Consolidation of Ethanol/Bentonite Slurry Injected in a Planar Fracture; Mathematical Modelling and Experiment
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Consolidation of Ethanol/Bentonite Slurry Injected in a Planar Fracture; Mathematical Modelling and Experiment
Hiroyasu Takase*, Kengo Iwasa**, Takashi Ishii**,Hiroyoshi Ueda***, Yasushi Sakabe***, Katsuhiko Ishiguro*** *Quintessa Japan, **Shimizu Corporation, Japan, ***Nuclear Waste Management Organization of Japan ABSTRACT Ethanol/bentonite slurry is an alternative to the conventional cement-based material that may cause chemical disturbances to the host rock and the buffer material. In this study we carried out an experiment in which the ethanol/bentonite slurry was injected into a planar synthetic fracture at a constant rate. The pressure build-up due to the increase in viscosity of the bentonite ethanol slurry was observed and, eventually, consolidation of the slurry led to termination of the injection. A mathematical model was also developed to simulate the advection, dispersion and matrix dispersion of ethanol that contributed to the increase in viscosity of the ethanol/bentonite slurry, together with a Stoke’s flow of the slurry before consolidation. Furthermore, applicability of the ethanol/bentonite slurry as a grouting material in a range of geological settings is discussed based on the result of a case study using the same model.
INTRODUCTION Transmissive fractures are likely to exist in any deep rock mass which hosts a Japanese repository for high-level radioactive waste (JNC, 2000). To ensure safety and efficiency of construction and operation of the repository, such fractures need to be grouted at points where they intersect the facility. Cement-based grouting materials that have been used extensively in the past may, however, react with groundwater and produce hyperalkaline leachate (e.g. Metcalfe and Walker, 2004). This solution may cause dissolution of aluminosilicate minerals in the host rock and the bentonite buffer material (Metcalfe and Walker, 2004), the integrity of which is essential for assuring long-term safety. An alternative to cement-based grout is bentonite-water slurry. However, at the desired density, the viscosity of such a material is too high to allow injection into small fractures (Asada, 2003). In contrast, ethanol/bentonite slurry, with a density of up to 0.6 Mg/m3 and with an ethanol concentration of 60%, exhibits much lower viscosity than the bentonite-water slurry, and hence can be injected into fractures with small apertures (0.1 mm or less) (Asada, 2003). Once ethanol/bentonite slurry is injected into the fractures, the ethanol concentration decreases due to diffusion, dispersion and consumption by microbial activity (Japan Alcohol Association, 2000). The decrease in the ethanol concentration produces the sealing capability required as a grouting material and this may be attributed to a possible increase in the viscosity of the ethanol/bentonite slurry (Asada, 2003). To support the argument concerning the mechanism for the ethanol/bentonite slurry injected into a small fracture to function as a grouting material, we measured the viscosity of the ethanol/bentonite slurry
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