The matrix volume of coal swells when CO2 / CH4 adsorb on the coal structure. In coalbed gas reservoirs, matrix swelling could cause the fracture aperture width to decrease, causing a considerable reduction in permeability. On a unit concentration basis, CO2 causes greater degree of coal matrix swelling compared to CH4. Much of this difference is attributable to the differing sorption capacity that coal has towards carbon dioxide and methane. This condition in a coal reservoir would lead to differential swelling. Differential swelling will have consequences in terms of porosity / permeability loss, with serious implication for the performance and implementation of carbon sequestration projects. Coal can be understood as a macromolecular cross-linked polymeric structure. An experimental effort has been made to measure the differential swelling effect of CO2 / CH4 on this macromolecular structure and to theoretically translate that effect in terms of porosity and permeability. A unique feature of this work is that, real time permeability measurements were done to see the true effect of differential strain from CH4 saturated coal core flooding experiments.
IntroductionCoal matrix is heterogeneous and is characterized by three different porosity systems -micropore, mesopore and macropore. The macropores are the cleats, which are sub-vertically oriented to the bedding plane in coal.The cleat system consists of the face cleats, continuous throughout the reservoir, and butt cleats, which are discontinuous and terminate against the face cleat.Permeability of coal is recognized as the most important parameter for fluid transport through the seam. Being normal to the bedding plane and orthogonal to each other, the face and the butt cleats in coal seams are usually sub-vertically oriented. Thus changes in the cleat permeability can be considered to be primarily controlled by the prevailing effective horizontal stresses that act across the cleats, rather than the effective vertical stress, defined as the difference between the overburden stress and the pore pressure. During primary methane production, two distinct phenomenons are associated with reservoir pressure depletion, with opposing effects on coal permeability. The first is an increase in the effective horizontal stress under uniaxial strain conditions (Jaeger and
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AbstractStorage of carbon dioxide in geological formations is for many countries the only option to reduce greenhouse gas emissions and thus to satisfy the Kyoto agreements. The CO 2 storage in unminable coal seams has the advantage that it can sequestrate CO 2 emissions from industrial processes and be used to enhance coal bed methane recovery (CO 2 -ECBM). For this purpose, the storage capacity of coal is an important economical parameter.An experimental set-up has been developed to determine the rate of adsorption of gases such as carbon dioxide and methane in coal. Sorption kinetic experiments were carried out at various pressures and on coal particles with varying grain sizes (40µm-2mm diameter). The particles were considered to consist of aromatic aggregates embedded in amorphous aliphatic coal, intersected by cleats. The characteristic time of adsorption increases with increasing pressure and increasing particle diameter. However the characteristic time increases only for very small particles. This is attributed to the presence of cleats inside the larger particles. From the experiments it appears that water inhibits diffusion due to its presence in the small particles or in the small cleats. The results of the diffusion experiments can be described by an upscaled diffusion -adsorption model. Based on this model and the experiments an accurate assessment of the storage capacity can be achieved.
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