Enhanced gas drainage from undersaturated coal bed methane reservoirs

Traditional methods of coal seam gas drainage rely on the use of reservoir pressure reduction to promote gas desorption from the coal matrix. Studies in coal mining gas drainage operations, particularly in coal seams that are deeply undersaturated and have low permeability, found the rate of reservoir pressure reduction was prohibitively slow. In such conditions, lengthy production delays have been experienced while additional gas drainage drilling is undertaken in an attempt to reduce seam gas content below specified threshold limits. Such drilling represents a high additional operating cost and yields low total gas production whilst adversely impacting the mine's gas drainage drilling schedule. In extreme cases known zones of difficult to drain coal are avoided resulting in a loss of potentially recoverable coal and gas reserves. An alternative method for enhancing coal seam production which does not rely on reservoir pressure reduction has been identified. This method, known as the cyclic inert gas injection method, involving the injection of nitrogen, has potential application in deeply undersaturated and low permeability coal seams enabling seam gas to be removed and permeability increased without the need to reduce reservoir pressure to the critical desorption point. The cyclic inert gas injection method is presented and discussed.