An Experimental Study on Normal Stiffness and Flow Conductance of a Small-scale Hydraulic Fracture in a Granite

Abstract

A hydraulic fracture was made in the laboratory parallel to the rift plane in a block of Inada granite. Cylindrical specimens containing a part of the fracture were then taken from the block by boring the block perpendicular to the fracture surfaces. Two kinds of the specimens were prepared, separated specimens called fracture specimen and non-separated specimens called process zone specimen, and both flow conductance and closure of the fracture were measured simultaneously under normal stress up to 25 MPa.Main results obtained in this study are summarized as follows: (1) Both the fracture closure and the flow conductance of the fracture specimen were much greater than those of the process zone specimen. (2) The fracture was dozed non-uniformly. The fracture closure of the fracture specimen increased with normal stress obeying to Goodman's empirical relation except for high levels of normal stress. (3) The flow conductance of the fracture specimen decreased with the fracture closure according to the so-called cubic law for normal stress up to 5∼7 MPa. For the process zone specimen, the cubic law did not hold for the whole range of this study. The flow conductance of the process zone specimen did not decrease appreciably when normal stress was greater than 8∼9 MPa. (4) The hydraulic initial aperture, calculated from the cubic law, of the fracture specimen increased both with the fractal dimension of the surface roughness and with the mean mechanical initial aperture. However, the hydraulic initial aperture was much smaller than the mechanical initial aperture. (5) Normal stiffness of the hydraulic fracture was not a simple function of the rms of the surface roughness. The flow conductance of both the fracture specimen and the process zone specimen increased with the slope B in the log (normal stress)-fracture closure diagram.