Abstract
The purpose of this study is to perform simulation studies for a specific coal bed
methane reservoir. First, the theory and reservoir engineering aspects of coal
bed methane reservoirs, such as dual porosity concept, permeability
characteristics of CBM reservoirs and mechanism of gas storage and gas
transportation in CBM reservoir have been discussed. Next, simulation results for
the CBM reservoir presented. Simulation studies were carried out by using the
CBM reservoir simulator, SIMED II. Injection/fall-off test pressure data were
interpreted based on the pressure history matching method. The interpretation
results include the determination of reservoir permeability and identification of
the reservoir altered zone. Also available production histories were used to
simulate the reservoir production behavior. Then the production model was used
to predict the reservoir future production and to carry out sensitivity analysis on
reservoir performance.
For natural pressure depletion, methane recovery was increased significantly as
reservoir permeability was increased. Well-bore fracturing creates a fractured
zone with higher permeability. This increases methane production rate during
early time of reservoir life. Reservoir matrix porosity has a significant effect on
the reservoir performance. Higher production peak rate and also higher methane
recovery was obtained for the reservoir with lower porosity values. Any increase
in the reservoir compressibility causes greater reduction in reservoir absolute
permeability as well as relative permeability to gas throughout the reservoir.
Therefore, methane recovery decreased as the reservoir compressibility
increased. The reservoir production behavior was strongly affected by changes in
reservoir size. The production peak rate was significantly postponed and lowered
as reservoir size was increased. The effect of reservoir initial pressure was
investigated and the results show that higher initial reservoir pressure leads to
higher production rate during early years of production. However, for the later
years of reservoir life, the production profile is almost identical for different
initial pressures. Coal desorption time constant affects the methane production
by its own scale. In this study, the range of desorption time did not exceed
longer than three days and therefore the difference in production rate was
observed only in the first few days of production.