Analysis of a Middle-Rank Coal Reservoir on a Triple Porosity/Dual Permeability Model

Middle-rank coal reservoirs in the Chinese coalbed methane (CBM) industry are receiving increasing interest due to their highly developed seepage pore, rendering the classic dual porosity/single permeability model insufficient for CBM well predictions and analysis. This research employed low-field nuclear magnetic resonance to examine the pore system of coal reservoirs and assess the impacts of 11 reservoir parameter variations on CBM well performance using a triple porosity/dual permeability model. Results indicated that the seepage pore constituted approximately 57% of the total pore volume, surpassing the adsorption pore (38%) and cleat (5%). Comparing simulation results for a well in the western Guizhou CBM basin revealed that the dual porosity/single permeability model yielded more gas than the triple porosity/dual permeability model, while the opposite held true for water production, implying that dual porosity/single permeability model is not suitable when seepage pore is developed. Sensitivity analysis based on the triple porosity/dual permeability model demonstrated that thicker coal seams significantly increase recoverable CBM resources, coal thickness exerted the most significant influence on CBM production, followed by seepage pore water saturation and cleat permeability/porosity. Factors such as critical desorption pressure, hydraulic fracture permeability, fracture half-length, Langmuir volume, and seepage pore permeability had comparatively less significant impacts. This study underscores the importance of accounting for seepage pore when developing middle-rank coal reservoirs for CBM production and can offer valuable insights for future CBM development in the study area.