The following Abstract explains this research update report by the Office of Groundwater Impact Assessment dated April 2023:
“Coal seam gas extraction involves two key subsurface processes: fluid flow primarily through coal fractures/cleats and gas desorption from the coal matrix. The former leads to the depressurisation of the coal seam and coal compaction, while the latter induces coal shrinkage. Both processes contribute to coal seam gas (CSG)-induced subsidence.
The amount of coal shrinkage in a given time period is proportional to the corresponding change in gas content. It also depends on coal properties, such as Langmuir isotherm and shrinkage strain parameters, which vary by coal composition and rank.
Past research has extensively addressed the effect of coal matrix shrinkage on coal permeability evolution through analysing measurements at both laboratory and field scales, as well as developing analytical and numerical models. Relatively little research, however, has been done on the effect of coal shrinkage on subsidence in CSG fields.
To explore the potential contribution of coal shrinkage to CSG-induced subsidence in the Surat Basin context, the Office of Groundwater Impact Assessment (OGIA) has developed, following the first principles, an analytical model for shrinkage strain, based on Langmuir-type relations between gas content and pore fluid pressure. This approach lends itself well to coupling with groundwater models, as the specific storage and pressure/head are the shared variables.
A bolt-on Monte Carlo uncertainty analysis shows that prior distributions of these parameters lend themselves to a range in shrinkage proportion of total subsidence (54-73% on average). These results suggest that coal shrinkage is likely to play an important role in the total subsidence realised at the surface. Prior distributions for the parameters result in a large range of uncertainty in the current analysis and further work to constrain these distributions can reduce predictive variability.
A subsequent sensitivity analysis on five key parameters for this model shows that the shrinkage proportion is sensitive to all five, however the top three relate to specific storages of interburden and coal and the Langmuir strain parameter.
Limited experimental data are currently available on the Langmuir strain and head parameters in the Surat Cumulative Management Area (Surat CMA). OGIA is exploring options to obtain measurements of Langmuir strain and head from core data in the Surat CMA to better constrain prior distributions for these parameters and to inform the development of predictive subsidence models.
OGIA’s subsidence modelling presented in the Underground Water Impact Report 2021 is likely to have implicitly included the effect of coal shrinkage because of calibration against historical ground motion data. In future, more explicit representation of the process will allow further improvements to spatial and temporal resolution in predictions.”