Early Permian Cattle Creek Coals in Fairview Field in Queensland Australia is an insitu multicomponent Coal Seam Gas Reservoir. The Early Permian East coals or Springwater play typically occur at depths between 1,100 m to 1,800 m. These medium volatile bituminous coals are characterized as having lower permeability (<5 mD), a high gas content (10 – 15 m3/t daf), an elevated concentration of ethane and heavier hydrocarbon components (2 – 14% C2+), and higher inerts concentration (0.2 – 12% CO2, < 3% N2) than found in many CSG fields. .

An analysis and history match of the production data from existing appraisal wells with a particular emphasis on the produced gas composition and its variation with depletion was performed using both an in-house compositional simulator which used the extended Langmuir Isotherm as the basis for the equilibrium relationship between the free and the sorbed gas phase and a commercial compositional simulator. Methane, Carbon Dioxide, Ethane, Propane and Nitrogen were simulated during this study.

The results showed potential differences between the produced gas composition obtained during well production (considering the adsorption/desorption behavior of a multicomponent gas in a coal seam reservoir) and the in-situ gas composition determined from gas sampling during desorption tests. Heating value profiles were generated and compared illustrating the potential impact of the reservoir dynamic behaviour.

The results also showed how the produced concentration of N2 (free gas phase) can be 4 to 7 times higher than its insitu concentration and that a variation of up to 75% of the initial concentration in free gas phase of CO2 and C2H6 concentration is possible due to the variability of the isotherms indicated from core.

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