Coal swelling strain and permeability change with injecting liquid/supercritical CO2 and N2 at stress-constrained conditions

CO2 sequestration in deep unmineable coalbeds is regarded as a viable option for carbon storage. On the other hand, many uncertainties still remain due to the fact that coal interacts with CO2 in a variety of ways. In Japan, the first CO2 Enhanced Coalbed Methane Recovery field trials at Yubari were carried out. CO2 was injected from an injection well into a coalbed at a depth of 900 m, and coalbed methane was collected from an observation well. Since the CO2 injection rate was an order of magnitude lower than that estimated by preliminary analyses, N-2 was injected in an attempt to improve it. However, this caused only a temporary increase in the CO2 injection rate. To better understand the phenomena observed in the Yubari field tests, two laboratory experiments were conducted under stress-constrained conditions. In Test I. liquid CO2 was injected into a water-saturated coal specimen and then heated and injected as supercritical CO2. This was to simulate the initial stage of CO2 injection at Yubari when the coal seam was saturated with water. In Test II, supercritical CO2 was injected into a coal specimen saturated with N-2, and then N-2 and CO2 were repeatedly injected. This test was to simulate the case of N-2 injection and CO2 re-injection at Yubari. In Test I, a volumetric swelling strain of 0.25 to 0.5% was observed after injecting liquid CO2. However, in Test II, the swelling strain was about 0.5 to 0.8% after injecting supercritical CO2. Following further injection of N-2 in Test II, slow strain recovery was observed in the coal. At an effective stress of 2 MPa, the permeability of the water-saturated coal specimen was 2 x 10(-6) darcy. In contrast, the permeability of the N-2-saturated coal specimen was originally 5 x 10(-4) to 9 x 10(-4) darcy, and after injection of supercritical CO2 it decreased to 2 x 10(-4) darcy. Further injections of N-2 and supercritical CO2 caused little subsequent change in permeability. These results suggest that when liquid CO2 was injected into the water-saturated coal specimen, it did not completely displace the water in the coal matrix. To further investigate the coal swelling and permeability behavior during gas injection, elastic wave velocity measurements were carried out and the results were found to validate those obtained using strain gauges. The results indicate that coal swelling is likely to be the main cause for the permeability change in the Yubari field tests and thus provide useful information for modeling the field trial. (C) 2010 Elsevier B.V. All rights reserved.