Natural Gas Production From Coal Seams
Authors: Chase Robert W. 
Published: December 2007

The estimated 4 trillion tons of coal which lie buried beneath the U.S. at depths of less than 3,000 feet are thought to contain nearly 300 trillion cubic feet of pipeline-quality natural gas. The mechanism by which this gas is released is believed to result from the diffusion of adsorbed gas through the microporous bulk matrix into the natural fractures where mass transport is governed by the principles of Darcy flow. This two-step process may ultimately be controlled by conditions of critical flow, or choke flow, in the micropores adjacent to the fractures.

Several techniques have been developed to determine the gas content of a coal seam based primarily on the analysis of exploratory cores. Such methods generally yield quite different results when applied to the same core sample. A new technique which relies upon the analysis of decline curves constructed from gas desorption data obtained from a core, appears to give the most reliable results, and indirectly indicates that the Darcy flow mechanism controls the desorption process.

Introduction

Several authors have noted the existence of vast amounts of methane, or natural gas, in coal seams. The reserves of natural gas in coal, estimated at nearly 300 trillion cubic feet, exceed current proved recoverable gas reserves in the U.S., and proved recoverable gas reserves in the U.S., and represent a possible supplement to dwindling supplies.

Methane is viewed exclusively as a hazard in mining operations and is treated as an undesirable by-product of the mining process. Each day over 200 million cubic feet of gas are vented to the atmosphere through ventilation systems in U.S. coal mines. Ventilation systems dilute the methane emitted from mine workings with large volumes of air, thereby reducing the concentration of methane in air to 1 percent by volume, well below the minimum explosive percent by volume, well below the minimum explosive limit of 5 percent methane-in-air.

The cost of maintaining an explosion-free mining environment using a ventilation system is estimated to increase the cost of coal extraction by as much as $0.50 per ton. Since the size and cost of a ventilation system is directly proportional to the volume of natural gas associated with a coal seam, an accurate estimation of its gas content is warranted. Overestimating the gas content will result in overdesigned ventilation systems and increased capital expenditures. Conversely, inadequate ventilation figures in creating an unsafe mining environment resulting in decreased productivity and mine shut-downs. productivity and mine shut-downs. Degasification of coal seams in advance of, or during mining operations represents an alternative, or supplement, to mine ventilation. Current technology has demonstrated the feasibility of using vertical, deviated, or horizontal borehole systems to recover natural gas from both mineable and unmineable coalbeds. Feasibility, however, has been overshadowed by less than favorable economics resulting from the low, regulated market price of natural gas. Ultimately, decontrol of the price of natural gas may provide oil, gas, and coal companies with the incentive necessary to exploit the coal-gas reserves of the U.S.

COAL RESERVES

The coal resources of the Unites States remaining in the ground as of January 1, 1974, have been estimated by Averitt to be in the neighborhood of 3,968 billion tons. This figure represents approximately 25 percent of the identified coal resources in the world. Of this total resource, 91 percent is coal found at a depth of less than 1,000 feet, 43 percent is bituminous coal, and 33 percent is present in what are considered to be percent is present in what are considered to be thick beds.

Coal-bearing rocks are present in 37 states underlying approximately 13 percent of the land area of the 50 United States. Coal deposits in the lower 48 states are depicted in Figure 1 according to rank (a measure of fixed carbon content and heat content) of the deposits.