A new concept for the extraction of gas from Permian ultra-deep coal seams of the Cooper Basin, Australia: Expanding Reservoir Boundary Theory
Erik C. Dunlop
+ Author Affiliations
- Author Affiliations
Australian School of Petroleum and Energy Resources, The University of Adelaide, Adelaide, SA 5005, Australia. Email: erik.dunlop@adelaide.edu.au
The APPEA Journal 60(1) 296-314 https://doi.org/10.1071/AJ19011
Submitted: 24 February 2020 Accepted: 30 March 2020 Published: 15 May 2020
Abstract
An alternative geomechanical reservoir boundary condition is proposed for ultra-deep coal seams of the Cooper Basin in central Australia. This new concept is embodied within the author’s ‘Expanding Reservoir Boundary (ERB) Theory’, which calls for a paradigm shift in gas extraction technology, diametrically opposed to current practices. As with shale, full-cycle, standalone commercial gas production from Cooper Basin ultra-deep coal seams requires a large stimulated reservoir volume (SRV) having high fracture surface area for gas desorption. This goal has not yet been achieved after 13 years of trials because, owing to the bipolar combination of shale-like reservoir properties and coal-like geomechanical properties, these poorly cleated, inertinitic coal seams exhibit ‘hybrid’ characteristics. Stimulation techniques adopted from other play types are incompatible with the highly unfavourable combination of nanoDarcy-scale permeability, ‘ductility’ and high stress. Nevertheless, gas flow potential counterintuitively increases with depth, contingent upon the creation of an effective SRV. Optimum reservoir conditions occur at depths beyond 9000 feet (2740 m), driven by dehydration, high gas content, gas oversaturation, overpressure and a rigid host rock framework. The physical response of ultra-deep coal seams and the surrounding host rock to pressure drawdown is inadequately characterised. It remains to be established how artificial fracture and coal fabric aperture width change due to the competition between desorption-induced coal matrix shrinkage and compaction caused by increasing effective stress. Studies by the author suggest that pressure arching may ultimately control gas extraction efficiency. Harnessing this geomechanical phenomenon could resolve the technical impasse that currently inhibits commercialisation. Pressure arching neutralises SRV compaction by deflecting stress to adjacent strata of greater integrity. These strata then function as an abutment for accommodating increased stress outside the SRV. This shielding effect allows producing ultra-deep coal seams to progressively de-stress and ‘self-fracture’ naturally, in an overall state of shrinkage-induced tensile failure. An ‘expanding reservoir boundary and decreasing confining stress’ condition is generated by the combined, mutually sustaining actions of coal matrix shrinkage and sympathetic pressure arch evolution. This causes the SRV to steadily increase in size and permeability. Cooper Basin ultra-deep coal seams may be effectively stimulated by harnessing this self-perpetuating, depth-resistant mechanism for creating permeability and surface area. The ultra-deep coal seams may be induced to pervasively ‘shatter’ or ‘self-fracture’ naturally during production, independent of ‘brittleness’, analogous to the manner in which shrinkage crack networks slowly form, in a state of intrinsic tension, within desiccating clay-rich surface sediment.
Keywords: adsorption, coal matrix shrinkage, Cooper Basin, deep coal, desorption, dynamic permeability, effective stress, fracture network, pressure arch, pressure transient analysis, rate transient analysis, source rock reservoir, stimulated reservoir volume, tensile dilation, tensile failure, unconventional reservoir.
Erik C. Dunlop is an Exploration Geoscientist with 33 years of experience in the upstream oil and gas industry. After graduating with a BSc (Hons) from The University of Adelaide in 1986, a stint of fieldwork in the Timor Sea and Cooper Basin led to employment at Santos Limited in 1988, where he remained until 2015. His activities at Santos included exploring for conventional hydrocarbon accumulations in the Cooper and Eromanga basins; initiating a variety of Cooper Basin shale, ultra-deep coal and tight sandstone ‘pathfinder’ projects; and internationally patenting a process for more accurately and cost-effectively quantifying the total gas content of source rock reservoirs and gas hydrates. In early 2020, he completed PhD studies at the Australian School of Petroleum and Energy Resources, The University of Adelaide, which investigated controls on the dynamic gas production behaviour of Cooper Basin ultra-deep coal seams. He is currently working with Deep Coal Technologies Pty Ltd and Gidgee Energy Pty Ltd to patent and implement a holistic, paradigm shift technique for drilling, completing and extracting gas from ultra-deep coal seams. Erik is currently a member of AAPG and PESA. |
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