Experimental and modeling studies on gas migration in Kunigel V1 bentonite

The knowledge obtained from previous studies are as follows, 1) The gas permeabilities are 10-17 m2 for the 30wt% sand mixtures at a dry density of 1.6 Mg m-3 and 10-20 to 10-21 m2 for the bentonite (100%) at a dry density of 1.8 Mg m-3, 2) The breakthrough pressure seems to be almost the same as the swelling pressure at constant volume condition, 3) Gas pathways created during the first gas injection period are closed due to bentonite swelling during the resaturation period. For the recent experiment, two peaks of gas flow rate are obtained. In particular, maximum flow rate at the secondary peak is obtained approximately 1,667 cc min-1. This peak is probably indicative of generation of cracks in the specimen by particle displacement. Breakthrough pressure (2.5 MPa) is lager than the swelling pressure of the bentonite (swelling pressure is approximately 1.0 MP at a dry density of 1.6 Mg m-3). It may be that there is a time lag between the gas pressure change in the clay and expansion of cracks due to large pumping rate (0.05 cc min-1). The gas migration pathways are unstable due to stress condition in bentonite specimen and/or heterogeneity of specimen. The distribution of bulk density in the specimen was measured to demonstrate that X-ray CT was reliable new technique for the non-destructive measurement of gas migration through bentonite specimen. The degree of X-ray attenuation depends on the bulk density of the bainite specimen. The change in bulk density in this test specimen is not clarified from this test case. These experimental results are probably indicative of migration along preferential pathway rather than uniform flow in the matrix of bentonite specimen. A modified TOUGH2 simulator which has a gas flow model based on Kozeny-Carman relationship was developed and applied to simulate of gas migration test in compacted bentonite. Although the results of the simulation reasonably agreed with obtained experimental data around gas breakthrough phenomenon, it has some difficulties to simulate 'burst flow' which has extremely large and instantaneous increase of gas out flow. From these results, despite these difficulties, the continuous approach which can be conveniently coupled with phenomena of mechanical nature is one of the potential methods of describing gas migration in clay material. (author)