The recovery of gas from coal beds is a two-step process. First, the gas diffuses through the matrix then, secondly, it flows through the cleats to the wellbore. If the release of gas from the matrix to the cleats is very rapid compared to the flow of gas and water in the cleats, the desorption kinetics are relatively unimportant in modeling coal bed methane production. If the coal is well cleated, it can be assumed for engineering purposes that the gas desorbs instantaneously from the matrix to the cleat when the pressure in the cleat decreases. This assumption allows the adsorption of gas on the surface of the coal to be modeled as gas dissolved in an immobile oil. Conventional reservoir simulators can then be used for coal bed methane modeling purposes. The solution gas-oil ratio of this immobile "pseudo" oil is calculated from the Langmuir adsorption isotherm constants and coal bed properties. Additional modest modifications are required in the data describing the porosity and gas-water relative permeability curves to account for the presence of the "pseudo" oil. No code modification is required. This concept has been used with several different simulators to successfully model both single well and 3-D, multiwell coal bed methane problems. A coal well simulation using this method and COMETPC, a simulator developed by ICF-Lewin, are compared.
References and illustrations at end of paper.
Coal beds are naturally fractured, low pressure, water saturated gas reservoirs. While some free gas may exist in a coal deposit, the majority of the gas is absorbed on the surface of the coal matrix. When water is removed from the natural fractures of the coal, the pressure is reduced and gas is released from the matrix into the fractures. Once in the fractures, the gas flows to the wellbore. Thus coal degasification is a two-step process: desorption of gas from the coat matrix followed by flow through the fractures.
The slower of these two processes will control the rate of gas production from a coal. For engineering purposes, gas production can be approximated by mathematics which focus on the dominant process. If the rate of gas desorption from the matrix is very slow compared to the rate of fluid transport in the fractures, diffusion equations need to be incorporated into a conventional simulator to describe gas production. If the release of gas from the matrix is very rapid compared to the time scale of fluid flow in the cleats, gas production can be modeled by Darcy's law only.
Times required for various coals to desorbs gas have been reported in the literature. In developing the Direct Method for determination of coal bed gas content, Bertard, et. al.1 measured methane desorption from several different European coals and found that different coals released methane at different rates. Desorption of 90% of the gas required between 34 and 90 hours. The desorption rate was round to increase with temperature, thus the in-situ desorption rate is faster than that measured at ambient temperatures.
