Unlocking Methane Desorption Effects in Reservoir Compaction and Subsidence Computations for a Coal Seam Gas Development

For shallow coal seam gas developments, regulatory authorities can mandate subsidence predictions as a license-to-operate requirement. It is therefore important to understand and capture the depletion-induced reservoir compaction in geomechanical models for better constraining and improving the subsidence forecast for future production scenarios. For a particular coal seam gas development coal compressibility measurements were done to understand and quantify the contribution of gas-desorption-induced shrinkage to the total reservoir compaction. The experiments showed that there is a substantial increase in the compressibility of the core material due to methane desorption from the pores of the samples as the pore pressure is decreased. These experimental results were taken into account to build a constitutive model for simultaneously acting shrinkage-related and poro-elastic deformation mechanisms. Following implementation of this constitutive model into a calibrated geomechanical model, we observe good agreement between modelled and InSAR observed field subsidence. Apart from a valuable and better constrained subsidence forecast for future production scenarios, our results further motivate to implement and deploy the workflow to other coal-bed methane developments.