A new laboratory test for shear fracture formation and its permeability measurement

摘要

Fractures in subsurface rocks should be closed and their surfaces contact each other due to rock stress in compression. However, the fractures can maintain their permeability, since they retain residual apertures caused by misfit of rough surfaces even at high normal stress. It is generally considered that if shear sliding of fractures occur, the shear displacement will enhance the misfit of fracture surfaces and it will lead to increase in fracture permeability. Actually it has been observed through laboratory experiments that permeability of fractures with shear displacements were increased by several orders compared with that of fractures with no shear displacement (e.g., Chen et al., 2000). In the experiments, a cylindrical specimen with a fracture was prepared by coring a rock block containing a natural or artificially-created fracture. The specimen with a fracture in shear condition was prepared as follows; temporarily separating the fracture surfaces, they were fitted again with a shear displacement of few millimeters in the axial direction. Then the specimen was supplied for fluid flow experiments on the fracture with a shear displacement to measure its permeability. However, under natural conditions in subsurface rocks, it should be hard for fracture surfaces to be separated completely as far as pore pressure in the fracture is not raised abnormally to be equal to normal stress on the fracture. Thus the shear sliding of fracture should occur actually while keeping contact of the fracture surfaces, and such a manner is different to that of laboratory experiment.