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

Following two decades of intensive exploration, CSG production in the Surat Basin has begun to dramatically increase to meet the capacity of three newly completed CSG-to-LNG export projects. As the industry’s focus shifts from appraisal to exploitation, the production forecasts underpinning these LNG projects are being tested. In some cases, predicted reservoir performance is found to be invalidated by observed production data, a condition which may require costly amendments to project schedule and scope. The deviation between actual and predicted reservoir performance can often be attributed to an incomplete understanding of parametric uncertainties present in static or dynamic reservoir models. To address this limitation, this study aims to explore the parametric controls upon CSG production behaviours via a series of simulation experiments. Distributions of reservoir parameters were compiled from 152 open-source well completion reports available in three areas along the eastern edge of the Surat Basin. These distributions were validated and then sampled to extract representative ranges for subsurface factors including gas content, permeability, net coal thickness, Langmuir pressure, Langmuir volume, and drainage area. These inputs were used to construct single well radial models which were then simulated to generate predictions of monthly and cumulative produced fluid volumes. The results of this study indicate that net coal thickness and lateral coal connectivity are the most sensitive factors with respect to cumulative gas production while permeability was the single most influential parameter affecting the rate of gas production.