Coalbed methane (CBM) geology dominance in Surat basin is a coal-based fluvial depositional system. For CBM subsurface modelling, it is still primarily driven by simple deterministic static model and building on the understanding of long extends of coal continuity in lateral distribution. In reality, coal seams (or plies) tend to split or merge laterally across a large distance of few kilometers. Hence, the extend flow contribution or net coal distributions are not homogenous and can changed quite significantly, which we believe using a proper calibrated facies model will be able to predict such behaviors for both coals and its boundary lithology. In our facies modelling work, the coal swamp and shaly swamps depositions are primarily targets for CBM reservoirs and other facies in juxtaposition to them will also influence the lateral continuation of the coal swamp, the heterogeneity of swamp distribution and gas drainage.
This paper summarized four-step procedures to address swamp-based fluvial depositional facies modelling for an integrated approach to identify micro-facies, upscale the coal continuity and sand body, model the lithofacies and estimate the different potential flow patterns based on the multi-realization. This study involved the investigation of a region of 24,000 km2 and the log normalization on the available logs for a quantitative lithofacies interpretation firstly. Then the division of the fining-up sequence cycles, individual ply formation and the boundaries of top of sandstone (together 80,000 tops) have been carefully conducted to derive the variograms and 2D distributions of channels for the description of the coal flow drainage extension. The study focused on the creation of the fluvial facies and five micro-facies of channels, flooding plains, lacustrine, swamp and shaly swamp with geometry parameters, such as the coal thickness, percentage and the channels width, orientation, amplitude, wavelength from the generated 2D maps. The coals continuity and sand body were also thoroughly investigated and upscaled. After structure modeling, the team conducted three-order object facies modeling in reasonable stochastic modelling of fluvial system to descript 3D continuity of the flow patterns of coal swamp. Finally, the coal volume calculation of P10, P50 and P90 has been obtained through the multi-realization of property modeling, which was based on the facies modeling.
The study combines the knowledge of both conventional and unconventional reservoir modelling techniques and the work will be used for better reservoir simulation, as well as basin wide field development planning. In this study, the project team investigated ~1200 wells with geophysical logs, >50 wells with core data, conducted ply formation picking (~40,000 tops) and stochastic static modelling, which successfully correlated and outlined the statistics of sand geometry and ply lengths (areas) to build the concept of fluvial depositional system.
The integrated facies modelling workflow is a novel approach in CBM industry to better understand the coal heterogeneity both lateral continuation and vertical distributions. The geo-statistics outcome will provide multi-realizations for coal and gas production predictions.