Extensive drilling of the Walloon Subgroup for coal seam gas (CSG) during the last decade has revealed a world-class CSG play on the northern flank of the Surat Basin. Resources discovered in the Walloon Subgroup exceed 30 TCF; this gas now underpins four CSG-to-liquefied natural gas (LNG) projects. Results to date have revealed the highly heterogeneous nature of the Walloon Subgroup and its associated coal properties. The Walloon Subgroup is typically 350 m thick and contains an average of 30 m of net coal that is interbedded with a range of clay-rich, fluvio-lacustrine lithologies. The most prospective area of the play occurs down-dip and adjacent to the Walloon subcrop edge, where high permeability exists combined with a thick section of net pay. Coals in the Walloon Subgroup are low rank (0.35–0.65% Ro) with gas contents ranging between 1–15 m3/tonne (dry ash-free). Average coal ply thickness is 30 cm, making correlation and prediction of reservoir properties difficult. Reservoir properties—including permeability, gas content and saturation—differ as a result of compositional variability of the coal seams and also the tectonic history. Mapping of sparse 2D seismic data has highlighted the distribution of major structural features throughout the basin. Coal fracture permeability ranges from less than 0.1 mD to more than 2,000 mD, and mapping has identified areas where permeability appears to be enhanced on structures that have undergone mid Cretaceous–Eocene deformation.

Coal measures (coal bearing rock strata) can contain large reserves of methane. These reserves are being exploited at a rapidly increasing rate in many parts of the world. To extract coal seam gas, thousands of wells are drilled at relatively small spacing to depressurize coal seams to induce desorption and allow subsequent capture of the gas. To manage this process effectively, the effect of coal bed methane (CBM) extraction on regional aquifer systems must be properly understood and managed. Groundwater modeling is an integral part of this management process. However, modeling of CBM impacts presents some unique challenges, as processes that are operative at two very different scales must be adequately represented in the models. The impacts of large-scale gas extraction may be felt over a large area, yet despite the significant upscaling that accompanies construction of a regional model, near-well conditions and processes cannot be ignored. These include the highly heterogeneous nature of many coal measures, and the dual-phase flow of water and gas that is induced by coal seam depressurization. To understand these challenges, a fine-scale model was constructed incorporating a detailed representation of lithological heterogeneity to ensure that near-well processes and conditions could be examined. The detail of this heterogeneity was at a level not previously employed in models built to assess groundwater impacts arising from CBM extraction. A dual-phase reservoir simulator was used to examine depressurization and water desaturation processes in the vicinity of an extractive wellfield within this fine-scale model. A single-phase simulator was then employed so that depressurization errors incurred by neglecting near-well, dual-phase flow could be explored. Two models with fewer lithological details were then constructed in order to examine the nature of depressurization errors incurred by upscaling and to assess the interaction of the upscaling process with the requirement for adequate representation of near-source, dual-phase processes.

In 2009, QGC (a BG Group business) first planned to produce coal seam gas (CSG) in the Surat Basin as feedstock for a new Liquefied Natural Gas (LNG) plant. Subsurface models and associated field development plans were generated to underpin the investment case for the Queensland Curtis LNG (QCLNG) Project. This paper discusses history matching experience from QGC's Surat Basin reservoir simulation models, the challenges involved, and how these challenges have been overcome. Similar to conventional reservoirs, full-field numerical simulation is necessary to accurately account for well interference and broader reservoir connectivity. Simulationalso fully integrates modelled 3D static property variations and honours the physics of multi-phase flow through porous media. CSG reservoirs possess unique characteristics which differ from conventional reservoirs and tend to increase the challenge of history matching. Some of these challenges include the physics of diffusive flow from the matrix into fractures that combines with relative permeability to create a more complex multi-phase flow problem. Another challenge is the requirement to de-water the coal before gas production commences and the associated stress dependency of coal properties. At the same time, the nature of CSG developments makes conventional history-matching approaches impractical given the large number of wells and the need for quick turnaround and fast-cycle decision making, especially during development ramp-up. These demands are set against a lean business environment where cost efficiency is paramount. The properties of Walloon Subgroup (WSG) coals in the Surat Basin are unique compared to other CSG basin plays worldwide. The WSG is characterised by low to moderate coal rank, highly interbedded seams distributed through an extremely low permeability inter-burden, and highly variable coal permeability throughout the basin. Some of these differentiating properties of the WSG make history matching these reservoirs very challenging. This paper presents guidelines for overcoming these challenges and history-matching CSG production in a dynamic simulation model of the Surat basin WSG. Handling of uncertainty is discussed to consider a range of possible history-matches for 345 wells with 9 years of production history. The history matching guidelines that have been developed are enabling a faster turnaround of model predictions, capturing key uncertainty parameters and informing field development decision-making.