ABSTRACT
The recovery of gas from coalbeds 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 coalbed 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 coalbed 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 coalbed methane problems. A coal well simulation using this method and COMETPC, a simulator developed by ICF-Lewin, are compared.
