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Abstract

The recovery of gas from coalbeds is a two-step process. First, the gas diffuses through the matrix process. First, the gas diffuses through the matrix then, secondly, it flows through the cleats to the wellbore. If the release of gas fro. 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 production. If the coal is well cleated, it can be assumed for engineering purposes that the gas desorbs instantaneously fro. the matrix to the cleat when the pressure in the cleat decreases. This assumption alloys the adsorption of gas on the surface of she coal to be modeled as gas dissolved in an immobile oil. Conventional reservoir simulators can then be used for coalbed-methane modeling purposes. The solution gas-oil ratio of this immobile 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 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.

Introduction

Coalbeds 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 sorbed 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 coal 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.

PREVIOUS STUDIES OF CHARACTERISTIC DIFFUSION TIMES PREVIOUS STUDIES OF CHARACTERISTIC DIFFUSION TIMES Times required for various coals to desorb gas have been reported in the literature.

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