3D Permeability Characterisation of Large Coal Samples

ACARP, Newlands and Kestrel have supported a University of Queensland project investigating the 3-dimensional permeability characteristics of coal under true triaxial stress conditions. This is achieved by using the world-first True Triaxial Stress Coal Permeameter (TTSCP), recently-developed at UQ. This facility can test coal samples as large as 200mm, with in-situ stresses as high as 28 MPa and internal pore pressures as high as 14 MPa. It can deliver anisotropic stresses equal to in-situ conditions, whilst measuring permeability in each of the three principal directions.

A preliminary technical report was presented to an ACARP In-seam Drilling and Gas Seminar on 25 October 2002. A draft of the Final Report was submitted in February 2003. This report contains those initial results and additional insights gathered while testing a large 200mm coal cube.

The experimental investigations were carried out on three 40 mm cubes, one 80mm cube and the 200mm cube, all at restored state and in-situ conditions. These specimens were cut from two large coal blocks sourced from the Newlands underground mine. In all, 31 separate experiments were conducted; our findings are summarised below under the headings of the principal aims of the project.

Summary of Outcomes

1. This project has improved the understanding of mechanisms controlling gas flow in coal seams, for realistic anisotropic stress fields and for dynamic conditions as gas is pressure-depleted from coal seams.
2. The results can now be incorporated in updated gas flow prediction models and coal seam reservoir simulators to deliver much increased accuracy of expected seam gas flows under various operational conditions, by factors of 10 to 100, especially in the latter stages of gas depletion, where the directional net stresses can be seen to increase permeability, against a 10 to 100 fold decrease under isotropic stress conditions.The results can now be incorporated in updated gas flow prediction models and coal seam reservoir simulators to deliver much increased accuracy of expected seam gas flows under various operational conditions, by factors of 10 to 100, especially in the latter stages of gas depletion, where the directional net stresses can be seen to increase permeability, against a 10 to 100 fold decrease under isotropic stress conditions.
3. The results point to a range of scale-up factors that can be applied to laboratory-derived permeabilities and permeability profiles over time, to use in the gas flow calculations and prediction models, without having to perform expensive and often unreliable in-situ flow tests (injection fall-off or production build-up), which can cost over $1m.
4. The project has developed and proved equipment and procedures that can test for permeability anisotropy ratios, proved to be significant in gas flow predictions and for gas depressurisation procedures, such as confirming the need for directional drilling across the face cleats. This will result in substantial cost savings to the industry compared with performing multi-well tests to determine the permeability anisotropy ratios, which cost upwards of $3 million (minimum 3 wells).
5. The project has confirmed that the smaller 61mm HQ cores recovered from drill wells can continue to be used for mine development planning, by applying the appropriate scale-up factors, which will be site/coal rank dependent.
6. The world's first True Triaxial Stress Coal Permeameter has been proven reliable and versatile to test the varied 4-D permeability aspects of coals and is available to perform further investigations for industry on a research or contract lab testing basis, with the University of Queensland having hired a full-time technician to oversee its operation.

The above findings and capabilities will lead to:

• Improved mine safety, minimising the risk to both equipment, personnel and mine assets;
• Improved design of de-methanation/de-pressurisation procedures, leading to better engineering design and cost reductions in ventilation and in-seam drilling systems;
• Improved and accelerated methane recovery from coal seams, allowing accelerated coal production and less greenhouse gas emissions.