Swelling of Coal in Response to CO2 Sequestration for ECBM and Its Effect on Fracture Permeability

Abstract

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.