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Summary The "swelling" of coal by a penetrant refers to an increase in the volume occupied by the coal as a result of the viscoelastic relaxation of its highly crosslinked macromolecular structure. Projects relating to CO2 sequestration in coal seams suffer a serious setback in terms of injectivity loss resulting from the swelling of coal. Volumetric swelling associated with CO2 sorption on coal has a significant influence on the fracture porosity and permeability of the coal. Two coal samples differing in rank were used for volumetric strain measurements. With CO2-, the high-rank Selar Cornish coal showed a maximum volumetric strain of 1.48% corresponding to an average pore pressure of 13 MPa. A matrix swelling coefficient (Cm) of 1.77× 10-4 MPa-1 was calculated for this Selar Cornish coal. The low-rank Warndt Luisenthal coal exhibited higher strain of 1.6%, and a matrix swelling coefficient (Cm) of 8.98×10-5 MPa-1 was calculated. The rank dependence of swelling holds true in this set of experiments. Repeat volumetric strain measurement on the same Warndt Luisenthal coal core shows higher volumetric strain values for all pressure steps. A volumetric strain of 1.9% corresponding to a mean pore pressure of 14 MPa was measured. This confirms the process of sequential swelling. A unique feature of this work is that real-time permeability measurements were done under unconstrained conditions. Permeabilities were measured, reducing the pore pressure from 16 to 1 MPa at constant flow rate. Although measured permeability increased with increasing pore pressure under unconstrained swelling, in-situ permeability will actually decrease because of fracture closure in a constrained coal. To validate the permeability swelling relationship, both permeability measurements under unconstrained conditions and volumetric strain measurements were used. Introduction Maturation of coalbed methane (CBM) production operations in some basins, the emergence of injection schemes for enhanced coalbed methane (ECBM), and carbon sequestration of greenhouse gases has led to renewed focus on the behavior of coalbed reservoir properties under these conditions. Cleat permeability of coal is the most important parameter for coalbed methane production. Being normal to the bedding plane and orthogonal to each other, the face and butt cleats in coal seams are usually subvertically oriented. Thus, changes in the cleat permeability are 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 pore pressure (Harpalani and Chen 1997). Coal swelling accompanying CO2 sorption would decrease the permeability of the coal as the volume increase is compensated within the fracture porosity.

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Enhanced coalbed-methane (ECBM) recovery combines recovery of methane (CH 4) from coal seams with storage of carbon dioxide (CO 2). The efficiency of ECBM recovery depends on the CO 2 transfer rate between the macrocleats, via the microcleats to the coal matrix. Diffusive transport of CO 2 in the small cleats is enhanced when the coal is CO 2-wet. Indeed, for water-wet conditions, the small fracture system is filled with water and the rate of CO 2 sorption and CH 4 desorption is affected by slow diffusion of CO 2. This work investigates the wetting behavior of coal using capillary pressures between CO 2 and water, measured continuously as a function of water saturation at in-situ conditions. To facilitate the interpretation of the coal measurements, we also obtain capillary pressure curves for unconsolidated-sand samples. For medium-and high-rank coal, the primary drainage capillary pressure curves show a water-wet behavior. Secondary forcedimbibition experiments show that the medium-rank coal becomes CO 2-wet as the CO 2 pressure increases. High-rank coal is CO 2-wet during primary imbibition. The imbibition behavior is in agreement with contact-angle measurements. Hence, we conclude that imbibition tests provide the practically relevant data to evaluate the wetting properties of coal.

A large diameter (∼70 mm) dry coal sample was used to study the competitive displacement of CH 4 by injection of supercritical CO 2 , and CO 2 -CH 4 counter-diffusion in coal matrix. During the test, a staged loading procedure, which allows the calibration of the key reservoir modelling parameters in a sequential and progressive manner, was employed. The core-flooding test was history matched using an Enhanced Coalbed Methane (ECBM) simulator, in which Fick's Law for mixed gas diffusion and the extended Langmuir equations are implemented. The system pressure rise during the two loading stages and the CO 2 breakthrough time in the final production stage were matched by using the pair of constant sorption times (9 and 3.2 days) for CH 4 and CO 2 , respectively. The corresponding diffusion coefficients for CH 4 and CO 2 were estimated to be 1.6 × 10 −12 and 4.6 × 10 −12 m 2 /s, respectively. Comparison was made with published gas diffusion coefficients for dry ground samples (ranging from <0.063 to ∼3 mm) of the same coal at relatively low pressures (<4 MPa). The CO 2 /CH 4 gas diffusion coefficient ratio was well within the reported range (2-3), whereas the CH 4 diffusion coefficient obtained from history matching of the core-flooding test is approximately 15 times smaller than that arrived by curve-fitting the measured sorption uptake rate using a unipore diffusion model. The calibrated model prediction of the effluent gas composition was in good agreement with the test data for CO 2 mole fraction of up to 20%.

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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