We characterize the mechanical properties of coal samples from the Powder River Basin (Wyoming, USA) by conducting laboratory experiments. We present results from laboratory measurements of adsorption, static and dynamic elastic moduli, and permeability as a function of effective stress, pore pressure, and gas species. Notably, we observe that CO2 adsorption causes the static bulk modulus to decrease by a factor of two, while the dynamic bulk modulus remains essentially unchanged. Permeability of both intact and powdered samples decreases by approximately an order of magnitude in the presence of CO2, which is consistent with observations of adsorption-related swelling of the coal matrix. Interestingly, CO2 appears to change the constitutive behavior of coal; Helium saturated samples exhibit elastic behavior, while CO2 saturated samples exhibit viscous, anelastic behavior, as evidenced by creep strain observations.
Coal bed methane (CBM) production from unmineable coal seams is considered to be an unconventional gas resource; methane is typically generated during the coal maturation process and resides in the coal matrix as an immobile, adsorbed phase. During CBM production, the pressure inside a coal seam is reduced and methane desorbs from the coal matrix, at which point it exists as a free gas and can flow through the cleat (natural fracture) system to the producing wells. According to the U.S. Energy Information Agency (EIA), coal bed methane production currently accounts for 10% of the domestic natural gas supply, and is anticipated to increase significantly over the next several decades as conventional gas supplies continue to decline [1]. Coal exhibits the interesting property of selectively adsorbing certain gases. In particular, many coals have been observed to preferentially adsorb carbon dioxide over methane [2], making coal an attractive candidate for geological sequestration of CO2, since adsorbed gases are essentially immobile [3,4]. In addition, because the adsorption of carbon dioxide forces desorption of methane, it is possible that CO2 can be used to enhance coal bed methane (ECBM) production [5-7]. While several small-scale field studies have been performed or are currently underway in various coal seams around the world [8-11], the feasibility of ECBM or geological sequestration of CO2 for a given site is still largely dependent on predictions from numerical modeling tools such as reservoir simulators that have been modified for CBM [12,13]. These numerical models require values for numerous input parameters, many of which can be derived from laboratory data. Furthermore, predictions from reservoir simulators can only be as accurate and realistic as the underlying theoretical and mathematical models allow. Laboratory studies are needed to develop and verify theoretical models of the complex mechanical and chemical behavior of coal. For example, the adsorption of gases onto the surfaces of the coal matrix has been observed to cause volumetric swelling of the coal, while desorption of gases causes volumetric shrinkage [14,15]. This swelling and shrinkage of the matrix changes the width of the cleats and natural fractures in the coal, which in turn causes changes in cleat permeability [16,17,18].
