CBM reservoir simulation is one of specialities of Austar Gas. Reservoir simulation is a versatile and powerful tool which can yield insight into gas well performance in complex coal seam gas reservoirs. In particular, the models of SIMED II, taking into account shrinkage of matrix and cleat closure with pressure depletion, have allowed this study to investigate the change in permeability during CBM production.
Austar Gas has run reservoir simulation study to investigate the effects of desorption pressure, geological setting and coal rank on the variation in permeability under in-situ conditions during coalbed methane production, based on a study in the Hedong area, Ordos Basin, China. The simulations allowed history matching of gas and water production from 12 wells with the actual well conditions specified as the model pressure. Good agreement was achieved between the model yields and the actual production data, suggesting that the changing permeability interpreted from the simulation is a realistic representation of the in-situ reservoir properties.
The reservoir simulation study found that the decreases in permeability with production exceeded the increase in permeability caused by matrix shrinkage for nearly all wells in the Hedong area. The magnitude of the decrease in permeability increases as the gap between initial pressure and desorption pressure increases. The decrease in permeability is slower in the zone closest to the fault. The reservoir simulation has demonstrated that coal rank influences significantly the change in permeability during coalbed methane production.