (FMM), hydraulic fracturing diagnostics to improve barrier placement implementation and verification, and recommendations for multi-seam scenarios and operational implementation.
Keyword Barriers
Barrier placement
Coal pre-drainage
Fugitive gas
Fugitive emissions
Coal mining
Hydraulic fracturing
Hydraulic fracturing modelling
Reservoir modelling
Economic modelling

Document type: Thesis
Collections: UQ Theses (RHD) - Official
UQ Theses (RHD) - Open Access
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Created: Tue, 11 Mar 2014, 08:55:16 EST by Raymond Johnson on behalf of Learning and Research Services (UQ Library)