Production of Methane and Water from Coal Seams and the Associated Ground Surface Subsidence- a Numerical Model

Abstract

Production of methane from coal seems is achieved by pumping out large quantities of water leading to cleat pressure drawdown and hence methane desorption from the matrix, its diffusion through the micropores to the cleat system and out of the wellbore. A coupled numerical model is developed within FLAC3D to simulate the production of methane and water from coal seams through depressurization and to investigate possibilities of land subsidence around the wellbore. The axisymmetric model simulates the drilling of a vertical wellbore through a formation containing 2 coal seams. The model couples Geomechanics and fluid flow. Langmuir desorption isotherms control the maximum volume of gas desorbed in the coal matrix at any specific pressure. The cleats are assumed to be initially full of water. Desorption of methane from the coal matrix and its diffusion into the cleat system is assumed to be instantaneous. The geomechanical behaviour of the formations follows a Mohr Coulomb yield criterion.The model qualitatively predicts the quantities of produced methane and water from coal seams separated, overlain or underlain by lithologies of contrasting permeabilities. The presence of an aquitard between both seams ensures higher methane production and less water production. The highest methane production and least amounts of produced water were predicted for seams separated, overlain and underlain by aquitards. The model can predict the amount of ground surface subsidence in the region around the wellbore. Sunsidence up to 11 cm was predicted near the wellbore.