Triple Porosity/Dual Permeability Model Study And Application For Coalbed Methane Reservoir Simulation

Based on the theory of coalbed methane geology, seepage mechanics, fractaltheory and numerical simulation for oil and gas reservoir, this paper proposed acomprehensive classification method for identifying migration pore, permeation poreand fracture, established models for calculating reservoir parameters of permeationpore and fracture, and finally took well QSDU01in Qinshui basin as an example tosimulate its drainage performance under triple porosity/dual permeability (TPDP)model. The following cognitions are obtained.Based on the classification results of coal pore systems under mercury intrusionporosimetry (MIP), nuclear magnetic resonance (NMR) and nitrogen adsorption underlow temperature (NALT), a comprehensive classification method for identifying coalpore systems is proposed. The results show that the dividing radius of migration poreand permeation pore is about64nm, and the dividing radius of permeation pore andfracture is about600~700nm. The results also indicate that the volumetric proportionof migration pore increases along with vitrinite reflectance increasing, that ofpermeation pore decreases with vitrinite reflectance increasing, and fracture contenthas little relationship with vitrinite reflectance.Porosity of different pore systems can be arranged into two types. The first typeincludes irreducible porosity of each pore system, and this type has little impact onpermeability. The second type represents reducible porosity of permeation pore andfracture, and this type is the main fact influencing coal permeability. Coal samples canbe divided into three types based on the relative amount of porosity of permeationpore and fracture, and they are fracture-dominated samples, permeation-dominatedsamples and hybrid samples. The modified matchstick and Coates models can then beestablished by fitting porosity of fracture-dominated and permeation-dominatedsamples with permeability, respectively. The two models can be used to predictpermeability of permeation pore and fracture systems.Historical matching under TPDP and dual porosity/single permeability modelsfor well QSDU01indicates that the TPDP model is more suitable for describing CBMflow process. Sensitivity analysis indicates that porosity has a negative influence ongas production, and the negative influence vanishes if porosity is lower than0.1%.Hence, invalid critical porosity is defined, of which constant value is0.1%.Gas and water linkages of permeation pore and fracture systems reveal that the features of gas and water transmitted from permeation pore to fracture control theperformances of gas and water output from well bore respectively. Drainage offracture water is the main reason for reservoir pressure decreasing at the early stage,while the output of water of permeation pore system becomes more and moreimportant as the drainage processing. The first peak of gas production is controlled bydesorption gas from desorption pore system, and the second peak is influenced by thegas transmitted from permeation pore to fracture system.Drainage performance simulation under TPDP model shows that the drainageperiod for well QSDU01can be divided into two parts. The first part refers the earlier310days, of which the reservoir parameters change a little. The second part refers thetime after310days, when reservoir parameters decrease rapidly.Finally, a gas production prediction model is established under TPDP model.