An investigation of numerical facies models for coal seam gas reservoirs; Walloon Subgroup, Surat Basin

Abstract

The Walloon Subgroup (WSG) represents a prolific coal seam gas (CSG) resource which, in part, underpins three large CSG to liquefied natural gas (LNG) projects operating along the Australian east coast. These mega-projects require long term forecasts of gas production, predictions which are often derived via the dynamic simulation of numerical reservoir models. Reservoir models consist of hierarchical collections of inter-related rock and fluid parameters. The facies model, representing palaeogeography, serves as the foundation of this linkage as it defines the three-dimensional (3D) geometry and distribution of coal seams. This thesis is comprised of complimentary studies aimed at investigating and enhancing the predictive capability of numerical facies models in the Walloon Subgroup.A preliminary screening study investigates CSG production behaviours to understand how reservoir parameters, including the distribution and continuity of coal layers, impact the predictions output by reservoir models. A database of open-source reservoir properties was compiled and used to construct representative reservoir models for three CSG development areas present along the WSG productive fairway in the eastern Surat Basin. Simulation experiments using these models confirms that the distribution and lateral continuity of coal facies are a significant control upon the late-stage production behaviour of CSG wells -a finding which provides impetus for further detailed investigation of WSG facies models.Construction of predictive facies models requires a comprehensive understanding of regional depositional trends, local facies architectures and depositional processes as well as computer routines able to translate this information into numerical 3D grids. Present gaps in the industry's understanding of the WSG highlight the requirement for further studies aimed at 1) refinement of the WSG's regional stratigraphic and palaeogeographic models, 2) delineation of the WSG's reservoir scale internal alluvial architecture, 3) identification of the processes or mechanisms controlling reservoir-scale facies relationships within the WSG and 4) development of workflows better able to represent alluvial facies in numerical reservoir models. The focus of this dissertation is to explore these conceptual gaps leading to improved numerical facies models of the WSG.Investigation of WSG facies models began with a regional study integrating newly available borehole image log data with sedimentological interpretations from cored wells and regional correlations to test and refine existing basin-wide palaeogeographic models for the WSG. This analysis offers a glimpse into fluvial styles and palaeoflow orientations present within the basin's interior and was 2 used to identify regional trends describing the spatial and temporal distribution of alluvial depositional styles. Synthesis of stratigraphic, sedimentological and palaeoflow interpretations developed during this study suggests the presence of a SE oriented trunk drainage system during depo...