Estimation of permeability in depth of a single fracture in Neogene mudstone by using laboratory measurements

We operated laboratory permeability measurements with Neogene mudstone specimens in order to investigate an applicability of laboratory tests to estimate fracture permeability in depth. We collected rock samples from drill cores of Koetoi Formation diatomaceous mudstone and Wakkanai Formation siliceous mudstone at Horonobe area, northern Hokkaido, Japan. We prepared specimens with a saw-cut discontinuity, simulating a fracture in rock, and measured permeability in the direction parallel to the discontinuity under hydrostatic stress condition up to 80 MPa. The measured permeability of a specimen with a saw-cut discontinuity fracture was larger than that of an intact specimen when confining pressure is less than 1 MPa (comparable to the depth of approximately 150 m) in the case of Koetoi Formation diatomaceous mudstone, but, when confining pressure is 1 MPa or more, the permeability values of a saw-cut specimen and an intact specimen became a similar to each other. On the other hand, in the case of Wakkanai Formation siliceous mudstone, the confining pressure under which the permeability of a saw-cut specimen became similar to the permeability of an intact specimen was close to 80 MPa (comparable to the depth of several km). These results can explain the difference between Koetoi and Wakkanai Formations on distribution of in situ permeability measurements operated by Japan Atomic Energy Agency. Numerical simulations of a flow through a fracture under normal stress could explain the results of the laboratory test for Koetoi Formation diatomaceous mudstone, by using reasonable mechanical properties. These results support the possibility of estimating stress dependency of fracture permeability from rock mechanical properties and permeability of intact part. (author)