Study on flow and mass transport through fractured soft sedimentary rocks

The objective of this study is to establish a conceptual model for flow and transport in fractured soft sedimentary rock. In this fiscal year, this study was carried out for two tasks: (1) classification of soft sedimentary rocks by literature survey, (2) laboratory experiments on the flow and mass transport using rock specimens obtained at HDB-4 borehole in Horonobe Deep Underground Research Center. Classification of soft sedimentary rocks is carried out using two databases of rock properties collected by JNC and Geological Survey of Japan. In laboratory experiments, core samples of the Tertiary siliceous mud rock obtained from the HDB-4 borehole, which was excavated in the Horonobe area in this year, were studied. The matrix porosity of intact cores measured by wet and dry weight was ranging from 33% 41%. The hydraulic conductivities of rock matrix determined by the transient pulse test were in an order of 10-11 m/s. The effective diffusion coefficients obtained from the 'through diffusion test' were 0.8 to 2.5 X 10-11 m2/s. This suggested that, in these cores, the flow in the rock matrix is not significant comparing with the flow in fractures, and the matrix diffusion from fractures into rock matrix should cause a significant effect to the mass transport phenomenon. Non-sorbing tracer experiment was conducted in fractured cores. As a tracer, potassium iodide solution was injected at a constant rate and the tracer concentration change during injection was observed at the outlet. The obtained breakthrough curve was interpreted and fitted by using a numerical simulator for flow and mass transport called FRAC3DVS (Therrien et al., 1999), and finally, mass transport parameters are obtained. The result indicates that the matrix diffusion plays an important role for mass transport in the fractured soft sedimentary rocks. (author)