Gas production from coal seams may be interrupted during the production period due to different technical or non-technical reasons. Some free gas remains in the subsurface every time the well is shut-in, which may re-adsorb into coal or get produced in the next production period. This study aims to investigate the factors affecting the fate of residual free gas during and after shut-in periods.
A dual-porosity approach was used to model gas-water flow in a coal package built based on typical coal seam gas (CSG or CBM) well spacing in the Surat Basin. A heterogeneous model was created and populated with coal properties observed in the Surat Basin using a geostatistical tool developed at The University of Queensland. High and low methane adsorption capacities were assigned to coal and interburden grids, respectively. Six scenarios were defined based on production (pumping) pattern, aquifer strength and permeability anisotropy. In addition, the concepts of vertical and spatial coal connectivity were developed to help interpret gas-water flow behaviour in coal seams.
In a simple isotropic permeability case, the topmost layer produces up to three orders of magnitude more gas compared to the deeper layers. As the ratio of vertical to horizontal permeability decreases, the size of this effect reduces significantly. In all cases, saturation distribution in the post-production period shows that the residual free gas plume exists in a relatively small area in the proximity of the well. However, there are also cases where the lateral coal geometry and vertical connectivity lead to isolated ‘patches’ of high gas saturation in areas away from the well, though these saturations reduce over time. This may be important for any post-closure activities.
Unlike in the simulation cases with isotropic permeability, gas flow and adsorption rates in cases with vertical to horizontal permeability ratios lower than 0.0001 follow the same trend in all the layers. It is also shown that directional, areal, and volumetric coal (spatial) connectivity could be as informative as coal vertical connectivity in analysing gas generation and flow in coal seams. Results from the more representative, heterogeneous models also indicate the importance of improved modelling of heterogeneity for improvements in production forecasting.
Unlike most studies reported in the literature that have fully focused on gas flow during the production period, this research investigates the fate of gas beyond the production period. Analysing gas-water flow mechanisms after the shut-in period, this research adds novel information that could be of great interest for CSG companies with wells approaching their decommissioning time. This research also enables better static/dynamic reservoir characterisation and analysis by extending the concept of coal connectivity.