A confined Pittsburgh seam (DECS-12) coal sample was pressurized with carbon dioxide (CO2)
and the changes in the sorbed gas concentration and matrix properties were studied using dual-energy X-ray computed tomography (X-ray CT). The use of dual-energy technique enabled the
quantification of spatial and temporal variations in bulk density and effective atomic number.
These two quantities were used to calculate separately the amount of sorbed gas and the changes
in the coal matrix and to reveal the kinetics of the complex heterogeneous processes occurring
with CO2 injection in a consolidated bituminous coal sample kept under a constant effective stress.
The swelling and CO2 sorption in coal are heterogeneous processes and different parts of the
coal behaved differently. Vitrite, liptite, and clarite microlithotypes swelled due to dissolution of
CO2, while the clay + inertite region was compressed, even though it was the region that had
the highest CO2 concentration. The vitrite swelled and de-swelled on the time scale of the
experiment (5000−7000 min at each pressure) demonstrating that CO2 dissolution enabled rapid
coal structure changes. Sorption kinetics were also heterogeneous: CO2 uptake was fastest for
the clay + inertite region. Vitrite swelling reached a maximum then decreased with the expulsion
of CO2, a behavior that has been observed in polymers.
We have investigated porosity and permeability damage around perforations using a combination of transient analysis and X-ray CT. The method applied allowed us to perform the entire experiments on samples under simulated in-situ stress conditions and to map variations in permeability along the length of the core as well as with radial distance from the perforation. Berea (10.2-cm (4-in.) dia) cores saturated with low-viscosity silicone oil were perforated using conventional-shaped charges (6-g HMX) and API RP43 procedures by using 6.88-MPa (1000-psi) effective stress and 5.16-MPa (750-psi) and 2.61-MPa (350-psi) underbalance. Low-permeability Torrey Buff Sandstone was also perforated using 5.16-MPa (750-psi) underbalance. After sufficiently flowing the perforations, higher-viscosity silicone oil was injected. The movement of fluids was tracked using X-ray CT to measure the local velocity of the viscous fluid front at different locations along the perforation. Results of these tests were compared in terms of permeability and porosity damage. Quantitative analysis on Berea cores show, for the specific charge and test conditions used, that damage extends approximately 2 cm (0.78 in.) from the center of the perforation. Comparison of tests performed with 2.41-MPa (350-psi) and 5.16-MPa (750-psi) underbalance show a clear increase in permeability near the tunnel wall at the higher underbalance. A zone of somewhat-reduced permeability exists at approximately 1.7 cm from the perforation center in the latter case. Porosity profiles calculated show that porosity is almost uniform out from the tunnel and there is no compacted zone near the tunnel wall in liquid-saturated cores. However, there is a high-porosity zone from the tunnel wall out about 2 mm. This may be due to a region of circumferential partings and small cracks that lead to high porosity or due to the possible artifacts discussed in the paper. Qualitative results have also been obtained for a tight sandstone for which underbalance was insufficient to remove debris from the perforation tunnel. CT images reveal that the plugged tunnel acts as a conduit for fluid flow, showing that the plugging material has significantly higher permeability than the surrounding rock.
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