Discrete Net-to-Gross Truncated Gaussian Simulation: An Alternative Modelling Approach for CSG Unconventional Reservoirs, Bowen Basin, Eastern Australia

Abstract

Building a representative static model for predicting and monitoring performance of coal seam gas fields presents several complex and unique challenges. The individual reservoirs possess very different coal architectures, often with highly complex seam splitting, amalgamating and structural deformation. The objective was to develop an alternative approach which honoured log and core data capturing both the lateral heterogeneity and the vertical signature of the Bowen Basin coals, Central Queensland. In some areas of the Bowen Basin, coals can be thick and laterally continuous; picking the top and base of each seam works well in small models with homogeneous coals. As seam geometries begin to increase in complexity and coals become more heterogeneous in nature with thinner seams in multiple packages, then a net-to-gross (NTG) approach is often more appropriate. Each method has its merits. The former approach describes the reservoir architecture but implies a certain degree of confidence in coal correlation; in a vast field with complex seam splitting and merging with abundant drilling data, it may not be a practical technique. The later method (NTG) disregards coal seam architecture and reservoir connectivity. The proposed workflow is designed to take advantage of both NTG characterization and facies modelling technique using a combined hybrid approach. The process is operating on a relatively coarse layered chronostratigraphic framework in which coal is captured as contiguous discrete-NTG "facies". The utilization of the Truncated Gaussian model ensures the contiguity of facies and mimics transitions between coals and carbonaceous mudstones (or other transitional interburdens). With the adoption of facies vertical proportion trends we are able to replicate a similar coal seam signature laterally away from the well bore. The definition of a categorical coal model allows the proper scaling of seams with different coal quality characteristics. With the successful geocellular model re-construction of two historical Coal Seam Gas (CSG) fields in the Bowen Basin, the discrete-NTG Truncated Gaussian Simulation approach has proven to be a valid alternative CSG modelling technique.