Coal bed methane reservoir simulation studies

The purpose of this study is to perform simulation studies for a specific coal bedmethane reservoir. First, the theory and reservoir engineering aspects of coalbed methane reservoirs, such as dual porosity concept, permeabilitycharacteristics of CBM reservoirs and mechanism of gas storage and gastransportation in CBM reservoir have been discussed. Next, simulation results forthe CBM reservoir presented. Simulation studies were carried out by using theCBM reservoir simulator, SIMED II. Injection/fall-off test pressure data wereinterpreted based on the pressure history matching method. The interpretationresults include the determination of reservoir permeability and identification ofthe reservoir altered zone. Also available production histories were used tosimulate the reservoir production behavior. Then the production model was usedto predict the reservoir future production and to carry out sensitivity analysis onreservoir performance.For natural pressure depletion, methane recovery was increased significantly asreservoir permeability was increased. Well-bore fracturing creates a fracturedzone with higher permeability. This increases methane production rate duringearly time of reservoir life. Reservoir matrix porosity has a significant effect onthe reservoir performance. Higher production peak rate and also higher methanerecovery was obtained for the reservoir with lower porosity values. Any increasein the reservoir compressibility causes greater reduction in reservoir absolutepermeability as well as relative permeability to gas throughout the reservoir.Therefore, methane recovery decreased as the reservoir compressibilityincreased. The reservoir production behavior was strongly affected by changes inreservoir size. The production peak rate was significantly postponed and loweredas reservoir size was increased. The effect of reservoir initial pressure wasinvestigated and the results show that higher initial reservoir pressure leads tohigher production rate during early years of production. However, for the lateryears of reservoir life, the production profile is almost identical for differentinitial pressures. Coal desorption time constant affects the methane productionby its own scale. In this study, the range of desorption time did not exceedlonger than three days and therefore the difference in production rate wasobserved only in the first few days of production